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2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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9 <title>D-Bus Specification</title>
10 <releaseinfo>Version 0.12</releaseinfo>
11 <date>7 November 2006</date>
14 <firstname>Havoc</firstname>
15 <surname>Pennington</surname>
17 <orgname>Red Hat, Inc.</orgname>
19 <email>hp@pobox.com</email>
24 <firstname>Anders</firstname>
25 <surname>Carlsson</surname>
27 <orgname>CodeFactory AB</orgname>
29 <email>andersca@codefactory.se</email>
34 <firstname>Alexander</firstname>
35 <surname>Larsson</surname>
37 <orgname>Red Hat, Inc.</orgname>
39 <email>alexl@redhat.com</email>
46 <sect1 id="introduction">
47 <title>Introduction</title>
49 D-Bus is a system for low-latency, low-overhead, easy to use
50 interprocess communication (IPC). In more detail:
54 D-Bus is <emphasis>low-latency</emphasis> because it is designed
55 to avoid round trips and allow asynchronous operation, much like
61 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
62 binary protocol, and does not have to convert to and from a text
63 format such as XML. Because D-Bus is intended for potentially
64 high-resolution same-machine IPC, not primarily for Internet IPC,
65 this is an interesting optimization.
70 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
71 of <firstterm>messages</firstterm> rather than byte streams, and
72 automatically handles a lot of the hard IPC issues. Also, the D-Bus
73 library is designed to be wrapped in a way that lets developers use
74 their framework's existing object/type system, rather than learning
75 a new one specifically for IPC.
82 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
83 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
84 a system for one application to talk to a single other
85 application. However, the primary intended application of the protocol is the
86 D-Bus <firstterm>message bus</firstterm>, specified in <xref
87 linkend="message-bus"/>. The message bus is a special application that
88 accepts connections from multiple other applications, and forwards
93 Uses of D-Bus include notification of system changes (notification of when
94 a camera is plugged in to a computer, or a new version of some software
95 has been installed), or desktop interoperability, for example a file
96 monitoring service or a configuration service.
100 D-Bus is designed for two specific use cases:
104 A "system bus" for notifications from the system to user sessions,
105 and to allow the system to request input from user sessions.
110 A "session bus" used to implement desktop environments such as
115 D-Bus is not intended to be a generic IPC system for any possible
116 application, and intentionally omits many features found in other
117 IPC systems for this reason.
121 At the same time, the bus daemons offer a number of features not found in
122 other IPC systems, such as single-owner "bus names" (similar to X
123 selections), on-demand startup of services, and security policies.
124 In many ways, these features are the primary motivation for developing
125 D-Bus; other systems would have sufficed if IPC were the only goal.
129 D-Bus may turn out to be useful in unanticipated applications, but future
130 versions of this spec and the reference implementation probably will not
131 incorporate features that interfere with the core use cases.
135 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
136 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
137 document are to be interpreted as described in RFC 2119. However, the
138 document could use a serious audit to be sure it makes sense to do
139 so. Also, they are not capitalized.
144 <sect1 id="message-protocol">
145 <title>Message Protocol</title>
148 A <firstterm>message</firstterm> consists of a
149 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
150 think of a message as a package, the header is the address, and the body
151 contains the package contents. The message delivery system uses the header
152 information to figure out where to send the message and how to interpret
153 it; the recipient interprets the body of the message.
157 The body of the message is made up of zero or more
158 <firstterm>arguments</firstterm>, which are typed values, such as an
159 integer or a byte array.
163 Both header and body use the same type system and format for
164 serializing data. Each type of value has a wire format.
165 Converting a value from some other representation into the wire
166 format is called <firstterm>marshaling</firstterm> and converting
167 it back from the wire format is <firstterm>unmarshaling</firstterm>.
170 <sect2 id="message-protocol-signatures">
171 <title>Type Signatures</title>
174 The D-Bus protocol does not include type tags in the marshaled data; a
175 block of marshaled values must have a known <firstterm>type
176 signature</firstterm>. The type signature is made up of <firstterm>type
177 codes</firstterm>. A type code is an ASCII character representing the
178 type of a value. Because ASCII characters are used, the type signature
179 will always form a valid ASCII string. A simple string compare
180 determines whether two type signatures are equivalent.
184 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
185 the ASCII character 'i'. So the signature for a block of values
186 containing a single <literal>INT32</literal> would be:
190 A block of values containing two <literal>INT32</literal> would have this signature:
197 All <firstterm>basic</firstterm> types work like
198 <literal>INT32</literal> in this example. To marshal and unmarshal
199 basic types, you simply read one value from the data
200 block corresponding to each type code in the signature.
201 In addition to basic types, there are four <firstterm>container</firstterm>
202 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
203 and <literal>DICT_ENTRY</literal>.
207 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
208 code does not appear in signatures. Instead, ASCII characters
209 '(' and ')' are used to mark the beginning and end of the struct.
210 So for example, a struct containing two integers would have this
215 Structs can be nested, so for example a struct containing
216 an integer and another struct:
220 The value block storing that struct would contain three integers; the
221 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
226 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
227 but is useful in code that implements the protocol. This type code
228 is specified to allow such code to interoperate in non-protocol contexts.
232 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
233 followed by a <firstterm>single complete type</firstterm>. The single
234 complete type following the array is the type of each array element. So
235 the simple example is:
239 which is an array of 32-bit integers. But an array can be of any type,
240 such as this array-of-struct-with-two-int32-fields:
244 Or this array of array of integer:
251 The phrase <firstterm>single complete type</firstterm> deserves some
252 definition. A single complete type is a basic type code, a variant type code,
253 an array with its element type, or a struct with its fields.
254 So the following signatures are not single complete types:
264 And the following signatures contain multiple complete types:
274 Note however that a single complete type may <emphasis>contain</emphasis>
275 multiple other single complete types.
279 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
280 type <literal>VARIANT</literal> will have the signature of a single complete type as part
281 of the <emphasis>value</emphasis>. This signature will be followed by a
282 marshaled value of that type.
286 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
287 than parentheses it uses curly braces, and it has more restrictions.
288 The restrictions are: it occurs only as an array element type; it has
289 exactly two single complete types inside the curly braces; the first
290 single complete type (the "key") must be a basic type rather than a
291 container type. Implementations must not accept dict entries outside of
292 arrays, must not accept dict entries with zero, one, or more than two
293 fields, and must not accept dict entries with non-basic-typed keys. A
294 dict entry is always a key-value pair.
298 The first field in the <literal>DICT_ENTRY</literal> is always the key.
299 A message is considered corrupt if the same key occurs twice in the same
300 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
301 implementations are not required to reject dicts with duplicate keys.
305 In most languages, an array of dict entry would be represented as a
306 map, hash table, or dict object.
310 The following table summarizes the D-Bus types.
315 <entry>Conventional Name</entry>
317 <entry>Description</entry>
322 <entry><literal>INVALID</literal></entry>
323 <entry>0 (ASCII NUL)</entry>
324 <entry>Not a valid type code, used to terminate signatures</entry>
326 <entry><literal>BYTE</literal></entry>
327 <entry>121 (ASCII 'y')</entry>
328 <entry>8-bit unsigned integer</entry>
330 <entry><literal>BOOLEAN</literal></entry>
331 <entry>98 (ASCII 'b')</entry>
332 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
334 <entry><literal>INT16</literal></entry>
335 <entry>110 (ASCII 'n')</entry>
336 <entry>16-bit signed integer</entry>
338 <entry><literal>UINT16</literal></entry>
339 <entry>113 (ASCII 'q')</entry>
340 <entry>16-bit unsigned integer</entry>
342 <entry><literal>INT32</literal></entry>
343 <entry>105 (ASCII 'i')</entry>
344 <entry>32-bit signed integer</entry>
346 <entry><literal>UINT32</literal></entry>
347 <entry>117 (ASCII 'u')</entry>
348 <entry>32-bit unsigned integer</entry>
350 <entry><literal>INT64</literal></entry>
351 <entry>120 (ASCII 'x')</entry>
352 <entry>64-bit signed integer</entry>
354 <entry><literal>UINT64</literal></entry>
355 <entry>116 (ASCII 't')</entry>
356 <entry>64-bit unsigned integer</entry>
358 <entry><literal>DOUBLE</literal></entry>
359 <entry>100 (ASCII 'd')</entry>
360 <entry>IEEE 754 double</entry>
362 <entry><literal>STRING</literal></entry>
363 <entry>115 (ASCII 's')</entry>
364 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated.</entry>
366 <entry><literal>OBJECT_PATH</literal></entry>
367 <entry>111 (ASCII 'o')</entry>
368 <entry>Name of an object instance</entry>
370 <entry><literal>SIGNATURE</literal></entry>
371 <entry>103 (ASCII 'g')</entry>
372 <entry>A type signature</entry>
374 <entry><literal>ARRAY</literal></entry>
375 <entry>97 (ASCII 'a')</entry>
378 <entry><literal>STRUCT</literal></entry>
379 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
380 <entry>Struct</entry>
382 <entry><literal>VARIANT</literal></entry>
383 <entry>118 (ASCII 'v') </entry>
384 <entry>Variant type (the type of the value is part of the value itself)</entry>
386 <entry><literal>DICT_ENTRY</literal></entry>
387 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
388 <entry>Entry in a dict or map (array of key-value pairs)</entry>
397 <sect2 id="message-protocol-marshaling">
398 <title>Marshaling (Wire Format)</title>
401 Given a type signature, a block of bytes can be converted into typed
402 values. This section describes the format of the block of bytes. Byte
403 order and alignment issues are handled uniformly for all D-Bus types.
407 A block of bytes has an associated byte order. The byte order
408 has to be discovered in some way; for D-Bus messages, the
409 byte order is part of the message header as described in
410 <xref linkend="message-protocol-messages"/>. For now, assume
411 that the byte order is known to be either little endian or big
416 Each value in a block of bytes is aligned "naturally," for example
417 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
418 8-byte boundary. To properly align a value, <firstterm>alignment
419 padding</firstterm> may be necessary. The alignment padding must always
420 be the minimum required padding to properly align the following value;
421 and it must always be made up of nul bytes. The alignment padding must
422 not be left uninitialized (it can't contain garbage), and more padding
423 than required must not be used.
427 Given all this, the types are marshaled on the wire as follows:
432 <entry>Conventional Name</entry>
433 <entry>Encoding</entry>
434 <entry>Alignment</entry>
439 <entry><literal>INVALID</literal></entry>
440 <entry>Not applicable; cannot be marshaled.</entry>
443 <entry><literal>BYTE</literal></entry>
444 <entry>A single 8-bit byte.</entry>
447 <entry><literal>BOOLEAN</literal></entry>
448 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
451 <entry><literal>INT16</literal></entry>
452 <entry>16-bit signed integer in the message's byte order.</entry>
455 <entry><literal>UINT16</literal></entry>
456 <entry>16-bit unsigned integer in the message's byte order.</entry>
459 <entry><literal>INT32</literal></entry>
460 <entry>32-bit signed integer in the message's byte order.</entry>
463 <entry><literal>UINT32</literal></entry>
464 <entry>32-bit unsigned integer in the message's byte order.</entry>
467 <entry><literal>INT64</literal></entry>
468 <entry>64-bit signed integer in the message's byte order.</entry>
471 <entry><literal>UINT64</literal></entry>
472 <entry>64-bit unsigned integer in the message's byte order.</entry>
475 <entry><literal>DOUBLE</literal></entry>
476 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
479 <entry><literal>STRING</literal></entry>
480 <entry>A <literal>UINT32</literal> indicating the string's
481 length in bytes excluding its terminating nul, followed by
482 string data of the given length, followed by a terminating nul
489 <entry><literal>OBJECT_PATH</literal></entry>
490 <entry>Exactly the same as <literal>STRING</literal> except the
491 content must be a valid object path (see below).
497 <entry><literal>SIGNATURE</literal></entry>
498 <entry>The same as <literal>STRING</literal> except the length is a single
499 byte (thus signatures have a maximum length of 255)
500 and the content must be a valid signature (see below).
506 <entry><literal>ARRAY</literal></entry>
508 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
509 alignment padding to the alignment boundary of the array element type,
510 followed by each array element. The array length is from the
511 end of the alignment padding to the end of the last element,
512 i.e. it does not include the padding after the length,
513 or any padding after the last element.
514 Arrays have a maximum length defined to be 2 to the 26th power or
515 67108864. Implementations must not send or accept arrays exceeding this
522 <entry><literal>STRUCT</literal></entry>
524 A struct must start on an 8-byte boundary regardless of the
525 type of the struct fields. The struct value consists of each
526 field marshaled in sequence starting from that 8-byte
533 <entry><literal>VARIANT</literal></entry>
535 A variant type has a marshaled <literal>SIGNATURE</literal>
536 followed by a marshaled value with the type
537 given in the signature.
538 Unlike a message signature, the variant signature
539 can contain only a single complete type.
540 So "i" is OK, "ii" is not.
543 1 (alignment of the signature)
546 <entry><literal>DICT_ENTRY</literal></entry>
559 <sect3 id="message-protocol-marshaling-object-path">
560 <title>Valid Object Paths</title>
563 An object path is a name used to refer to an object instance.
564 Conceptually, each participant in a D-Bus message exchange may have
565 any number of object instances (think of C++ or Java objects) and each
566 such instance will have a path. Like a filesystem, the object
567 instances in an application form a hierarchical tree.
571 The following rules define a valid object path. Implementations must
572 not send or accept messages with invalid object paths.
576 The path may be of any length.
581 The path must begin with an ASCII '/' (integer 47) character,
582 and must consist of elements separated by slash characters.
587 Each element must only contain the ASCII characters
593 No element may be the empty string.
598 Multiple '/' characters cannot occur in sequence.
603 A trailing '/' character is not allowed unless the
604 path is the root path (a single '/' character).
613 <sect3 id="message-protocol-marshaling-signature">
614 <title>Valid Signatures</title>
616 An implementation must not send or accept invalid signatures.
617 Valid signatures will conform to the following rules:
621 The signature ends with a nul byte.
626 The signature is a list of single complete types.
627 Arrays must have element types, and structs must
628 have both open and close parentheses.
633 Only type codes and open and close parentheses are
634 allowed in the signature. The <literal>STRUCT</literal> type code
635 is not allowed in signatures, because parentheses
641 The maximum depth of container type nesting is 32 array type
642 codes and 32 open parentheses. This implies that the maximum
643 total depth of recursion is 64, for an "array of array of array
644 of ... struct of struct of struct of ..." where there are 32
650 The maximum length of a signature is 255.
655 Signatures must be nul-terminated.
664 <sect2 id="message-protocol-messages">
665 <title>Message Format</title>
668 A message consists of a header and a body. The header is a block of
669 values with a fixed signature and meaning. The body is a separate block
670 of values, with a signature specified in the header.
674 The length of the header must be a multiple of 8, allowing the body to
675 begin on an 8-byte boundary when storing the entire message in a single
676 buffer. If the header does not naturally end on an 8-byte boundary
677 up to 7 bytes of nul-initialized alignment padding must be added.
681 The message body need not end on an 8-byte boundary.
685 The maximum length of a message, including header, header alignment padding,
686 and body is 2 to the 27th power or 134217728. Implementations must not
687 send or accept messages exceeding this size.
691 The signature of the header is:
695 Written out more readably, this is:
697 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
702 These values have the following meanings:
708 <entry>Description</entry>
713 <entry>1st <literal>BYTE</literal></entry>
714 <entry>Endianness flag; ASCII 'l' for little-endian
715 or ASCII 'B' for big-endian. Both header and body are
716 in this endianness.</entry>
719 <entry>2nd <literal>BYTE</literal></entry>
720 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
721 Currently-defined types are described below.
725 <entry>3rd <literal>BYTE</literal></entry>
726 <entry>Bitwise OR of flags. Unknown flags
727 must be ignored. Currently-defined flags are described below.
731 <entry>4th <literal>BYTE</literal></entry>
732 <entry>Major protocol version of the sending application. If
733 the major protocol version of the receiving application does not
734 match, the applications will not be able to communicate and the
735 D-Bus connection must be disconnected. The major protocol
736 version for this version of the specification is 0.
737 FIXME this field is stupid and pointless to put in
742 <entry>1st <literal>UINT32</literal></entry>
743 <entry>Length in bytes of the message body, starting
744 from the end of the header. The header ends after
745 its alignment padding to an 8-boundary.
749 <entry>2nd <literal>UINT32</literal></entry>
750 <entry>The serial of this message, used as a cookie
751 by the sender to identify the reply corresponding
756 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
757 <entry>An array of zero or more <firstterm>header
758 fields</firstterm> where the byte is the field code, and the
759 variant is the field value. The message type determines
760 which fields are required.
768 <firstterm>Message types</firstterm> that can appear in the second byte
774 <entry>Conventional name</entry>
775 <entry>Decimal value</entry>
776 <entry>Description</entry>
781 <entry><literal>INVALID</literal></entry>
783 <entry>This is an invalid type.</entry>
786 <entry><literal>METHOD_CALL</literal></entry>
788 <entry>Method call.</entry>
791 <entry><literal>METHOD_RETURN</literal></entry>
793 <entry>Method reply with returned data.</entry>
796 <entry><literal>ERROR</literal></entry>
798 <entry>Error reply. If the first argument exists and is a
799 string, it is an error message.</entry>
802 <entry><literal>SIGNAL</literal></entry>
804 <entry>Signal emission.</entry>
811 Flags that can appear in the third byte of the header:
816 <entry>Conventional name</entry>
817 <entry>Hex value</entry>
818 <entry>Description</entry>
823 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
825 <entry>This message does not expect method return replies or
826 error replies; the reply can be omitted as an
827 optimization. However, it is compliant with this specification
828 to return the reply despite this flag and the only harm
829 from doing so is extra network traffic.
833 <entry><literal>NO_AUTO_START</literal></entry>
835 <entry>The bus must not launch an owner
836 for the destination name in response to this message.
844 <sect3 id="message-protocol-header-fields">
845 <title>Header Fields</title>
848 The array at the end of the header contains <firstterm>header
849 fields</firstterm>, where each field is a 1-byte field code followed
850 by a field value. A header must contain the required header fields for
851 its message type, and zero or more of any optional header
852 fields. Future versions of this protocol specification may add new
853 fields. Implementations must ignore fields they do not
854 understand. Implementations must not invent their own header fields;
855 only changes to this specification may introduce new header fields.
859 Again, if an implementation sees a header field code that it does not
860 expect, it must ignore that field, as it will be part of a new
861 (but compatible) version of this specification. This also applies
862 to known header fields appearing in unexpected messages, for
863 example: if a signal has a reply serial it must be ignored
864 even though it has no meaning as of this version of the spec.
868 However, implementations must not send or accept known header fields
869 with the wrong type stored in the field value. So for example a
870 message with an <literal>INTERFACE</literal> field of type
871 <literal>UINT32</literal> would be considered corrupt.
875 Here are the currently-defined header fields:
880 <entry>Conventional Name</entry>
881 <entry>Decimal Code</entry>
883 <entry>Required In</entry>
884 <entry>Description</entry>
889 <entry><literal>INVALID</literal></entry>
892 <entry>not allowed</entry>
893 <entry>Not a valid field name (error if it appears in a message)</entry>
896 <entry><literal>PATH</literal></entry>
898 <entry><literal>OBJECT_PATH</literal></entry>
899 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
900 <entry>The object to send a call to,
901 or the object a signal is emitted from.
905 <entry><literal>INTERFACE</literal></entry>
907 <entry><literal>STRING</literal></entry>
908 <entry><literal>SIGNAL</literal></entry>
910 The interface to invoke a method call on, or
911 that a signal is emitted from. Optional for
912 method calls, required for signals.
916 <entry><literal>MEMBER</literal></entry>
918 <entry><literal>STRING</literal></entry>
919 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
920 <entry>The member, either the method name or signal name.</entry>
923 <entry><literal>ERROR_NAME</literal></entry>
925 <entry><literal>STRING</literal></entry>
926 <entry><literal>ERROR</literal></entry>
927 <entry>The name of the error that occurred, for errors</entry>
930 <entry><literal>REPLY_SERIAL</literal></entry>
932 <entry><literal>UINT32</literal></entry>
933 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
934 <entry>The serial number of the message this message is a reply
935 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
938 <entry><literal>DESTINATION</literal></entry>
940 <entry><literal>STRING</literal></entry>
941 <entry>optional</entry>
942 <entry>The name of the connection this message is intended for.
943 Only used in combination with the message bus, see
944 <xref linkend="message-bus"/>.</entry>
947 <entry><literal>SENDER</literal></entry>
949 <entry><literal>STRING</literal></entry>
950 <entry>optional</entry>
951 <entry>Unique name of the sending connection.
952 The message bus fills in this field so it is reliable; the field is
953 only meaningful in combination with the message bus.</entry>
956 <entry><literal>SIGNATURE</literal></entry>
958 <entry><literal>SIGNATURE</literal></entry>
959 <entry>optional</entry>
960 <entry>The signature of the message body.
961 If omitted, it is assumed to be the
962 empty signature "" (i.e. the body must be 0-length).</entry>
971 <sect2 id="message-protocol-names">
972 <title>Valid Names</title>
974 The various names in D-Bus messages have some restrictions.
977 There is a <firstterm>maximum name length</firstterm>
978 of 255 which applies to bus names, interfaces, and members.
980 <sect3 id="message-protocol-names-interface">
981 <title>Interface names</title>
983 Interfaces have names with type <literal>STRING</literal>, meaning that
984 they must be valid UTF-8. However, there are also some
985 additional restrictions that apply to interface names
988 <listitem><para>Interface names are composed of 1 or more elements separated by
989 a period ('.') character. All elements must contain at least
993 <listitem><para>Each element must only contain the ASCII characters
994 "[A-Z][a-z][0-9]_" and must not begin with a digit.
998 <listitem><para>Interface names must contain at least one '.' (period)
999 character (and thus at least two elements).
1002 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1003 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1007 <sect3 id="message-protocol-names-bus">
1008 <title>Bus names</title>
1010 Connections have one or more bus names associated with them.
1011 A connection has exactly one bus name that is a unique connection
1012 name. The unique connection name remains with the connection for
1013 its entire lifetime.
1014 A bus name is of type <literal>STRING</literal>,
1015 meaning that it must be valid UTF-8. However, there are also
1016 some additional restrictions that apply to bus names
1019 <listitem><para>Bus names that start with a colon (':')
1020 character are unique connection names.
1023 <listitem><para>Bus names are composed of 1 or more elements separated by
1024 a period ('.') character. All elements must contain at least
1028 <listitem><para>Each element must only contain the ASCII characters
1029 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1030 connection name may begin with a digit, elements in
1031 other bus names must not begin with a digit.
1035 <listitem><para>Bus names must contain at least one '.' (period)
1036 character (and thus at least two elements).
1039 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1040 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1044 Note that the hyphen ('-') character is allowed in bus names but
1045 not in interface names.
1048 <sect3 id="message-protocol-names-member">
1049 <title>Member names</title>
1051 Member (i.e. method or signal) names:
1053 <listitem><para>Must only contain the ASCII characters
1054 "[A-Z][a-z][0-9]_" and may not begin with a
1055 digit.</para></listitem>
1056 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1057 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1058 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1062 <sect3 id="message-protocol-names-error">
1063 <title>Error names</title>
1065 Error names have the same restrictions as interface names.
1070 <sect2 id="message-protocol-types">
1071 <title>Message Types</title>
1073 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1074 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1075 This section describes these conventions.
1077 <sect3 id="message-protocol-types-method">
1078 <title>Method Calls</title>
1080 Some messages invoke an operation on a remote object. These are
1081 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1082 messages map naturally to methods on objects in a typical program.
1085 A method call message is required to have a <literal>MEMBER</literal> header field
1086 indicating the name of the method. Optionally, the message has an
1087 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1088 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1089 a method with the same name, it is undefined which of the two methods
1090 will be invoked. Implementations may also choose to return an error in
1091 this ambiguous case. However, if a method name is unique
1092 implementations must not require an interface field.
1095 Method call messages also include a <literal>PATH</literal> field
1096 indicating the object to invoke the method on. If the call is passing
1097 through a message bus, the message will also have a
1098 <literal>DESTINATION</literal> field giving the name of the connection
1099 to receive the message.
1102 When an application handles a method call message, it is required to
1103 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1104 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1105 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1108 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1109 are the return value(s) or "out parameters" of the method call.
1110 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1111 and the call fails; no return value will be provided. It makes
1112 no sense to send multiple replies to the same method call.
1115 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1116 reply is required, so the caller will know the method
1117 was successfully processed.
1120 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1124 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1125 then as an optimization the application receiving the method
1126 call may choose to omit the reply message (regardless of
1127 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1128 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1129 flag and reply anyway.
1132 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1133 destination name does not exist then a program to own the destination
1134 name will be started before the message is delivered. The message
1135 will be held until the new program is successfully started or has
1136 failed to start; in case of failure, an error will be returned. This
1137 flag is only relevant in the context of a message bus, it is ignored
1138 during one-to-one communication with no intermediate bus.
1140 <sect4 id="message-protocol-types-method-apis">
1141 <title>Mapping method calls to native APIs</title>
1143 APIs for D-Bus may map method calls to a method call in a specific
1144 programming language, such as C++, or may map a method call written
1145 in an IDL to a D-Bus message.
1148 In APIs of this nature, arguments to a method are often termed "in"
1149 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1150 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1151 "inout" arguments, which are both sent and received, i.e. the caller
1152 passes in a value which is modified. Mapped to D-Bus, an "inout"
1153 argument is equivalent to an "in" argument, followed by an "out"
1154 argument. You can't pass things "by reference" over the wire, so
1155 "inout" is purely an illusion of the in-process API.
1158 Given a method with zero or one return values, followed by zero or more
1159 arguments, where each argument may be "in", "out", or "inout", the
1160 caller constructs a message by appending each "in" or "inout" argument,
1161 in order. "out" arguments are not represented in the caller's message.
1164 The recipient constructs a reply by appending first the return value
1165 if any, then each "out" or "inout" argument, in order.
1166 "in" arguments are not represented in the reply message.
1169 Error replies are normally mapped to exceptions in languages that have
1173 In converting from native APIs to D-Bus, it is perhaps nice to
1174 map D-Bus naming conventions ("FooBar") to native conventions
1175 such as "fooBar" or "foo_bar" automatically. This is OK
1176 as long as you can say that the native API is one that
1177 was specifically written for D-Bus. It makes the most sense
1178 when writing object implementations that will be exported
1179 over the bus. Object proxies used to invoke remote D-Bus
1180 objects probably need the ability to call any D-Bus method,
1181 and thus a magic name mapping like this could be a problem.
1184 This specification doesn't require anything of native API bindings;
1185 the preceding is only a suggested convention for consistency
1191 <sect3 id="message-protocol-types-signal">
1192 <title>Signal Emission</title>
1194 Unlike method calls, signal emissions have no replies.
1195 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1196 It must have three header fields: <literal>PATH</literal> giving the object
1197 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1198 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1199 for signals, though it is optional for method calls.
1203 <sect3 id="message-protocol-types-errors">
1204 <title>Errors</title>
1206 Messages of type <literal>ERROR</literal> are most commonly replies
1207 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1208 to any kind of message. The message bus for example
1209 will return an <literal>ERROR</literal> in reply to a signal emission if
1210 the bus does not have enough memory to send the signal.
1213 An <literal>ERROR</literal> may have any arguments, but if the first
1214 argument is a <literal>STRING</literal>, it must be an error message.
1215 The error message may be logged or shown to the user
1220 <sect3 id="message-protocol-types-notation">
1221 <title>Notation in this document</title>
1223 This document uses a simple pseudo-IDL to describe particular method
1224 calls and signals. Here is an example of a method call:
1226 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1227 out UINT32 resultcode)
1229 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1230 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1231 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1232 characters so it's known that the last part of the name in
1233 the "IDL" is the member name.
1236 In C++ that might end up looking like this:
1238 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1239 unsigned int flags);
1241 or equally valid, the return value could be done as an argument:
1243 void org::freedesktop::DBus::StartServiceByName (const char *name,
1245 unsigned int *resultcode);
1247 It's really up to the API designer how they want to make
1248 this look. You could design an API where the namespace wasn't used
1249 in C++, using STL or Qt, using varargs, or whatever you wanted.
1252 Signals are written as follows:
1254 org.freedesktop.DBus.NameLost (STRING name)
1256 Signals don't specify "in" vs. "out" because only
1257 a single direction is possible.
1260 It isn't especially encouraged to use this lame pseudo-IDL in actual
1261 API implementations; you might use the native notation for the
1262 language you're using, or you might use COM or CORBA IDL, for example.
1267 <sect2 id="message-protocol-handling-invalid">
1268 <title>Invalid Protocol and Spec Extensions</title>
1271 For security reasons, the D-Bus protocol should be strictly parsed and
1272 validated, with the exception of defined extension points. Any invalid
1273 protocol or spec violations should result in immediately dropping the
1274 connection without notice to the other end. Exceptions should be
1275 carefully considered, e.g. an exception may be warranted for a
1276 well-understood idiosyncrasy of a widely-deployed implementation. In
1277 cases where the other end of a connection is 100% trusted and known to
1278 be friendly, skipping validation for performance reasons could also make
1279 sense in certain cases.
1283 Generally speaking violations of the "must" requirements in this spec
1284 should be considered possible attempts to exploit security, and violations
1285 of the "should" suggestions should be considered legitimate (though perhaps
1286 they should generate an error in some cases).
1290 The following extension points are built in to D-Bus on purpose and must
1291 not be treated as invalid protocol. The extension points are intended
1292 for use by future versions of this spec, they are not intended for third
1293 parties. At the moment, the only way a third party could extend D-Bus
1294 without breaking interoperability would be to introduce a way to negotiate new
1295 feature support as part of the auth protocol, using EXTENSION_-prefixed
1296 commands. There is not yet a standard way to negotiate features.
1300 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1301 commands result in an ERROR rather than a disconnect. This enables
1302 future extensions to the protocol. Commands starting with EXTENSION_ are
1303 reserved for third parties.
1308 The authentication protocol supports pluggable auth mechanisms.
1313 The address format (see <xref linkend="addresses"/>) supports new
1319 Messages with an unknown type (something other than
1320 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1321 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1322 Unknown-type messages must still be well-formed in the same way
1323 as the known messages, however. They still have the normal
1329 Header fields with an unknown or unexpected field code must be ignored,
1330 though again they must still be well-formed.
1335 New standard interfaces (with new methods and signals) can of course be added.
1345 <sect1 id="auth-protocol">
1346 <title>Authentication Protocol</title>
1348 Before the flow of messages begins, two applications must
1349 authenticate. A simple plain-text protocol is used for
1350 authentication; this protocol is a SASL profile, and maps fairly
1351 directly from the SASL specification. The message encoding is
1352 NOT used here, only plain text messages.
1355 In examples, "C:" and "S:" indicate lines sent by the client and
1356 server respectively.
1358 <sect2 id="auth-protocol-overview">
1359 <title>Protocol Overview</title>
1361 The protocol is a line-based protocol, where each line ends with
1362 \r\n. Each line begins with an all-caps ASCII command name containing
1363 only the character range [A-Z_], a space, then any arguments for the
1364 command, then the \r\n ending the line. The protocol is
1365 case-sensitive. All bytes must be in the ASCII character set.
1367 Commands from the client to the server are as follows:
1370 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1371 <listitem><para>CANCEL</para></listitem>
1372 <listitem><para>BEGIN</para></listitem>
1373 <listitem><para>DATA <data in hex encoding></para></listitem>
1374 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1377 From server to client are as follows:
1380 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1381 <listitem><para>OK <GUID in hex></para></listitem>
1382 <listitem><para>DATA <data in hex encoding></para></listitem>
1383 <listitem><para>ERROR</para></listitem>
1387 Unofficial extensions to the command set must begin with the letters
1388 "EXTENSION_", to avoid conflicts with future official commands.
1389 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1392 <sect2 id="auth-nul-byte">
1393 <title>Special credentials-passing nul byte</title>
1395 Immediately after connecting to the server, the client must send a
1396 single nul byte. This byte may be accompanied by credentials
1397 information on some operating systems that use sendmsg() with
1398 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1399 sockets. However, the nul byte must be sent even on other kinds of
1400 socket, and even on operating systems that do not require a byte to be
1401 sent in order to transmit credentials. The text protocol described in
1402 this document begins after the single nul byte. If the first byte
1403 received from the client is not a nul byte, the server may disconnect
1407 A nul byte in any context other than the initial byte is an error;
1408 the protocol is ASCII-only.
1411 The credentials sent along with the nul byte may be used with the
1412 SASL mechanism EXTERNAL.
1415 <sect2 id="auth-command-auth">
1416 <title>AUTH command</title>
1418 If an AUTH command has no arguments, it is a request to list
1419 available mechanisms. The server must respond with a REJECTED
1420 command listing the mechanisms it understands, or with an error.
1423 If an AUTH command specifies a mechanism, and the server supports
1424 said mechanism, the server should begin exchanging SASL
1425 challenge-response data with the client using DATA commands.
1428 If the server does not support the mechanism given in the AUTH
1429 command, it must send either a REJECTED command listing the mechanisms
1430 it does support, or an error.
1433 If the [initial-response] argument is provided, it is intended for use
1434 with mechanisms that have no initial challenge (or an empty initial
1435 challenge), as if it were the argument to an initial DATA command. If
1436 the selected mechanism has an initial challenge and [initial-response]
1437 was provided, the server should reject authentication by sending
1441 If authentication succeeds after exchanging DATA commands,
1442 an OK command must be sent to the client.
1445 The first octet received by the client after the \r\n of the OK
1446 command must be the first octet of the authenticated/encrypted
1447 stream of D-Bus messages.
1450 The first octet received by the server after the \r\n of the BEGIN
1451 command from the client must be the first octet of the
1452 authenticated/encrypted stream of D-Bus messages.
1455 <sect2 id="auth-command-cancel">
1456 <title>CANCEL Command</title>
1458 At any time up to sending the BEGIN command, the client may send a
1459 CANCEL command. On receiving the CANCEL command, the server must
1460 send a REJECTED command and abort the current authentication
1464 <sect2 id="auth-command-data">
1465 <title>DATA Command</title>
1467 The DATA command may come from either client or server, and simply
1468 contains a hex-encoded block of data to be interpreted
1469 according to the SASL mechanism in use.
1472 Some SASL mechanisms support sending an "empty string";
1473 FIXME we need some way to do this.
1476 <sect2 id="auth-command-begin">
1477 <title>BEGIN Command</title>
1479 The BEGIN command acknowledges that the client has received an
1480 OK command from the server, and that the stream of messages
1484 The first octet received by the server after the \r\n of the BEGIN
1485 command from the client must be the first octet of the
1486 authenticated/encrypted stream of D-Bus messages.
1489 <sect2 id="auth-command-rejected">
1490 <title>REJECTED Command</title>
1492 The REJECTED command indicates that the current authentication
1493 exchange has failed, and further exchange of DATA is inappropriate.
1494 The client would normally try another mechanism, or try providing
1495 different responses to challenges.
1497 Optionally, the REJECTED command has a space-separated list of
1498 available auth mechanisms as arguments. If a server ever provides
1499 a list of supported mechanisms, it must provide the same list
1500 each time it sends a REJECTED message. Clients are free to
1501 ignore all lists received after the first.
1504 <sect2 id="auth-command-ok">
1505 <title>OK Command</title>
1507 The OK command indicates that the client has been authenticated,
1508 and that further communication will be a stream of D-Bus messages
1509 (optionally encrypted, as negotiated) rather than this protocol.
1512 The first octet received by the client after the \r\n of the OK
1513 command must be the first octet of the authenticated/encrypted
1514 stream of D-Bus messages.
1517 The client must respond to the OK command by sending a BEGIN
1518 command, followed by its stream of messages, or by disconnecting.
1519 The server must not accept additional commands using this protocol
1520 after the OK command has been sent.
1523 The OK command has one argument, which is the GUID of the server.
1524 See <xref linkend="addresses"/> for more on server GUIDs.
1527 <sect2 id="auth-command-error">
1528 <title>ERROR Command</title>
1530 The ERROR command indicates that either server or client did not
1531 know a command, does not accept the given command in the current
1532 context, or did not understand the arguments to the command. This
1533 allows the protocol to be extended; a client or server can send a
1534 command present or permitted only in new protocol versions, and if
1535 an ERROR is received instead of an appropriate response, fall back
1536 to using some other technique.
1539 If an ERROR is sent, the server or client that sent the
1540 error must continue as if the command causing the ERROR had never been
1541 received. However, the the server or client receiving the error
1542 should try something other than whatever caused the error;
1543 if only canceling/rejecting the authentication.
1546 If the D-Bus protocol changes incompatibly at some future time,
1547 applications implementing the new protocol would probably be able to
1548 check for support of the new protocol by sending a new command and
1549 receiving an ERROR from applications that don't understand it. Thus the
1550 ERROR feature of the auth protocol is an escape hatch that lets us
1551 negotiate extensions or changes to the D-Bus protocol in the future.
1554 <sect2 id="auth-examples">
1555 <title>Authentication examples</title>
1559 <title>Example of successful magic cookie authentication</title>
1561 (MAGIC_COOKIE is a made up mechanism)
1563 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1569 <title>Example of finding out mechanisms then picking one</title>
1572 S: REJECTED KERBEROS_V4 SKEY
1573 C: AUTH SKEY 7ab83f32ee
1574 S: DATA 8799cabb2ea93e
1575 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1581 <title>Example of client sends unknown command then falls back to regular auth</title>
1585 C: AUTH MAGIC_COOKIE 3736343435313230333039
1591 <title>Example of server doesn't support initial auth mechanism</title>
1593 C: AUTH MAGIC_COOKIE 3736343435313230333039
1594 S: REJECTED KERBEROS_V4 SKEY
1595 C: AUTH SKEY 7ab83f32ee
1596 S: DATA 8799cabb2ea93e
1597 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1603 <title>Example of wrong password or the like followed by successful retry</title>
1605 C: AUTH MAGIC_COOKIE 3736343435313230333039
1606 S: REJECTED KERBEROS_V4 SKEY
1607 C: AUTH SKEY 7ab83f32ee
1608 S: DATA 8799cabb2ea93e
1609 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1611 C: AUTH SKEY 7ab83f32ee
1612 S: DATA 8799cabb2ea93e
1613 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1619 <title>Example of skey cancelled and restarted</title>
1621 C: AUTH MAGIC_COOKIE 3736343435313230333039
1622 S: REJECTED KERBEROS_V4 SKEY
1623 C: AUTH SKEY 7ab83f32ee
1624 S: DATA 8799cabb2ea93e
1627 C: AUTH SKEY 7ab83f32ee
1628 S: DATA 8799cabb2ea93e
1629 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1636 <sect2 id="auth-states">
1637 <title>Authentication state diagrams</title>
1640 This section documents the auth protocol in terms of
1641 a state machine for the client and the server. This is
1642 probably the most robust way to implement the protocol.
1645 <sect3 id="auth-states-client">
1646 <title>Client states</title>
1649 To more precisely describe the interaction between the
1650 protocol state machine and the authentication mechanisms the
1651 following notation is used: MECH(CHALL) means that the
1652 server challenge CHALL was fed to the mechanism MECH, which
1658 CONTINUE(RESP) means continue the auth conversation
1659 and send RESP as the response to the server;
1665 OK(RESP) means that after sending RESP to the server
1666 the client side of the auth conversation is finished
1667 and the server should return "OK";
1673 ERROR means that CHALL was invalid and could not be
1679 Both RESP and CHALL may be empty.
1683 The Client starts by getting an initial response from the
1684 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
1685 the mechanism did not provide an initial response. If the
1686 mechanism returns CONTINUE, the client starts in state
1687 <emphasis>WaitingForData</emphasis>, if the mechanism
1688 returns OK the client starts in state
1689 <emphasis>WaitingForOK</emphasis>.
1693 The client should keep track of available mechanisms and
1694 which it mechanisms it has already attempted. This list is
1695 used to decide which AUTH command to send. When the list is
1696 exhausted, the client should give up and close the
1701 <title><emphasis>WaitingForData</emphasis></title>
1709 MECH(CHALL) returns CONTINUE(RESP) → send
1711 <emphasis>WaitingForData</emphasis>
1715 MECH(CHALL) returns OK(RESP) → send DATA
1716 RESP, goto <emphasis>WaitingForOK</emphasis>
1720 MECH(CHALL) returns ERROR → send ERROR
1721 [msg], goto <emphasis>WaitingForData</emphasis>
1729 Receive REJECTED [mechs] →
1730 send AUTH [next mech], goto
1731 WaitingForData or <emphasis>WaitingForOK</emphasis>
1736 Receive ERROR → send
1738 <emphasis>WaitingForReject</emphasis>
1743 Receive OK → send
1744 BEGIN, terminate auth
1745 conversation, authenticated
1750 Receive anything else → send
1752 <emphasis>WaitingForData</emphasis>
1760 <title><emphasis>WaitingForOK</emphasis></title>
1765 Receive OK → send BEGIN, terminate auth
1766 conversation, <emphasis>authenticated</emphasis>
1771 Receive REJECT [mechs] → send AUTH [next mech],
1772 goto <emphasis>WaitingForData</emphasis> or
1773 <emphasis>WaitingForOK</emphasis>
1779 Receive DATA → send CANCEL, goto
1780 <emphasis>WaitingForReject</emphasis>
1786 Receive ERROR → send CANCEL, goto
1787 <emphasis>WaitingForReject</emphasis>
1793 Receive anything else → send ERROR, goto
1794 <emphasis>WaitingForOK</emphasis>
1802 <title><emphasis>WaitingForReject</emphasis></title>
1807 Receive REJECT [mechs] → send AUTH [next mech],
1808 goto <emphasis>WaitingForData</emphasis> or
1809 <emphasis>WaitingForOK</emphasis>
1815 Receive anything else → terminate auth
1816 conversation, disconnect
1825 <sect3 id="auth-states-server">
1826 <title>Server states</title>
1829 For the server MECH(RESP) means that the client response
1830 RESP was fed to the the mechanism MECH, which returns one of
1835 CONTINUE(CHALL) means continue the auth conversation and
1836 send CHALL as the challenge to the client;
1842 OK means that the client has been successfully
1849 REJECT means that the client failed to authenticate or
1850 there was an error in RESP.
1855 The server starts out in state
1856 <emphasis>WaitingForAuth</emphasis>. If the client is
1857 rejected too many times the server must disconnect the
1862 <title><emphasis>WaitingForAuth</emphasis></title>
1868 Receive AUTH → send REJECTED [mechs], goto
1869 <emphasis>WaitingForAuth</emphasis>
1875 Receive AUTH MECH RESP
1879 MECH not valid mechanism → send REJECTED
1881 <emphasis>WaitingForAuth</emphasis>
1885 MECH(RESP) returns CONTINUE(CHALL) → send
1887 <emphasis>WaitingForData</emphasis>
1891 MECH(RESP) returns OK → send OK, goto
1892 <emphasis>WaitingForBegin</emphasis>
1896 MECH(RESP) returns REJECT → send REJECTED
1898 <emphasis>WaitingForAuth</emphasis>
1906 Receive BEGIN → terminate
1907 auth conversation, disconnect
1913 Receive ERROR → send REJECTED [mechs], goto
1914 <emphasis>WaitingForAuth</emphasis>
1920 Receive anything else → send
1922 <emphasis>WaitingForAuth</emphasis>
1931 <title><emphasis>WaitingForData</emphasis></title>
1939 MECH(RESP) returns CONTINUE(CHALL) → send
1941 <emphasis>WaitingForData</emphasis>
1945 MECH(RESP) returns OK → send OK, goto
1946 <emphasis>WaitingForBegin</emphasis>
1950 MECH(RESP) returns REJECT → send REJECTED
1952 <emphasis>WaitingForAuth</emphasis>
1960 Receive BEGIN → terminate auth conversation,
1967 Receive CANCEL → send REJECTED [mechs], goto
1968 <emphasis>WaitingForAuth</emphasis>
1974 Receive ERROR → send REJECTED [mechs], goto
1975 <emphasis>WaitingForAuth</emphasis>
1981 Receive anything else → send ERROR, goto
1982 <emphasis>WaitingForData</emphasis>
1990 <title><emphasis>WaitingForBegin</emphasis></title>
1995 Receive BEGIN → terminate auth conversation,
1996 client authenticated
2002 Receive CANCEL → send REJECTED [mechs], goto
2003 <emphasis>WaitingForAuth</emphasis>
2009 Receive ERROR → send REJECTED [mechs], goto
2010 <emphasis>WaitingForAuth</emphasis>
2016 Receive anything else → send ERROR, goto
2017 <emphasis>WaitingForBegin</emphasis>
2027 <sect2 id="auth-mechanisms">
2028 <title>Authentication mechanisms</title>
2030 This section describes some new authentication mechanisms.
2031 D-Bus also allows any standard SASL mechanism of course.
2033 <sect3 id="auth-mechanisms-sha">
2034 <title>DBUS_COOKIE_SHA1</title>
2036 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2037 has the ability to read a private file owned by the user being
2038 authenticated. If the client can prove that it has access to a secret
2039 cookie stored in this file, then the client is authenticated.
2040 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2044 Authentication proceeds as follows:
2048 The client sends the username it would like to authenticate
2054 The server sends the name of its "cookie context" (see below); a
2055 space character; the integer ID of the secret cookie the client
2056 must demonstrate knowledge of; a space character; then a
2057 hex-encoded randomly-generated challenge string.
2062 The client locates the cookie, and generates its own hex-encoded
2063 randomly-generated challenge string. The client then
2064 concatenates the server's hex-encoded challenge, a ":"
2065 character, its own hex-encoded challenge, another ":" character,
2066 and the hex-encoded cookie. It computes the SHA-1 hash of this
2067 composite string. It sends back to the server the client's
2068 hex-encoded challenge string, a space character, and the SHA-1
2074 The server generates the same concatenated string used by the
2075 client and computes its SHA-1 hash. It compares the hash with
2076 the hash received from the client; if the two hashes match, the
2077 client is authenticated.
2083 Each server has a "cookie context," which is a name that identifies a
2084 set of cookies that apply to that server. A sample context might be
2085 "org_freedesktop_session_bus". Context names must be valid ASCII,
2086 nonzero length, and may not contain the characters slash ("/"),
2087 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2088 tab ("\t"), or period ("."). There is a default context,
2089 "org_freedesktop_general" that's used by servers that do not specify
2093 Cookies are stored in a user's home directory, in the directory
2094 <filename>~/.dbus-keyrings/</filename>. This directory must
2095 not be readable or writable by other users. If it is,
2096 clients and servers must ignore it. The directory
2097 contains cookie files named after the cookie context.
2100 A cookie file contains one cookie per line. Each line
2101 has three space-separated fields:
2105 The cookie ID number, which must be a non-negative integer and
2106 may not be used twice in the same file.
2111 The cookie's creation time, in UNIX seconds-since-the-epoch
2117 The cookie itself, a hex-encoded random block of bytes. The cookie
2118 may be of any length, though obviously security increases
2119 as the length increases.
2125 Only server processes modify the cookie file.
2126 They must do so with this procedure:
2130 Create a lockfile name by appending ".lock" to the name of the
2131 cookie file. The server should attempt to create this file
2132 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2133 fails, the lock fails. Servers should retry for a reasonable
2134 period of time, then they may choose to delete an existing lock
2135 to keep users from having to manually delete a stale
2136 lock. <footnote><para>Lockfiles are used instead of real file
2137 locking <literal>fcntl()</literal> because real locking
2138 implementations are still flaky on network
2139 filesystems.</para></footnote>
2144 Once the lockfile has been created, the server loads the cookie
2145 file. It should then delete any cookies that are old (the
2146 timeout can be fairly short), or more than a reasonable
2147 time in the future (so that cookies never accidentally
2148 become permanent, if the clock was set far into the future
2149 at some point). If no recent keys remain, the
2150 server may generate a new key.
2155 The pruned and possibly added-to cookie file
2156 must be resaved atomically (using a temporary
2157 file which is rename()'d).
2162 The lock must be dropped by deleting the lockfile.
2168 Clients need not lock the file in order to load it,
2169 because servers are required to save the file atomically.
2174 <sect1 id="addresses">
2175 <title>Server Addresses</title>
2177 Server addresses consist of a transport name followed by a colon, and
2178 then an optional, comma-separated list of keys and values in the form key=value.
2179 Each value is escaped.
2183 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2184 Which is the address to a unix socket with the path /tmp/dbus-test.
2187 Value escaping is similar to URI escaping but simpler.
2191 The set of optionally-escaped bytes is:
2192 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2193 <emphasis>byte</emphasis> (note, not character) which is not in the
2194 set of optionally-escaped bytes must be replaced with an ASCII
2195 percent (<literal>%</literal>) and the value of the byte in hex.
2196 The hex value must always be two digits, even if the first digit is
2197 zero. The optionally-escaped bytes may be escaped if desired.
2202 To unescape, append each byte in the value; if a byte is an ASCII
2203 percent (<literal>%</literal>) character then append the following
2204 hex value instead. It is an error if a <literal>%</literal> byte
2205 does not have two hex digits following. It is an error if a
2206 non-optionally-escaped byte is seen unescaped.
2210 The set of optionally-escaped bytes is intended to preserve address
2211 readability and convenience.
2215 A server may specify a key-value pair with the key <literal>guid</literal>
2216 and the value a hex-encoded 16-byte sequence. This globally unique ID must
2217 be created by filling the first 4 bytes with a 32-bit UNIX time since the
2218 epoch, and the remaining 12 bytes with random bytes. If present, the GUID
2219 may be used to distinguish one server from another. A server should use a
2220 different GUID for each address it listens on. For example, if a message
2221 bus daemon offers both UNIX domain socket and TCP connections, but treats
2222 clients the same regardless of how they connect, those two connections are
2223 equivalent post-connection but should have distinct GUIDs to distinguish
2224 the kinds of connection.
2228 The intent of the GUID feature is to allow a client to avoid opening
2229 multiple identical connections to the same server, by allowing the client
2230 to check whether an address corresponds to an already-existing connection.
2231 Comparing two addresses is insufficient, because addresses can be recycled
2232 by distinct servers.
2236 [FIXME clarify if attempting to connect to each is a requirement
2237 or just a suggestion]
2238 When connecting to a server, multiple server addresses can be
2239 separated by a semi-colon. The library will then try to connect
2240 to the first address and if that fails, it'll try to connect to
2241 the next one specified, and so forth. For example
2242 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2247 <sect1 id="transports">
2248 <title>Transports</title>
2250 [FIXME we need to specify in detail each transport and its possible arguments]
2252 Current transports include: unix domain sockets (including
2253 abstract namespace on linux), TCP/IP, and a debug/testing transport using
2254 in-process pipes. Future possible transports include one that
2255 tunnels over X11 protocol.
2258 <sect2 id="transports-unix-domain-sockets">
2259 <title>Unix Domain Sockets</title>
2261 Unix domain sockets can be either paths in the file system or on Linux
2262 kernels, they can be abstract which are similar to paths but
2263 do not show up in the file system.
2267 When a socket is opened by the D-Bus library it truncates the path
2268 name right before the first trailing Nul byte. This is true for both
2269 normal paths and abstract paths. Note that this is a departure from
2270 previous versions of D-Bus that would create sockets with a fixed
2271 length path name. Names which were shorter than the fixed length
2272 would be padded by Nul bytes.
2277 <sect1 id="naming-conventions">
2278 <title>Naming Conventions</title>
2281 D-Bus namespaces are all lowercase and correspond to reversed domain
2282 names, as with Java. e.g. "org.freedesktop"
2285 Interface, signal, method, and property names are "WindowsStyleCaps", note
2286 that the first letter is capitalized, unlike Java.
2289 Object paths are normally all lowercase with underscores used rather than
2294 <sect1 id="standard-interfaces">
2295 <title>Standard Interfaces</title>
2297 See <xref linkend="message-protocol-types-notation"/> for details on
2298 the notation used in this section. There are some standard interfaces
2299 that may be useful across various D-Bus applications.
2301 <sect2 id="standard-interfaces-peer">
2302 <title><literal>org.freedesktop.DBus.Peer</literal></title>
2304 The <literal>org.freedesktop.DBus.Peer</literal> interface
2307 org.freedesktop.DBus.Peer.Ping ()
2308 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
2312 On receipt of the <literal>METHOD_CALL</literal> message
2313 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
2314 nothing other than reply with a <literal>METHOD_RETURN</literal> as
2315 usual. It does not matter which object path a ping is sent to. The
2316 reference implementation handles this method automatically.
2319 On receipt of the <literal>METHOD_CALL</literal> message
2320 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
2321 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
2322 UUID representing the identity of the machine the process is running on.
2323 This UUID must be the same for all processes on a single system at least
2324 until that system next reboots. It should be the same across reboots
2325 if possible, but this is not always possible to implement and is not
2327 It does not matter which object path a GetMachineId is sent to. The
2328 reference implementation handles this method automatically.
2331 The UUID is intended to be per-instance-of-the-operating-system, so may represent
2332 a virtual machine running on a hypervisor, rather than a physical machine.
2333 Basically if two processes see the same UUID, they should also see the same
2334 shared memory, UNIX domain sockets, process IDs, and other features that require
2335 a running OS kernel in common between the processes.
2338 The UUID is often used where other programs might use a hostname. Hostnames
2339 can change without rebooting, however, or just be "localhost" - so the UUID
2343 The UUID must contain 128 bits of data and be hex-encoded (meaning, the hex
2344 string contains 32 ASCII characters). The hex-encoded string may not contain
2345 hyphens or other non-hex-digit characters, and it must be exactly 32 characters long.
2346 To generate a UUID, the recommended algorithm is to put the current time in seconds
2347 since the UNIX epoch in the last 32 bits of the UUID, and to put randomly-generated bits
2348 in the first 96 bits of the UUID.
2352 <sect2 id="standard-interfaces-introspectable">
2353 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
2355 This interface has one method:
2357 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
2361 Objects instances may implement
2362 <literal>Introspect</literal> which returns an XML description of
2363 the object, including its interfaces (with signals and methods), objects
2364 below it in the object path tree, and its properties.
2367 <xref linkend="introspection-format"/> describes the format of this XML string.
2370 <sect2 id="standard-interfaces-properties">
2371 <title><literal>org.freedesktop.DBus.Properties</literal></title>
2373 Many native APIs will have a concept of object <firstterm>properties</firstterm>
2374 or <firstterm>attributes</firstterm>. These can be exposed via the
2375 <literal>org.freedesktop.DBus.Properties</literal> interface.
2379 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
2380 in STRING property_name,
2382 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
2383 in STRING property_name,
2388 The available properties and whether they are writable can be determined
2389 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
2390 see <xref linkend="standard-interfaces-introspectable"/>.
2393 An empty string may be provided for the interface name; in this case,
2394 if there are multiple properties on an object with the same name,
2395 the results are undefined (picking one by according to an arbitrary
2396 deterministic rule, or returning an error, are the reasonable
2402 <sect1 id="introspection-format">
2403 <title>Introspection Data Format</title>
2405 As described in <xref linkend="standard-interfaces-introspectable"/>,
2406 objects may be introspected at runtime, returning an XML string
2407 that describes the object. The same XML format may be used in
2408 other contexts as well, for example as an "IDL" for generating
2409 static language bindings.
2412 Here is an example of introspection data:
2414 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
2415 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
2416 <node name="/org/freedesktop/sample_object">
2417 <interface name="org.freedesktop.SampleInterface">
2418 <method name="Frobate">
2419 <arg name="foo" type="i" direction="in"/>
2420 <arg name="bar" type="s" direction="out"/>
2421 <arg name="baz" type="a{us}" direction="out"/>
2422 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
2424 <method name="Bazify">
2425 <arg name="bar" type="(iiu)" direction="in"/>
2426 <arg name="bar" type="v" direction="out"/>
2428 <method name="Mogrify">
2429 <arg name="bar" type="(iiav)" direction="in"/>
2431 <signal name="Changed">
2432 <arg name="new_value" type="b"/>
2434 <property name="Bar" type="y" access="readwrite"/>
2436 <node name="child_of_sample_object"/>
2437 <node name="another_child_of_sample_object"/>
2442 A more formal DTD and spec needs writing, but here are some quick notes.
2446 Only the root <node> element can omit the node name, as it's
2447 known to be the object that was introspected. If the root
2448 <node> does have a name attribute, it must be an absolute
2449 object path. If child <node> have object paths, they must be
2455 If a child <node> has any sub-elements, then they
2456 must represent a complete introspection of the child.
2457 If a child <node> is empty, then it may or may
2458 not have sub-elements; the child must be introspected
2459 in order to find out. The intent is that if an object
2460 knows that its children are "fast" to introspect
2461 it can go ahead and return their information, but
2462 otherwise it can omit it.
2467 The direction element on <arg> may be omitted,
2468 in which case it defaults to "in" for method calls
2469 and "out" for signals. Signals only allow "out"
2470 so while direction may be specified, it's pointless.
2475 The possible directions are "in" and "out",
2476 unlike CORBA there is no "inout"
2481 The possible property access flags are
2482 "readwrite", "read", and "write"
2487 Multiple interfaces can of course be listed for
2493 The "name" attribute on arguments is optional.
2499 Method, interface, property, and signal elements may have
2500 "annotations", which are generic key/value pairs of metadata.
2501 They are similar conceptually to Java's annotations and C# attributes.
2502 Well-known annotations:
2509 <entry>Values (separated by ,)</entry>
2510 <entry>Description</entry>
2515 <entry>org.freedesktop.DBus.Deprecated</entry>
2516 <entry>true,false</entry>
2517 <entry>Whether or not the entity is deprecated; defaults to false</entry>
2520 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
2521 <entry>(string)</entry>
2522 <entry>The C symbol; may be used for methods and interfaces</entry>
2525 <entry>org.freedesktop.DBus.Method.NoReply</entry>
2526 <entry>true,false</entry>
2527 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
2533 <sect1 id="message-bus">
2534 <title>Message Bus Specification</title>
2535 <sect2 id="message-bus-overview">
2536 <title>Message Bus Overview</title>
2538 The message bus accepts connections from one or more applications.
2539 Once connected, applications can exchange messages with other
2540 applications that are also connected to the bus.
2543 In order to route messages among connections, the message bus keeps a
2544 mapping from names to connections. Each connection has one
2545 unique-for-the-lifetime-of-the-bus name automatically assigned.
2546 Applications may request additional names for a connection. Additional
2547 names are usually "well-known names" such as
2548 "org.freedesktop.TextEditor". When a name is bound to a connection,
2549 that connection is said to <firstterm>own</firstterm> the name.
2552 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
2553 This name routes messages to the bus, allowing applications to make
2554 administrative requests. For example, applications can ask the bus
2555 to assign a name to a connection.
2558 Each name may have <firstterm>queued owners</firstterm>. When an
2559 application requests a name for a connection and the name is already in
2560 use, the bus will optionally add the connection to a queue waiting for
2561 the name. If the current owner of the name disconnects or releases
2562 the name, the next connection in the queue will become the new owner.
2566 This feature causes the right thing to happen if you start two text
2567 editors for example; the first one may request "org.freedesktop.TextEditor",
2568 and the second will be queued as a possible owner of that name. When
2569 the first exits, the second will take over.
2573 Messages may have a <literal>DESTINATION</literal> field (see <xref
2574 linkend="message-protocol-header-fields"/>). If the
2575 <literal>DESTINATION</literal> field is present, it specifies a message
2576 recipient by name. Method calls and replies normally specify this field.
2580 Signals normally do not specify a destination; they are sent to all
2581 applications with <firstterm>message matching rules</firstterm> that
2586 When the message bus receives a method call, if the
2587 <literal>DESTINATION</literal> field is absent, the call is taken to be
2588 a standard one-to-one message and interpreted by the message bus
2589 itself. For example, sending an
2590 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
2591 <literal>DESTINATION</literal> will cause the message bus itself to
2592 reply to the ping immediately; the message bus will not make this
2593 message visible to other applications.
2597 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
2598 the ping message were sent with a <literal>DESTINATION</literal> name of
2599 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
2600 forwarded, and the Yoyodyne Corporation screensaver application would be
2601 expected to reply to the ping.
2605 <sect2 id="message-bus-names">
2606 <title>Message Bus Names</title>
2608 Each connection has at least one name, assigned at connection time and
2609 returned in response to the
2610 <literal>org.freedesktop.DBus.Hello</literal> method call. This
2611 automatically-assigned name is called the connection's <firstterm>unique
2612 name</firstterm>. Unique names are never reused for two different
2613 connections to the same bus.
2616 Ownership of a unique name is a prerequisite for interaction with
2617 the message bus. It logically follows that the unique name is always
2618 the first name that an application comes to own, and the last
2619 one that it loses ownership of.
2622 Unique connection names must begin with the character ':' (ASCII colon
2623 character); bus names that are not unique names must not begin
2624 with this character. (The bus must reject any attempt by an application
2625 to manually request a name beginning with ':'.) This restriction
2626 categorically prevents "spoofing"; messages sent to a unique name
2627 will always go to the expected connection.
2630 When a connection is closed, all the names that it owns are deleted (or
2631 transferred to the next connection in the queue if any).
2634 A connection can request additional names to be associated with it using
2635 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
2636 linkend="message-protocol-names-bus"/> describes the format of a valid
2637 name. These names can be released again using the
2638 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
2641 <sect3 id="bus-messages-request-name">
2642 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
2646 UINT32 RequestName (in STRING name, in UINT32 flags)
2653 <entry>Argument</entry>
2655 <entry>Description</entry>
2661 <entry>STRING</entry>
2662 <entry>Name to request</entry>
2666 <entry>UINT32</entry>
2667 <entry>Flags</entry>
2677 <entry>Argument</entry>
2679 <entry>Description</entry>
2685 <entry>UINT32</entry>
2686 <entry>Return value</entry>
2693 This method call should be sent to
2694 <literal>org.freedesktop.DBus</literal> and asks the message bus to
2695 assign the given name to the method caller. Each name maintains a
2696 queue of possible owners, where the head of the queue is the primary
2697 or current owner of the name. Each potential owner in the queue
2698 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
2699 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
2700 call. When RequestName is invoked the following occurs:
2704 If the method caller is currently the primary owner of the name,
2705 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
2706 values are updated with the values from the new RequestName call,
2707 and nothing further happens.
2713 If the current primary owner (head of the queue) has
2714 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
2715 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
2716 the caller of RequestName replaces the current primary owner at
2717 the head of the queue and the current primary owner moves to the
2718 second position in the queue. If the caller of RequestName was
2719 in the queue previously its flags are updated with the values from
2720 the new RequestName in addition to moving it to the head of the queue.
2726 If replacement is not possible, and the method caller is
2727 currently in the queue but not the primary owner, its flags are
2728 updated with the values from the new RequestName call.
2734 If replacement is not possible, and the method caller is
2735 currently not in the queue, the method caller is appended to the
2742 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
2743 set and is not the primary owner, it is removed from the
2744 queue. This can apply to the previous primary owner (if it
2745 was replaced) or the method caller (if it updated the
2746 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
2747 queue, or if it was just added to the queue with that flag set).
2753 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
2754 queue," even if another application already in the queue had specified
2755 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
2756 that does not allow replacement goes away, and the next primary owner
2757 does allow replacement. In this case, queued items that specified
2758 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
2759 automatically replace the new primary owner. In other words,
2760 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
2761 time RequestName is called. This is deliberate to avoid an infinite loop
2762 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
2763 and DBUS_NAME_FLAG_REPLACE_EXISTING.
2766 The flags argument contains any of the following values logically ORed
2773 <entry>Conventional Name</entry>
2774 <entry>Value</entry>
2775 <entry>Description</entry>
2780 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
2784 If an application A specifies this flag and succeeds in
2785 becoming the owner of the name, and another application B
2786 later calls RequestName with the
2787 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
2788 will lose ownership and receive a
2789 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
2790 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
2791 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
2792 is not specified by application B, then application B will not replace
2793 application A as the owner.
2798 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
2802 Try to replace the current owner if there is one. If this
2803 flag is not set the application will only become the owner of
2804 the name if there is no current owner. If this flag is set,
2805 the application will replace the current owner if
2806 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
2811 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
2815 Without this flag, if an application requests a name that is
2816 already owned, the application will be placed in a queue to
2817 own the name when the current owner gives it up. If this
2818 flag is given, the application will not be placed in the
2819 queue, the request for the name will simply fail. This flag
2820 also affects behavior when an application is replaced as
2821 name owner; by default the application moves back into the
2822 waiting queue, unless this flag was provided when the application
2823 became the name owner.
2831 The return code can be one of the following values:
2837 <entry>Conventional Name</entry>
2838 <entry>Value</entry>
2839 <entry>Description</entry>
2844 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
2845 <entry>1</entry> <entry>The caller is now the primary owner of
2846 the name, replacing any previous owner. Either the name had no
2847 owner before, or the caller specified
2848 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
2849 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
2852 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
2855 <entry>The name already had an owner,
2856 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
2857 the current owner did not specify
2858 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
2859 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
2863 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
2864 <entry>The name already has an owner,
2865 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
2866 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
2867 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
2868 specified by the requesting application.</entry>
2871 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
2873 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
2881 <sect3 id="bus-messages-release-name">
2882 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
2886 UINT32 ReleaseName (in STRING name)
2893 <entry>Argument</entry>
2895 <entry>Description</entry>
2901 <entry>STRING</entry>
2902 <entry>Name to release</entry>
2912 <entry>Argument</entry>
2914 <entry>Description</entry>
2920 <entry>UINT32</entry>
2921 <entry>Return value</entry>
2928 This method call should be sent to
2929 <literal>org.freedesktop.DBus</literal> and asks the message bus to
2930 release the method caller's claim to the given name. If the caller is
2931 the primary owner, a new primary owner will be selected from the
2932 queue if any other owners are waiting. If the caller is waiting in
2933 the queue for the name, the caller will removed from the queue and
2934 will not be made an owner of the name if it later becomes available.
2935 If there are no other owners in the queue for the name, it will be
2936 removed from the bus entirely.
2938 The return code can be one of the following values:
2944 <entry>Conventional Name</entry>
2945 <entry>Value</entry>
2946 <entry>Description</entry>
2951 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
2952 <entry>1</entry> <entry>The caller has released his claim on
2953 the given name. Either the caller was the primary owner of
2954 the name, and the name is now unused or taken by somebody
2955 waiting in the queue for the name, or the caller was waiting
2956 in the queue for the name and has now been removed from the
2960 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
2962 <entry>The given name does not exist on this bus.</entry>
2965 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
2967 <entry>The caller was not the primary owner of this name,
2968 and was also not waiting in the queue to own this name.</entry>
2977 <sect2 id="message-bus-routing">
2978 <title>Message Bus Message Routing</title>
2982 <sect3 id="message-bus-routing-match-rules">
2983 <title>Match Rules</title>
2985 An important part of the message bus routing protocol is match
2986 rules. Match rules describe what messages can be sent to a client
2987 based on the contents of the message. When a message is routed
2988 through the bus it is compared to clients' match rules. If any
2989 of the rules match, the message is dispatched to the client.
2990 If none of the rules match the message never leaves the bus. This
2991 is an effective way to control traffic over the bus and to make sure
2992 only relevant message need to be processed by the client.
2995 Match rules are added using the AddMatch bus method
2996 (see xref linkend="bus-messages-add-match"/>). Rules are
2997 specified as a string of comma separated key/value pairs.
2998 Excluding a key from the rule indicates a wildcard match.
2999 For instance excluding the the member from a match rule but
3000 adding a sender would let all messages from that sender through.
3001 An example of a complete rule would be
3002 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
3005 The following table describes the keys that can be used to create
3007 The following table summarizes the D-Bus types.
3013 <entry>Possible Values</entry>
3014 <entry>Description</entry>
3019 <entry><literal>type</literal></entry>
3020 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
3021 <entry>Match on the message type. An example of a type match is type='signal'</entry>
3024 <entry><literal>sender</literal></entry>
3025 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
3026 and <xref linkend="term-unique-name"/> respectively)
3028 <entry>Match messages sent by a particular sender. An example of a sender match
3029 is sender='org.freedesktop.Hal'</entry>
3032 <entry><literal>interface</literal></entry>
3033 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
3034 <entry>Match messages sent over or to a particular interface. An example of an
3035 interface match is interface='org.freedesktop.Hal.Manager'.
3036 If a message omits the interface header, it must not match any rule
3037 that specifies this key.</entry>
3040 <entry><literal>member</literal></entry>
3041 <entry>Any valid method or signal name</entry>
3042 <entry>Matches messages which have the give method or signal name. An example of
3043 a member match is member='NameOwnerChanged'</entry>
3046 <entry><literal>path</literal></entry>
3047 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
3048 <entry>Matches messages which are sent from or to the given object. An example of a
3049 path match is path='/org/freedesktop/Hal/Manager'</entry>
3052 <entry><literal>destination</literal></entry>
3053 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
3054 <entry>Matches messages which are being sent to the given unique name. An
3055 example of a destination match is destination=':1.0'</entry>
3058 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
3059 <entry>Any string</entry>
3060 <entry>Arg matches are special and are used for further restricting the
3061 match based on the arguments in the body of a message. As of this time
3062 only string arguments can be matched. An example of an argument match
3063 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
3072 <sect2 id="message-bus-starting-services">
3073 <title>Message Bus Starting Services</title>
3075 The message bus can start applications on behalf of other applications.
3076 In CORBA terms, this would be called <firstterm>activation</firstterm>.
3077 An application that can be started in this way is called a
3078 <firstterm>service</firstterm>.
3081 With D-Bus, starting a service is normally done by name. That is,
3082 applications ask the message bus to start some program that will own a
3083 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
3084 This implies a contract documented along with the name
3085 <literal>org.freedesktop.TextEditor</literal> for which objects
3086 the owner of that name will provide, and what interfaces those
3090 To find an executable corresponding to a particular name, the bus daemon
3091 looks for <firstterm>service description files</firstterm>. Service
3092 description files define a mapping from names to executables. Different
3093 kinds of message bus will look for these files in different places, see
3094 <xref linkend="message-bus-types"/>.
3097 [FIXME the file format should be much better specified than "similar to
3098 .desktop entries" esp. since desktop entries are already
3099 badly-specified. ;-)] Service description files have the ".service" file
3100 extension. The message bus will only load service description files
3101 ending with .service; all other files will be ignored. The file format
3102 is similar to that of <ulink
3103 url="http://www.freedesktop.org/standards/desktop-entry-spec/desktop-entry-spec.html">desktop
3104 entries</ulink>. All service description files must be in UTF-8
3105 encoding. To ensure that there will be no name collisions, service files
3106 must be namespaced using the same mechanism as messages and service
3110 <title>Example service description file</title>
3112 # Sample service description file
3114 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
3115 Exec=/usr/libexec/gconfd-2
3120 When an application asks to start a service by name, the bus daemon tries to
3121 find a service that will own that name. It then tries to spawn the
3122 executable associated with it. If this fails, it will report an
3123 error. [FIXME what happens if two .service files offer the same service;
3124 what kind of error is reported, should we have a way for the client to
3128 The executable launched will have the environment variable
3129 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
3130 message bus so it can connect and request the appropriate names.
3133 The executable being launched may want to know whether the message bus
3134 starting it is one of the well-known message buses (see <xref
3135 linkend="message-bus-types"/>). To facilitate this, the bus must also set
3136 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
3137 of the well-known buses. The currently-defined values for this variable
3138 are <literal>system</literal> for the systemwide message bus,
3139 and <literal>session</literal> for the per-login-session message
3140 bus. The new executable must still connect to the address given
3141 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
3142 resulting connection is to the well-known bus.
3145 [FIXME there should be a timeout somewhere, either specified
3146 in the .service file, by the client, or just a global value
3147 and if the client being activated fails to connect within that
3148 timeout, an error should be sent back.]
3151 <sect3 id="message-bus-starting-services-scope">
3152 <title>Message Bus Service Scope</title>
3154 The "scope" of a service is its "per-", such as per-session,
3155 per-machine, per-home-directory, or per-display. The reference
3156 implementation doesn't yet support starting services in a different
3157 scope from the message bus itself. So e.g. if you start a service
3158 on the session bus its scope is per-session.
3161 We could add an optional scope to a bus name. For example, for
3162 per-(display,session pair), we could have a unique ID for each display
3163 generated automatically at login and set on screen 0 by executing a
3164 special "set display ID" binary. The ID would be stored in a
3165 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
3166 random bytes. This ID would then be used to scope names.
3167 Starting/locating a service could be done by ID-name pair rather than
3171 Contrast this with a per-display scope. To achieve that, we would
3172 want a single bus spanning all sessions using a given display.
3173 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
3174 property on screen 0 of the display, pointing to this bus.
3179 <sect2 id="message-bus-types">
3180 <title>Well-known Message Bus Instances</title>
3182 Two standard message bus instances are defined here, along with how
3183 to locate them and where their service files live.
3185 <sect3 id="message-bus-types-login">
3186 <title>Login session message bus</title>
3188 Each time a user logs in, a <firstterm>login session message
3189 bus</firstterm> may be started. All applications in the user's login
3190 session may interact with one another using this message bus.
3193 The address of the login session message bus is given
3194 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
3195 variable. If that variable is not set, applications may
3196 also try to read the address from the X Window System root
3197 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
3198 The root window property must have type <literal>STRING</literal>.
3199 The environment variable should have precedence over the
3200 root window property.
3203 [FIXME specify location of .service files, probably using
3204 DESKTOP_DIRS etc. from basedir specification, though login session
3205 bus is not really desktop-specific]
3208 <sect3 id="message-bus-types-system">
3209 <title>System message bus</title>
3211 A computer may have a <firstterm>system message bus</firstterm>,
3212 accessible to all applications on the system. This message bus may be
3213 used to broadcast system events, such as adding new hardware devices,
3214 changes in the printer queue, and so forth.
3217 The address of the system message bus is given
3218 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
3219 variable. If that variable is not set, applications should try
3220 to connect to the well-known address
3221 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
3224 The D-Bus reference implementation actually honors the
3225 <literal>$(localstatedir)</literal> configure option
3226 for this address, on both client and server side.
3231 [FIXME specify location of system bus .service files]
3236 <sect2 id="message-bus-messages">
3237 <title>Message Bus Messages</title>
3239 The special message bus name <literal>org.freedesktop.DBus</literal>
3240 responds to a number of additional messages.
3243 <sect3 id="bus-messages-hello">
3244 <title><literal>org.freedesktop.DBus.Hello</literal></title>
3255 <entry>Argument</entry>
3257 <entry>Description</entry>
3263 <entry>STRING</entry>
3264 <entry>Unique name assigned to the connection</entry>
3271 Before an application is able to send messages to other applications
3272 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
3273 to the message bus to obtain a unique name. If an application without
3274 a unique name tries to send a message to another application, or a
3275 message to the message bus itself that isn't the
3276 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
3277 disconnected from the bus.
3280 There is no corresponding "disconnect" request; if a client wishes to
3281 disconnect from the bus, it simply closes the socket (or other
3282 communication channel).
3285 <sect3 id="bus-messages-list-names">
3286 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
3290 ARRAY of STRING ListNames ()
3297 <entry>Argument</entry>
3299 <entry>Description</entry>
3305 <entry>ARRAY of STRING</entry>
3306 <entry>Array of strings where each string is a bus name</entry>
3313 Returns a list of all currently-owned names on the bus.
3316 <sect3 id="bus-messages-list-activatable-names">
3317 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
3321 ARRAY of STRING ListActivatableNames ()
3328 <entry>Argument</entry>
3330 <entry>Description</entry>
3336 <entry>ARRAY of STRING</entry>
3337 <entry>Array of strings where each string is a bus name</entry>
3344 Returns a list of all names that can be activated on the bus.
3347 <sect3 id="bus-messages-name-exists">
3348 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
3352 BOOLEAN NameHasOwner (in STRING name)
3359 <entry>Argument</entry>
3361 <entry>Description</entry>
3367 <entry>STRING</entry>
3368 <entry>Name to check</entry>
3378 <entry>Argument</entry>
3380 <entry>Description</entry>
3386 <entry>BOOLEAN</entry>
3387 <entry>Return value, true if the name exists</entry>
3394 Checks if the specified name exists (currently has an owner).
3398 <sect3 id="bus-messages-name-owner-changed">
3399 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
3403 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
3410 <entry>Argument</entry>
3412 <entry>Description</entry>
3418 <entry>STRING</entry>
3419 <entry>Name with a new owner</entry>
3423 <entry>STRING</entry>
3424 <entry>Old owner or empty string if none</entry>
3428 <entry>STRING</entry>
3429 <entry>New owner or empty string if none</entry>
3436 This signal indicates that the owner of a name has changed.
3437 It's also the signal to use to detect the appearance of
3438 new names on the bus.
3441 <sect3 id="bus-messages-name-lost">
3442 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
3446 NameLost (STRING name)
3453 <entry>Argument</entry>
3455 <entry>Description</entry>
3461 <entry>STRING</entry>
3462 <entry>Name which was lost</entry>
3469 This signal is sent to a specific application when it loses
3470 ownership of a name.
3474 <sect3 id="bus-messages-name-acquired">
3475 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
3479 NameAcquired (STRING name)
3486 <entry>Argument</entry>
3488 <entry>Description</entry>
3494 <entry>STRING</entry>
3495 <entry>Name which was acquired</entry>
3502 This signal is sent to a specific application when it gains
3503 ownership of a name.
3507 <sect3 id="bus-messages-start-service-by-name">
3508 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
3512 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
3519 <entry>Argument</entry>
3521 <entry>Description</entry>
3527 <entry>STRING</entry>
3528 <entry>Name of the service to start</entry>
3532 <entry>UINT32</entry>
3533 <entry>Flags (currently not used)</entry>
3543 <entry>Argument</entry>
3545 <entry>Description</entry>
3551 <entry>UINT32</entry>
3552 <entry>Return value</entry>
3557 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
3561 The return value can be one of the following values:
3566 <entry>Identifier</entry>
3567 <entry>Value</entry>
3568 <entry>Description</entry>
3573 <entry>DBUS_START_REPLY_SUCCESS</entry>
3575 <entry>The service was successfully started.</entry>
3578 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
3580 <entry>A connection already owns the given name.</entry>
3589 <sect3 id="bus-messages-get-name-owner">
3590 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
3594 STRING GetNameOwner (in STRING name)
3601 <entry>Argument</entry>
3603 <entry>Description</entry>
3609 <entry>STRING</entry>
3610 <entry>Name to get the owner of</entry>
3620 <entry>Argument</entry>
3622 <entry>Description</entry>
3628 <entry>STRING</entry>
3629 <entry>Return value, a unique connection name</entry>
3634 Returns the unique connection name of the primary owner of the name
3635 given. If the requested name doesn't have an owner, returns a
3636 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
3640 <sect3 id="bus-messages-get-connection-unix-user">
3641 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
3645 UINT32 GetConnectionUnixUser (in STRING connection_name)
3652 <entry>Argument</entry>
3654 <entry>Description</entry>
3660 <entry>STRING</entry>
3661 <entry>Name of the connection to query</entry>
3671 <entry>Argument</entry>
3673 <entry>Description</entry>
3679 <entry>UINT32</entry>
3680 <entry>unix user id</entry>
3685 Returns the unix uid of the process connected to the server. If unable to
3686 determine it, a <literal>org.freedesktop.DBus.Error.Failed</literal>
3691 <sect3 id="bus-messages-add-match">
3692 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
3696 AddMatch (in STRING rule)
3703 <entry>Argument</entry>
3705 <entry>Description</entry>
3711 <entry>STRING</entry>
3712 <entry>Match rule to add to the connection</entry>
3717 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
3718 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
3722 <sect3 id="bus-messages-remove-match">
3723 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
3727 RemoveMatch (in STRING rule)
3734 <entry>Argument</entry>
3736 <entry>Description</entry>
3742 <entry>STRING</entry>
3743 <entry>Match rule to remove from the connection</entry>
3748 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
3749 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
3758 <appendix id="implementation-notes">
3759 <title>Implementation notes</title>
3760 <sect1 id="implementation-notes-subsection">
3768 <glossary><title>Glossary</title>
3770 This glossary defines some of the terms used in this specification.
3773 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
3776 The message bus maintains an association between names and
3777 connections. (Normally, there's one connection per application.) A
3778 bus name is simply an identifier used to locate connections. For
3779 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
3780 name might be used to send a message to a screensaver from Yoyodyne
3781 Corporation. An application is said to <firstterm>own</firstterm> a
3782 name if the message bus has associated the application's connection
3783 with the name. Names may also have <firstterm>queued
3784 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
3785 The bus assigns a unique name to each connection,
3786 see <xref linkend="term-unique-name"/>. Other names
3787 can be thought of as "well-known names" and are
3788 used to find applications that offer specific functionality.
3793 <glossentry id="term-message"><glossterm>Message</glossterm>
3796 A message is the atomic unit of communication via the D-Bus
3797 protocol. It consists of a <firstterm>header</firstterm> and a
3798 <firstterm>body</firstterm>; the body is made up of
3799 <firstterm>arguments</firstterm>.
3804 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
3807 The message bus is a special application that forwards
3808 or routes messages between a group of applications
3809 connected to the message bus. It also manages
3810 <firstterm>names</firstterm> used for routing
3816 <glossentry id="term-name"><glossterm>Name</glossterm>
3819 See <xref linkend="term-bus-name"/>. "Name" may
3820 also be used to refer to some of the other names
3821 in D-Bus, such as interface names.
3826 <glossentry id="namespace"><glossterm>Namespace</glossterm>
3829 Used to prevent collisions when defining new interfaces or bus
3830 names. The convention used is the same one Java uses for defining
3831 classes: a reversed domain name.
3836 <glossentry id="term-object"><glossterm>Object</glossterm>
3839 Each application contains <firstterm>objects</firstterm>, which have
3840 <firstterm>interfaces</firstterm> and
3841 <firstterm>methods</firstterm>. Objects are referred to by a name,
3842 called a <firstterm>path</firstterm>.
3847 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
3850 An application talking directly to another application, without going
3851 through a message bus. One-to-one connections may be "peer to peer" or
3852 "client to server." The D-Bus protocol has no concept of client
3853 vs. server after a connection has authenticated; the flow of messages
3854 is symmetrical (full duplex).
3859 <glossentry id="term-path"><glossterm>Path</glossterm>
3862 Object references (object names) in D-Bus are organized into a
3863 filesystem-style hierarchy, so each object is named by a path. As in
3864 LDAP, there's no difference between "files" and "directories"; a path
3865 can refer to an object, while still having child objects below it.
3870 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
3873 Each bus name has a primary owner; messages sent to the name go to the
3874 primary owner. However, certain names also maintain a queue of
3875 secondary owners "waiting in the wings." If the primary owner releases
3876 the name, then the first secondary owner in the queue automatically
3877 becomes the new owner of the name.
3882 <glossentry id="term-service"><glossterm>Service</glossterm>
3885 A service is an executable that can be launched by the bus daemon.
3886 Services normally guarantee some particular features, for example they
3887 may guarantee that they will request a specific name such as
3888 "org.freedesktop.Screensaver", have a singleton object
3889 "/org/freedesktop/Application", and that object will implement the
3890 interface "org.freedesktop.ScreensaverControl".
3895 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
3898 ".service files" tell the bus about service applications that can be
3899 launched (see <xref linkend="term-service"/>). Most importantly they
3900 provide a mapping from bus names to services that will request those
3901 names when they start up.
3906 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
3909 The special name automatically assigned to each connection by the
3910 message bus. This name will never change owner, and will be unique
3911 (never reused during the lifetime of the message bus).
3912 It will begin with a ':' character.