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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.23</releaseinfo>
10 <date>2014-01-06</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
66 <orgname>Red Hat, Inc.</orgname>
68 <email>davidz@redhat.com</email>
75 <revnumber>0.23</revnumber>
76 <date>2014-01-06</date>
77 <authorinitials>SMcV, CY</authorinitials>
79 method call messages with no INTERFACE may be considered an error;
80 document tcp:bind=... and nonce-tcp:bind=...; define listenable
81 and connectable addresses
85 <revnumber>0.22</revnumber>
86 <date>2013-10-09</date>
87 <authorinitials></authorinitials>
88 <revremark>add GetConnectionCredentials, document
89 GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
90 document and correct .service file syntax and naming
94 <revnumber>0.21</revnumber>
95 <date>2013-04-25</date>
96 <authorinitials>smcv</authorinitials>
97 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
98 Corrigendum #9)</revremark>
101 <revnumber>0.20</revnumber>
102 <date>22 February 2013</date>
103 <authorinitials>smcv, walters</authorinitials>
104 <revremark>reorganise for clarity, remove false claims about
105 basic types, mention /o/fd/DBus</revremark>
108 <revnumber>0.19</revnumber>
109 <date>20 February 2012</date>
110 <authorinitials>smcv/lp</authorinitials>
111 <revremark>formally define unique connection names and well-known
112 bus names; document best practices for interface, bus, member and
113 error names, and object paths; document the search path for session
114 and system services on Unix; document the systemd transport</revremark>
117 <revnumber>0.18</revnumber>
118 <date>29 July 2011</date>
119 <authorinitials>smcv</authorinitials>
120 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
121 match keyword; promote type system to a top-level section</revremark>
124 <revnumber>0.17</revnumber>
125 <date>1 June 2011</date>
126 <authorinitials>smcv/davidz</authorinitials>
127 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
128 by GVariant</revremark>
131 <revnumber>0.16</revnumber>
132 <date>11 April 2011</date>
133 <authorinitials></authorinitials>
134 <revremark>add path_namespace, arg0namespace; argNpath matches object
138 <revnumber>0.15</revnumber>
139 <date>3 November 2010</date>
140 <authorinitials></authorinitials>
141 <revremark></revremark>
144 <revnumber>0.14</revnumber>
145 <date>12 May 2010</date>
146 <authorinitials></authorinitials>
147 <revremark></revremark>
150 <revnumber>0.13</revnumber>
151 <date>23 Dezember 2009</date>
152 <authorinitials></authorinitials>
153 <revremark></revremark>
156 <revnumber>0.12</revnumber>
157 <date>7 November, 2006</date>
158 <authorinitials></authorinitials>
159 <revremark></revremark>
162 <revnumber>0.11</revnumber>
163 <date>6 February 2005</date>
164 <authorinitials></authorinitials>
165 <revremark></revremark>
168 <revnumber>0.10</revnumber>
169 <date>28 January 2005</date>
170 <authorinitials></authorinitials>
171 <revremark></revremark>
174 <revnumber>0.9</revnumber>
175 <date>7 Januar 2005</date>
176 <authorinitials></authorinitials>
177 <revremark></revremark>
180 <revnumber>0.8</revnumber>
181 <date>06 September 2003</date>
182 <authorinitials></authorinitials>
183 <revremark>First released document.</revremark>
188 <sect1 id="introduction">
189 <title>Introduction</title>
191 D-Bus is a system for low-overhead, easy to use
192 interprocess communication (IPC). In more detail:
196 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
197 binary protocol, and does not have to convert to and from a text
198 format such as XML. Because D-Bus is intended for potentially
199 high-resolution same-machine IPC, not primarily for Internet IPC,
200 this is an interesting optimization. D-Bus is also designed to
201 avoid round trips and allow asynchronous operation, much like
207 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
208 of <firstterm>messages</firstterm> rather than byte streams, and
209 automatically handles a lot of the hard IPC issues. Also, the D-Bus
210 library is designed to be wrapped in a way that lets developers use
211 their framework's existing object/type system, rather than learning
212 a new one specifically for IPC.
219 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
220 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
221 a system for one application to talk to a single other
222 application. However, the primary intended application of the protocol is the
223 D-Bus <firstterm>message bus</firstterm>, specified in <xref
224 linkend="message-bus"/>. The message bus is a special application that
225 accepts connections from multiple other applications, and forwards
230 Uses of D-Bus include notification of system changes (notification of when
231 a camera is plugged in to a computer, or a new version of some software
232 has been installed), or desktop interoperability, for example a file
233 monitoring service or a configuration service.
237 D-Bus is designed for two specific use cases:
241 A "system bus" for notifications from the system to user sessions,
242 and to allow the system to request input from user sessions.
247 A "session bus" used to implement desktop environments such as
252 D-Bus is not intended to be a generic IPC system for any possible
253 application, and intentionally omits many features found in other
254 IPC systems for this reason.
258 At the same time, the bus daemons offer a number of features not found in
259 other IPC systems, such as single-owner "bus names" (similar to X
260 selections), on-demand startup of services, and security policies.
261 In many ways, these features are the primary motivation for developing
262 D-Bus; other systems would have sufficed if IPC were the only goal.
266 D-Bus may turn out to be useful in unanticipated applications, but future
267 versions of this spec and the reference implementation probably will not
268 incorporate features that interfere with the core use cases.
272 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
273 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
274 document are to be interpreted as described in RFC 2119. However, the
275 document could use a serious audit to be sure it makes sense to do
276 so. Also, they are not capitalized.
279 <sect2 id="stability">
280 <title>Protocol and Specification Stability</title>
282 The D-Bus protocol is frozen (only compatible extensions are allowed) as
283 of November 8, 2006. However, this specification could still use a fair
284 bit of work to make interoperable reimplementation possible without
285 reference to the D-Bus reference implementation. Thus, this
286 specification is not marked 1.0. To mark it 1.0, we'd like to see
287 someone invest significant effort in clarifying the specification
288 language, and growing the specification to cover more aspects of the
289 reference implementation's behavior.
292 Until this work is complete, any attempt to reimplement D-Bus will
293 probably require looking at the reference implementation and/or asking
294 questions on the D-Bus mailing list about intended behavior.
295 Questions on the list are very welcome.
298 Nonetheless, this document should be a useful starting point and is
299 to our knowledge accurate, though incomplete.
305 <sect1 id="type-system">
306 <title>Type System</title>
309 D-Bus has a type system, in which values of various types can be
310 serialized into a sequence of bytes referred to as the
311 <firstterm>wire format</firstterm> in a standard way.
312 Converting a value from some other representation into the wire
313 format is called <firstterm>marshaling</firstterm> and converting
314 it back from the wire format is <firstterm>unmarshaling</firstterm>.
318 The D-Bus protocol does not include type tags in the marshaled data; a
319 block of marshaled values must have a known <firstterm>type
320 signature</firstterm>. The type signature is made up of zero or more
321 <firstterm id="term-single-complete-type">single complete
322 types</firstterm>, each made up of one or more
323 <firstterm>type codes</firstterm>.
327 A type code is an ASCII character representing the
328 type of a value. Because ASCII characters are used, the type signature
329 will always form a valid ASCII string. A simple string compare
330 determines whether two type signatures are equivalent.
334 A single complete type is a sequence of type codes that fully describes
335 one type: either a basic type, or a single fully-described container type.
336 A single complete type is a basic type code, a variant type code,
337 an array with its element type, or a struct with its fields (all of which
338 are defined below). So the following signatures are not single complete
349 And the following signatures contain multiple complete types:
359 Note however that a single complete type may <emphasis>contain</emphasis>
360 multiple other single complete types, by containing a struct or dict
364 <sect2 id="basic-types">
365 <title>Basic types</title>
368 The simplest type codes are the <firstterm id="term-basic-type">basic
369 types</firstterm>, which are the types whose structure is entirely
370 defined by their 1-character type code. Basic types consist of
371 fixed types and string-like types.
375 The <firstterm id="term-fixed-type">fixed types</firstterm>
376 are basic types whose values have a fixed length, namely BYTE,
377 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
382 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
383 the ASCII character 'i'. So the signature for a block of values
384 containing a single <literal>INT32</literal> would be:
388 A block of values containing two <literal>INT32</literal> would have this signature:
395 The characteristics of the fixed types are listed in this table.
401 <entry>Conventional name</entry>
402 <entry>ASCII type-code</entry>
403 <entry>Encoding</entry>
408 <entry><literal>BYTE</literal></entry>
409 <entry><literal>y</literal> (121)</entry>
410 <entry>Unsigned 8-bit integer</entry>
413 <entry><literal>BOOLEAN</literal></entry>
414 <entry><literal>b</literal> (98)</entry>
415 <entry>Boolean value: 0 is false, 1 is true, any other value
416 allowed by the marshalling format is invalid</entry>
419 <entry><literal>INT16</literal></entry>
420 <entry><literal>n</literal> (110)</entry>
421 <entry>Signed (two's complement) 16-bit integer</entry>
424 <entry><literal>UINT16</literal></entry>
425 <entry><literal>q</literal> (113)</entry>
426 <entry>Unsigned 16-bit integer</entry>
429 <entry><literal>INT32</literal></entry>
430 <entry><literal>i</literal> (105)</entry>
431 <entry>Signed (two's complement) 32-bit integer</entry>
434 <entry><literal>UINT32</literal></entry>
435 <entry><literal>u</literal> (117)</entry>
436 <entry>Unsigned 32-bit integer</entry>
439 <entry><literal>INT64</literal></entry>
440 <entry><literal>x</literal> (120)</entry>
441 <entry>Signed (two's complement) 64-bit integer
442 (mnemonic: x and t are the first characters in "sixty" not
443 already used for something more common)</entry>
446 <entry><literal>UINT64</literal></entry>
447 <entry><literal>t</literal> (116)</entry>
448 <entry>Unsigned 64-bit integer</entry>
451 <entry><literal>DOUBLE</literal></entry>
452 <entry><literal>d</literal> (100)</entry>
453 <entry>IEEE 754 double-precision floating point</entry>
456 <entry><literal>UNIX_FD</literal></entry>
457 <entry><literal>h</literal> (104)</entry>
458 <entry>Unsigned 32-bit integer representing an index into an
459 out-of-band array of file descriptors, transferred via some
460 platform-specific mechanism (mnemonic: h for handle)</entry>
468 The <firstterm id="term-string-like-type">string-like types</firstterm>
469 are basic types with a variable length. The value of any string-like
470 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
471 none of which may be U+0000. The UTF-8 text must be validated
472 strictly: in particular, it must not contain overlong sequences
473 or codepoints above U+10FFFF.
477 Since D-Bus Specification version 0.21, in accordance with Unicode
478 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
479 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
480 D-Bus rejected these noncharacters).
484 The marshalling formats for the string-like types all end with a
485 single zero (NUL) byte, but that byte is not considered to be part of
490 The characteristics of the string-like types are listed in this table.
496 <entry>Conventional name</entry>
497 <entry>ASCII type-code</entry>
498 <entry>Validity constraints</entry>
503 <entry><literal>STRING</literal></entry>
504 <entry><literal>s</literal> (115)</entry>
505 <entry>No extra constraints</entry>
508 <entry><literal>OBJECT_PATH</literal></entry>
509 <entry><literal>o</literal> (111)</entry>
511 <link linkend="message-protocol-marshaling-object-path">a
512 syntactically valid object path</link></entry>
515 <entry><literal>SIGNATURE</literal></entry>
516 <entry><literal>g</literal> (103)</entry>
518 <firstterm linkend="term-single-complete-type">single
519 complete types</firstterm></entry>
526 <sect3 id="message-protocol-marshaling-object-path">
527 <title>Valid Object Paths</title>
530 An object path is a name used to refer to an object instance.
531 Conceptually, each participant in a D-Bus message exchange may have
532 any number of object instances (think of C++ or Java objects) and each
533 such instance will have a path. Like a filesystem, the object
534 instances in an application form a hierarchical tree.
538 Object paths are often namespaced by starting with a reversed
539 domain name and containing an interface version number, in the
541 <link linkend="message-protocol-names-interface">interface
543 <link linkend="message-protocol-names-bus">well-known
545 This makes it possible to implement more than one service, or
546 more than one version of a service, in the same process,
547 even if the services share a connection but cannot otherwise
548 co-operate (for instance, if they are implemented by different
553 For instance, if the owner of <literal>example.com</literal> is
554 developing a D-Bus API for a music player, they might use the
555 hierarchy of object paths that start with
556 <literal>/com/example/MusicPlayer1</literal> for its objects.
560 The following rules define a valid object path. Implementations must
561 not send or accept messages with invalid object paths.
565 The path may be of any length.
570 The path must begin with an ASCII '/' (integer 47) character,
571 and must consist of elements separated by slash characters.
576 Each element must only contain the ASCII characters
582 No element may be the empty string.
587 Multiple '/' characters cannot occur in sequence.
592 A trailing '/' character is not allowed unless the
593 path is the root path (a single '/' character).
601 <sect3 id="message-protocol-marshaling-signature">
602 <title>Valid Signatures</title>
604 An implementation must not send or accept invalid signatures.
605 Valid signatures will conform to the following rules:
609 The signature is a list of single complete types.
610 Arrays must have element types, and structs must
611 have both open and close parentheses.
616 Only type codes, open and close parentheses, and open and
617 close curly brackets are allowed in the signature. The
618 <literal>STRUCT</literal> type code
619 is not allowed in signatures, because parentheses
620 are used instead. Similarly, the
621 <literal>DICT_ENTRY</literal> type code is not allowed in
622 signatures, because curly brackets are used instead.
627 The maximum depth of container type nesting is 32 array type
628 codes and 32 open parentheses. This implies that the maximum
629 total depth of recursion is 64, for an "array of array of array
630 of ... struct of struct of struct of ..." where there are 32
636 The maximum length of a signature is 255.
643 When signatures appear in messages, the marshalling format
644 guarantees that they will be followed by a nul byte (which can
645 be interpreted as either C-style string termination or the INVALID
646 type-code), but this is not conceptually part of the signature.
652 <sect2 id="container-types">
653 <title>Container types</title>
656 In addition to basic types, there are four <firstterm>container</firstterm>
657 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
658 and <literal>DICT_ENTRY</literal>.
662 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
663 code does not appear in signatures. Instead, ASCII characters
664 '(' and ')' are used to mark the beginning and end of the struct.
665 So for example, a struct containing two integers would have this
670 Structs can be nested, so for example a struct containing
671 an integer and another struct:
675 The value block storing that struct would contain three integers; the
676 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
681 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
682 but is useful in code that implements the protocol. This type code
683 is specified to allow such code to interoperate in non-protocol contexts.
687 Empty structures are not allowed; there must be at least one
688 type code between the parentheses.
692 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
693 followed by a <firstterm>single complete type</firstterm>. The single
694 complete type following the array is the type of each array element. So
695 the simple example is:
699 which is an array of 32-bit integers. But an array can be of any type,
700 such as this array-of-struct-with-two-int32-fields:
704 Or this array of array of integer:
711 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
712 type <literal>VARIANT</literal> will have the signature of a single complete type as part
713 of the <emphasis>value</emphasis>. This signature will be followed by a
714 marshaled value of that type.
718 Unlike a message signature, the variant signature can
719 contain only a single complete type. So "i", "ai"
720 or "(ii)" is OK, but "ii" is not. Use of variants may not
721 cause a total message depth to be larger than 64, including
722 other container types such as structures.
726 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
727 than parentheses it uses curly braces, and it has more restrictions.
728 The restrictions are: it occurs only as an array element type; it has
729 exactly two single complete types inside the curly braces; the first
730 single complete type (the "key") must be a basic type rather than a
731 container type. Implementations must not accept dict entries outside of
732 arrays, must not accept dict entries with zero, one, or more than two
733 fields, and must not accept dict entries with non-basic-typed keys. A
734 dict entry is always a key-value pair.
738 The first field in the <literal>DICT_ENTRY</literal> is always the key.
739 A message is considered corrupt if the same key occurs twice in the same
740 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
741 implementations are not required to reject dicts with duplicate keys.
745 In most languages, an array of dict entry would be represented as a
746 map, hash table, or dict object.
751 <title>Summary of types</title>
754 The following table summarizes the D-Bus types.
759 <entry>Conventional Name</entry>
761 <entry>Description</entry>
766 <entry><literal>INVALID</literal></entry>
767 <entry>0 (ASCII NUL)</entry>
768 <entry>Not a valid type code, used to terminate signatures</entry>
770 <entry><literal>BYTE</literal></entry>
771 <entry>121 (ASCII 'y')</entry>
772 <entry>8-bit unsigned integer</entry>
774 <entry><literal>BOOLEAN</literal></entry>
775 <entry>98 (ASCII 'b')</entry>
776 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
778 <entry><literal>INT16</literal></entry>
779 <entry>110 (ASCII 'n')</entry>
780 <entry>16-bit signed integer</entry>
782 <entry><literal>UINT16</literal></entry>
783 <entry>113 (ASCII 'q')</entry>
784 <entry>16-bit unsigned integer</entry>
786 <entry><literal>INT32</literal></entry>
787 <entry>105 (ASCII 'i')</entry>
788 <entry>32-bit signed integer</entry>
790 <entry><literal>UINT32</literal></entry>
791 <entry>117 (ASCII 'u')</entry>
792 <entry>32-bit unsigned integer</entry>
794 <entry><literal>INT64</literal></entry>
795 <entry>120 (ASCII 'x')</entry>
796 <entry>64-bit signed integer</entry>
798 <entry><literal>UINT64</literal></entry>
799 <entry>116 (ASCII 't')</entry>
800 <entry>64-bit unsigned integer</entry>
802 <entry><literal>DOUBLE</literal></entry>
803 <entry>100 (ASCII 'd')</entry>
804 <entry>IEEE 754 double</entry>
806 <entry><literal>STRING</literal></entry>
807 <entry>115 (ASCII 's')</entry>
808 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
810 <entry><literal>OBJECT_PATH</literal></entry>
811 <entry>111 (ASCII 'o')</entry>
812 <entry>Name of an object instance</entry>
814 <entry><literal>SIGNATURE</literal></entry>
815 <entry>103 (ASCII 'g')</entry>
816 <entry>A type signature</entry>
818 <entry><literal>ARRAY</literal></entry>
819 <entry>97 (ASCII 'a')</entry>
822 <entry><literal>STRUCT</literal></entry>
823 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
824 <entry>Struct; type code 114 'r' is reserved for use in
825 bindings and implementations to represent the general
826 concept of a struct, and must not appear in signatures
827 used on D-Bus.</entry>
829 <entry><literal>VARIANT</literal></entry>
830 <entry>118 (ASCII 'v') </entry>
831 <entry>Variant type (the type of the value is part of the value itself)</entry>
833 <entry><literal>DICT_ENTRY</literal></entry>
834 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
835 <entry>Entry in a dict or map (array of key-value pairs).
836 Type code 101 'e' is reserved for use in bindings and
837 implementations to represent the general concept of a
838 dict or dict-entry, and must not appear in signatures
839 used on D-Bus.</entry>
841 <entry><literal>UNIX_FD</literal></entry>
842 <entry>104 (ASCII 'h')</entry>
843 <entry>Unix file descriptor</entry>
846 <entry>(reserved)</entry>
847 <entry>109 (ASCII 'm')</entry>
848 <entry>Reserved for <ulink
849 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
850 'maybe' type compatible with the one in GVariant</ulink>,
851 and must not appear in signatures used on D-Bus until
852 specified here</entry>
855 <entry>(reserved)</entry>
856 <entry>42 (ASCII '*')</entry>
857 <entry>Reserved for use in bindings/implementations to
858 represent any <firstterm>single complete type</firstterm>,
859 and must not appear in signatures used on D-Bus.</entry>
862 <entry>(reserved)</entry>
863 <entry>63 (ASCII '?')</entry>
864 <entry>Reserved for use in bindings/implementations to
865 represent any <firstterm>basic type</firstterm>, and must
866 not appear in signatures used on D-Bus.</entry>
869 <entry>(reserved)</entry>
870 <entry>64 (ASCII '@'), 38 (ASCII '&'),
871 94 (ASCII '^')</entry>
872 <entry>Reserved for internal use by bindings/implementations,
873 and must not appear in signatures used on D-Bus.
874 GVariant uses these type-codes to encode calling
885 <sect1 id="message-protocol-marshaling">
886 <title>Marshaling (Wire Format)</title>
889 D-Bus defines a marshalling format for its type system, which is
890 used in D-Bus messages. This is not the only possible marshalling
891 format for the type system: for instance, GVariant (part of GLib)
892 re-uses the D-Bus type system but implements an alternative marshalling
897 <title>Byte order and alignment</title>
900 Given a type signature, a block of bytes can be converted into typed
901 values. This section describes the format of the block of bytes. Byte
902 order and alignment issues are handled uniformly for all D-Bus types.
906 A block of bytes has an associated byte order. The byte order
907 has to be discovered in some way; for D-Bus messages, the
908 byte order is part of the message header as described in
909 <xref linkend="message-protocol-messages"/>. For now, assume
910 that the byte order is known to be either little endian or big
915 Each value in a block of bytes is aligned "naturally," for example
916 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
917 8-byte boundary. To properly align a value, <firstterm>alignment
918 padding</firstterm> may be necessary. The alignment padding must always
919 be the minimum required padding to properly align the following value;
920 and it must always be made up of nul bytes. The alignment padding must
921 not be left uninitialized (it can't contain garbage), and more padding
922 than required must not be used.
926 As an exception to natural alignment, <literal>STRUCT</literal> and
927 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
928 boundary, regardless of the alignments of their contents.
933 <title>Marshalling basic types</title>
936 To marshal and unmarshal fixed types, you simply read one value
937 from the data block corresponding to each type code in the signature.
938 All signed integer values are encoded in two's complement, DOUBLE
939 values are IEEE 754 double-precision floating-point, and BOOLEAN
940 values are encoded in 32 bits (of which only the least significant
945 The string-like types are all marshalled as a
946 fixed-length unsigned integer <varname>n</varname> giving the
947 length of the variable part, followed by <varname>n</varname>
948 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
949 which is not considered to be part of the text. The alignment
950 of the string-like type is the same as the alignment of
951 <varname>n</varname>.
955 For the STRING and OBJECT_PATH types, <varname>n</varname> is
956 encoded in 4 bytes, leading to 4-byte alignment.
957 For the SIGNATURE type, <varname>n</varname> is encoded as a single
958 byte. As a result, alignment padding is never required before a
964 <title>Marshalling containers</title>
967 Arrays are marshalled as a <literal>UINT32</literal>
968 <varname>n</varname> giving the length of the array data in bytes,
969 followed by alignment padding to the alignment boundary of the array
970 element type, followed by the <varname>n</varname> bytes of the
971 array elements marshalled in sequence. <varname>n</varname> does not
972 include the padding after the length, or any padding after the
977 For instance, if the current position in the message is a multiple
978 of 8 bytes and the byte-order is big-endian, an array containing only
979 the 64-bit integer 5 would be marshalled as:
982 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
983 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
984 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
989 Arrays have a maximum length defined to be 2 to the 26th power or
990 67108864. Implementations must not send or accept arrays exceeding this
995 Structs and dict entries are marshalled in the same way as their
996 contents, but their alignment is always to an 8-byte boundary,
997 even if their contents would normally be less strictly aligned.
1001 Variants are marshalled as the <literal>SIGNATURE</literal> of
1002 the contents (which must be a single complete type), followed by a
1003 marshalled value with the type given by that signature. The
1004 variant has the same 1-byte alignment as the signature, which means
1005 that alignment padding before a variant is never needed.
1006 Use of variants may not cause a total message depth to be larger
1007 than 64, including other container types such as structures.
1012 <title>Summary of D-Bus marshalling</title>
1015 Given all this, the types are marshaled on the wire as follows:
1020 <entry>Conventional Name</entry>
1021 <entry>Encoding</entry>
1022 <entry>Alignment</entry>
1027 <entry><literal>INVALID</literal></entry>
1028 <entry>Not applicable; cannot be marshaled.</entry>
1031 <entry><literal>BYTE</literal></entry>
1032 <entry>A single 8-bit byte.</entry>
1035 <entry><literal>BOOLEAN</literal></entry>
1036 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1039 <entry><literal>INT16</literal></entry>
1040 <entry>16-bit signed integer in the message's byte order.</entry>
1043 <entry><literal>UINT16</literal></entry>
1044 <entry>16-bit unsigned integer in the message's byte order.</entry>
1047 <entry><literal>INT32</literal></entry>
1048 <entry>32-bit signed integer in the message's byte order.</entry>
1051 <entry><literal>UINT32</literal></entry>
1052 <entry>32-bit unsigned integer in the message's byte order.</entry>
1055 <entry><literal>INT64</literal></entry>
1056 <entry>64-bit signed integer in the message's byte order.</entry>
1059 <entry><literal>UINT64</literal></entry>
1060 <entry>64-bit unsigned integer in the message's byte order.</entry>
1063 <entry><literal>DOUBLE</literal></entry>
1064 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1067 <entry><literal>STRING</literal></entry>
1068 <entry>A <literal>UINT32</literal> indicating the string's
1069 length in bytes excluding its terminating nul, followed by
1070 non-nul string data of the given length, followed by a terminating nul
1077 <entry><literal>OBJECT_PATH</literal></entry>
1078 <entry>Exactly the same as <literal>STRING</literal> except the
1079 content must be a valid object path (see above).
1085 <entry><literal>SIGNATURE</literal></entry>
1086 <entry>The same as <literal>STRING</literal> except the length is a single
1087 byte (thus signatures have a maximum length of 255)
1088 and the content must be a valid signature (see above).
1094 <entry><literal>ARRAY</literal></entry>
1096 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1097 alignment padding to the alignment boundary of the array element type,
1098 followed by each array element.
1104 <entry><literal>STRUCT</literal></entry>
1106 A struct must start on an 8-byte boundary regardless of the
1107 type of the struct fields. The struct value consists of each
1108 field marshaled in sequence starting from that 8-byte
1115 <entry><literal>VARIANT</literal></entry>
1117 The marshaled <literal>SIGNATURE</literal> of a single
1118 complete type, followed by a marshaled value with the type
1119 given in the signature.
1122 1 (alignment of the signature)
1125 <entry><literal>DICT_ENTRY</literal></entry>
1127 Identical to STRUCT.
1133 <entry><literal>UNIX_FD</literal></entry>
1134 <entry>32-bit unsigned integer in the message's byte
1135 order. The actual file descriptors need to be
1136 transferred out-of-band via some platform specific
1137 mechanism. On the wire, values of this type store the index to the
1138 file descriptor in the array of file descriptors that
1139 accompany the message.</entry>
1151 <sect1 id="message-protocol">
1152 <title>Message Protocol</title>
1155 A <firstterm>message</firstterm> consists of a
1156 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1157 think of a message as a package, the header is the address, and the body
1158 contains the package contents. The message delivery system uses the header
1159 information to figure out where to send the message and how to interpret
1160 it; the recipient interprets the body of the message.
1164 The body of the message is made up of zero or more
1165 <firstterm>arguments</firstterm>, which are typed values, such as an
1166 integer or a byte array.
1170 Both header and body use the D-Bus <link linkend="type-system">type
1171 system</link> and format for serializing data.
1174 <sect2 id="message-protocol-messages">
1175 <title>Message Format</title>
1178 A message consists of a header and a body. The header is a block of
1179 values with a fixed signature and meaning. The body is a separate block
1180 of values, with a signature specified in the header.
1184 The length of the header must be a multiple of 8, allowing the body to
1185 begin on an 8-byte boundary when storing the entire message in a single
1186 buffer. If the header does not naturally end on an 8-byte boundary
1187 up to 7 bytes of nul-initialized alignment padding must be added.
1191 The message body need not end on an 8-byte boundary.
1195 The maximum length of a message, including header, header alignment padding,
1196 and body is 2 to the 27th power or 134217728. Implementations must not
1197 send or accept messages exceeding this size.
1201 The signature of the header is:
1205 Written out more readably, this is:
1207 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1212 These values have the following meanings:
1217 <entry>Value</entry>
1218 <entry>Description</entry>
1223 <entry>1st <literal>BYTE</literal></entry>
1224 <entry>Endianness flag; ASCII 'l' for little-endian
1225 or ASCII 'B' for big-endian. Both header and body are
1226 in this endianness.</entry>
1229 <entry>2nd <literal>BYTE</literal></entry>
1230 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1231 Currently-defined types are described below.
1235 <entry>3rd <literal>BYTE</literal></entry>
1236 <entry>Bitwise OR of flags. Unknown flags
1237 must be ignored. Currently-defined flags are described below.
1241 <entry>4th <literal>BYTE</literal></entry>
1242 <entry>Major protocol version of the sending application. If
1243 the major protocol version of the receiving application does not
1244 match, the applications will not be able to communicate and the
1245 D-Bus connection must be disconnected. The major protocol
1246 version for this version of the specification is 1.
1250 <entry>1st <literal>UINT32</literal></entry>
1251 <entry>Length in bytes of the message body, starting
1252 from the end of the header. The header ends after
1253 its alignment padding to an 8-boundary.
1257 <entry>2nd <literal>UINT32</literal></entry>
1258 <entry>The serial of this message, used as a cookie
1259 by the sender to identify the reply corresponding
1260 to this request. This must not be zero.
1264 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1265 <entry>An array of zero or more <firstterm>header
1266 fields</firstterm> where the byte is the field code, and the
1267 variant is the field value. The message type determines
1268 which fields are required.
1276 <firstterm>Message types</firstterm> that can appear in the second byte
1282 <entry>Conventional name</entry>
1283 <entry>Decimal value</entry>
1284 <entry>Description</entry>
1289 <entry><literal>INVALID</literal></entry>
1291 <entry>This is an invalid type.</entry>
1294 <entry><literal>METHOD_CALL</literal></entry>
1296 <entry>Method call.</entry>
1299 <entry><literal>METHOD_RETURN</literal></entry>
1301 <entry>Method reply with returned data.</entry>
1304 <entry><literal>ERROR</literal></entry>
1306 <entry>Error reply. If the first argument exists and is a
1307 string, it is an error message.</entry>
1310 <entry><literal>SIGNAL</literal></entry>
1312 <entry>Signal emission.</entry>
1319 Flags that can appear in the third byte of the header:
1324 <entry>Conventional name</entry>
1325 <entry>Hex value</entry>
1326 <entry>Description</entry>
1331 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1333 <entry>This message does not expect method return replies or
1334 error replies; the reply can be omitted as an
1335 optimization. However, it is compliant with this specification
1336 to return the reply despite this flag and the only harm
1337 from doing so is extra network traffic.
1341 <entry><literal>NO_AUTO_START</literal></entry>
1343 <entry>The bus must not launch an owner
1344 for the destination name in response to this message.
1352 <sect3 id="message-protocol-header-fields">
1353 <title>Header Fields</title>
1356 The array at the end of the header contains <firstterm>header
1357 fields</firstterm>, where each field is a 1-byte field code followed
1358 by a field value. A header must contain the required header fields for
1359 its message type, and zero or more of any optional header
1360 fields. Future versions of this protocol specification may add new
1361 fields. Implementations must ignore fields they do not
1362 understand. Implementations must not invent their own header fields;
1363 only changes to this specification may introduce new header fields.
1367 Again, if an implementation sees a header field code that it does not
1368 expect, it must ignore that field, as it will be part of a new
1369 (but compatible) version of this specification. This also applies
1370 to known header fields appearing in unexpected messages, for
1371 example: if a signal has a reply serial it must be ignored
1372 even though it has no meaning as of this version of the spec.
1376 However, implementations must not send or accept known header fields
1377 with the wrong type stored in the field value. So for example a
1378 message with an <literal>INTERFACE</literal> field of type
1379 <literal>UINT32</literal> would be considered corrupt.
1383 Here are the currently-defined header fields:
1388 <entry>Conventional Name</entry>
1389 <entry>Decimal Code</entry>
1391 <entry>Required In</entry>
1392 <entry>Description</entry>
1397 <entry><literal>INVALID</literal></entry>
1400 <entry>not allowed</entry>
1401 <entry>Not a valid field name (error if it appears in a message)</entry>
1404 <entry><literal>PATH</literal></entry>
1406 <entry><literal>OBJECT_PATH</literal></entry>
1407 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1408 <entry>The object to send a call to,
1409 or the object a signal is emitted from.
1411 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1412 implementations should not send messages with this path,
1413 and the reference implementation of the bus daemon will
1414 disconnect any application that attempts to do so.
1418 <entry><literal>INTERFACE</literal></entry>
1420 <entry><literal>STRING</literal></entry>
1421 <entry><literal>SIGNAL</literal></entry>
1423 The interface to invoke a method call on, or
1424 that a signal is emitted from. Optional for
1425 method calls, required for signals.
1426 The special interface
1427 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1428 implementations should not send messages with this
1429 interface, and the reference implementation of the bus
1430 daemon will disconnect any application that attempts to
1435 <entry><literal>MEMBER</literal></entry>
1437 <entry><literal>STRING</literal></entry>
1438 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1439 <entry>The member, either the method name or signal name.</entry>
1442 <entry><literal>ERROR_NAME</literal></entry>
1444 <entry><literal>STRING</literal></entry>
1445 <entry><literal>ERROR</literal></entry>
1446 <entry>The name of the error that occurred, for errors</entry>
1449 <entry><literal>REPLY_SERIAL</literal></entry>
1451 <entry><literal>UINT32</literal></entry>
1452 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1453 <entry>The serial number of the message this message is a reply
1454 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1457 <entry><literal>DESTINATION</literal></entry>
1459 <entry><literal>STRING</literal></entry>
1460 <entry>optional</entry>
1461 <entry>The name of the connection this message is intended for.
1462 Only used in combination with the message bus, see
1463 <xref linkend="message-bus"/>.</entry>
1466 <entry><literal>SENDER</literal></entry>
1468 <entry><literal>STRING</literal></entry>
1469 <entry>optional</entry>
1470 <entry>Unique name of the sending connection.
1471 The message bus fills in this field so it is reliable; the field is
1472 only meaningful in combination with the message bus.</entry>
1475 <entry><literal>SIGNATURE</literal></entry>
1477 <entry><literal>SIGNATURE</literal></entry>
1478 <entry>optional</entry>
1479 <entry>The signature of the message body.
1480 If omitted, it is assumed to be the
1481 empty signature "" (i.e. the body must be 0-length).</entry>
1484 <entry><literal>UNIX_FDS</literal></entry>
1486 <entry><literal>UINT32</literal></entry>
1487 <entry>optional</entry>
1488 <entry>The number of Unix file descriptors that
1489 accompany the message. If omitted, it is assumed
1490 that no Unix file descriptors accompany the
1491 message. The actual file descriptors need to be
1492 transferred via platform specific mechanism
1493 out-of-band. They must be sent at the same time as
1494 part of the message itself. They may not be sent
1495 before the first byte of the message itself is
1496 transferred or after the last byte of the message
1506 <sect2 id="message-protocol-names">
1507 <title>Valid Names</title>
1509 The various names in D-Bus messages have some restrictions.
1512 There is a <firstterm>maximum name length</firstterm>
1513 of 255 which applies to bus names, interfaces, and members.
1515 <sect3 id="message-protocol-names-interface">
1516 <title>Interface names</title>
1518 Interfaces have names with type <literal>STRING</literal>, meaning that
1519 they must be valid UTF-8. However, there are also some
1520 additional restrictions that apply to interface names
1523 <listitem><para>Interface names are composed of 1 or more elements separated by
1524 a period ('.') character. All elements must contain at least
1528 <listitem><para>Each element must only contain the ASCII characters
1529 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1533 <listitem><para>Interface names must contain at least one '.' (period)
1534 character (and thus at least two elements).
1537 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1538 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1543 Interface names should start with the reversed DNS domain name of
1544 the author of the interface (in lower-case), like interface names
1545 in Java. It is conventional for the rest of the interface name
1546 to consist of words run together, with initial capital letters
1547 on all words ("CamelCase"). Several levels of hierarchy can be used.
1548 It is also a good idea to include the major version of the interface
1549 in the name, and increment it if incompatible changes are made;
1550 this way, a single object can implement several versions of an
1551 interface in parallel, if necessary.
1555 For instance, if the owner of <literal>example.com</literal> is
1556 developing a D-Bus API for a music player, they might define
1557 interfaces called <literal>com.example.MusicPlayer1</literal>,
1558 <literal>com.example.MusicPlayer1.Track</literal> and
1559 <literal>com.example.MusicPlayer1.Seekable</literal>.
1563 D-Bus does not distinguish between the concepts that would be
1564 called classes and interfaces in Java: either can be identified on
1565 D-Bus by an interface name.
1568 <sect3 id="message-protocol-names-bus">
1569 <title>Bus names</title>
1571 Connections have one or more bus names associated with them.
1572 A connection has exactly one bus name that is a <firstterm>unique
1573 connection name</firstterm>. The unique connection name remains
1574 with the connection for its entire lifetime.
1575 A bus name is of type <literal>STRING</literal>,
1576 meaning that it must be valid UTF-8. However, there are also
1577 some additional restrictions that apply to bus names
1580 <listitem><para>Bus names that start with a colon (':')
1581 character are unique connection names. Other bus names
1582 are called <firstterm>well-known bus names</firstterm>.
1585 <listitem><para>Bus names are composed of 1 or more elements separated by
1586 a period ('.') character. All elements must contain at least
1590 <listitem><para>Each element must only contain the ASCII characters
1591 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1592 connection name may begin with a digit, elements in
1593 other bus names must not begin with a digit.
1597 <listitem><para>Bus names must contain at least one '.' (period)
1598 character (and thus at least two elements).
1601 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1602 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1606 Note that the hyphen ('-') character is allowed in bus names but
1607 not in interface names.
1611 Like <link linkend="message-protocol-names-interface">interface
1612 names</link>, well-known bus names should start with the
1613 reversed DNS domain name of the author of the interface (in
1614 lower-case), and it is conventional for the rest of the well-known
1615 bus name to consist of words run together, with initial
1616 capital letters. As with interface names, including a version
1617 number in well-known bus names is a good idea; it's possible to
1618 have the well-known bus name for more than one version
1619 simultaneously if backwards compatibility is required.
1623 If a well-known bus name implies the presence of a "main" interface,
1624 that "main" interface is often given the same name as
1625 the well-known bus name, and situated at the corresponding object
1626 path. For instance, if the owner of <literal>example.com</literal>
1627 is developing a D-Bus API for a music player, they might define
1628 that any application that takes the well-known name
1629 <literal>com.example.MusicPlayer1</literal> should have an object
1630 at the object path <literal>/com/example/MusicPlayer1</literal>
1631 which implements the interface
1632 <literal>com.example.MusicPlayer1</literal>.
1635 <sect3 id="message-protocol-names-member">
1636 <title>Member names</title>
1638 Member (i.e. method or signal) names:
1640 <listitem><para>Must only contain the ASCII characters
1641 "[A-Z][a-z][0-9]_" and may not begin with a
1642 digit.</para></listitem>
1643 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1644 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1645 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1650 It is conventional for member names on D-Bus to consist of
1651 capitalized words with no punctuation ("camel-case").
1652 Method names should usually be verbs, such as
1653 <literal>GetItems</literal>, and signal names should usually be
1654 a description of an event, such as <literal>ItemsChanged</literal>.
1657 <sect3 id="message-protocol-names-error">
1658 <title>Error names</title>
1660 Error names have the same restrictions as interface names.
1664 Error names have the same naming conventions as interface
1665 names, and often contain <literal>.Error.</literal>; for instance,
1666 the owner of <literal>example.com</literal> might define the
1667 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1668 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1669 The errors defined by D-Bus itself, such as
1670 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1676 <sect2 id="message-protocol-types">
1677 <title>Message Types</title>
1679 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1680 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1681 This section describes these conventions.
1683 <sect3 id="message-protocol-types-method">
1684 <title>Method Calls</title>
1686 Some messages invoke an operation on a remote object. These are
1687 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1688 messages map naturally to methods on objects in a typical program.
1691 A method call message is required to have a <literal>MEMBER</literal> header field
1692 indicating the name of the method. Optionally, the message has an
1693 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1694 Including the <literal>INTERFACE</literal> in all method call
1695 messages is strongly recommended.
1698 In the absence of an <literal>INTERFACE</literal> field, if two
1699 or more interfaces on the same object have a method with the same
1700 name, it is undefined which of those methods will be invoked.
1701 Implementations may choose to either return an error, or deliver the
1702 message as though it had an arbitrary one of those interfaces.
1705 In some situations (such as the well-known system bus), messages
1706 are filtered through an access-control list external to the
1707 remote object implementation. If that filter rejects certain
1708 messages by matching their interface, or accepts only messages
1709 to specific interfaces, it must also reject messages that have no
1710 <literal>INTERFACE</literal>: otherwise, malicious
1711 applications could use this to bypass the filter.
1714 Method call messages also include a <literal>PATH</literal> field
1715 indicating the object to invoke the method on. If the call is passing
1716 through a message bus, the message will also have a
1717 <literal>DESTINATION</literal> field giving the name of the connection
1718 to receive the message.
1721 When an application handles a method call message, it is required to
1722 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1723 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1724 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1727 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1728 are the return value(s) or "out parameters" of the method call.
1729 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1730 and the call fails; no return value will be provided. It makes
1731 no sense to send multiple replies to the same method call.
1734 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1735 reply is required, so the caller will know the method
1736 was successfully processed.
1739 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1743 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1744 then as an optimization the application receiving the method
1745 call may choose to omit the reply message (regardless of
1746 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1747 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1748 flag and reply anyway.
1751 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1752 destination name does not exist then a program to own the destination
1753 name will be started before the message is delivered. The message
1754 will be held until the new program is successfully started or has
1755 failed to start; in case of failure, an error will be returned. This
1756 flag is only relevant in the context of a message bus, it is ignored
1757 during one-to-one communication with no intermediate bus.
1759 <sect4 id="message-protocol-types-method-apis">
1760 <title>Mapping method calls to native APIs</title>
1762 APIs for D-Bus may map method calls to a method call in a specific
1763 programming language, such as C++, or may map a method call written
1764 in an IDL to a D-Bus message.
1767 In APIs of this nature, arguments to a method are often termed "in"
1768 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1769 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1770 "inout" arguments, which are both sent and received, i.e. the caller
1771 passes in a value which is modified. Mapped to D-Bus, an "inout"
1772 argument is equivalent to an "in" argument, followed by an "out"
1773 argument. You can't pass things "by reference" over the wire, so
1774 "inout" is purely an illusion of the in-process API.
1777 Given a method with zero or one return values, followed by zero or more
1778 arguments, where each argument may be "in", "out", or "inout", the
1779 caller constructs a message by appending each "in" or "inout" argument,
1780 in order. "out" arguments are not represented in the caller's message.
1783 The recipient constructs a reply by appending first the return value
1784 if any, then each "out" or "inout" argument, in order.
1785 "in" arguments are not represented in the reply message.
1788 Error replies are normally mapped to exceptions in languages that have
1792 In converting from native APIs to D-Bus, it is perhaps nice to
1793 map D-Bus naming conventions ("FooBar") to native conventions
1794 such as "fooBar" or "foo_bar" automatically. This is OK
1795 as long as you can say that the native API is one that
1796 was specifically written for D-Bus. It makes the most sense
1797 when writing object implementations that will be exported
1798 over the bus. Object proxies used to invoke remote D-Bus
1799 objects probably need the ability to call any D-Bus method,
1800 and thus a magic name mapping like this could be a problem.
1803 This specification doesn't require anything of native API bindings;
1804 the preceding is only a suggested convention for consistency
1810 <sect3 id="message-protocol-types-signal">
1811 <title>Signal Emission</title>
1813 Unlike method calls, signal emissions have no replies.
1814 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1815 It must have three header fields: <literal>PATH</literal> giving the object
1816 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1817 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1818 for signals, though it is optional for method calls.
1822 <sect3 id="message-protocol-types-errors">
1823 <title>Errors</title>
1825 Messages of type <literal>ERROR</literal> are most commonly replies
1826 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1827 to any kind of message. The message bus for example
1828 will return an <literal>ERROR</literal> in reply to a signal emission if
1829 the bus does not have enough memory to send the signal.
1832 An <literal>ERROR</literal> may have any arguments, but if the first
1833 argument is a <literal>STRING</literal>, it must be an error message.
1834 The error message may be logged or shown to the user
1839 <sect3 id="message-protocol-types-notation">
1840 <title>Notation in this document</title>
1842 This document uses a simple pseudo-IDL to describe particular method
1843 calls and signals. Here is an example of a method call:
1845 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1846 out UINT32 resultcode)
1848 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1849 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1850 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1851 characters so it's known that the last part of the name in
1852 the "IDL" is the member name.
1855 In C++ that might end up looking like this:
1857 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1858 unsigned int flags);
1860 or equally valid, the return value could be done as an argument:
1862 void org::freedesktop::DBus::StartServiceByName (const char *name,
1864 unsigned int *resultcode);
1866 It's really up to the API designer how they want to make
1867 this look. You could design an API where the namespace wasn't used
1868 in C++, using STL or Qt, using varargs, or whatever you wanted.
1871 Signals are written as follows:
1873 org.freedesktop.DBus.NameLost (STRING name)
1875 Signals don't specify "in" vs. "out" because only
1876 a single direction is possible.
1879 It isn't especially encouraged to use this lame pseudo-IDL in actual
1880 API implementations; you might use the native notation for the
1881 language you're using, or you might use COM or CORBA IDL, for example.
1886 <sect2 id="message-protocol-handling-invalid">
1887 <title>Invalid Protocol and Spec Extensions</title>
1890 For security reasons, the D-Bus protocol should be strictly parsed and
1891 validated, with the exception of defined extension points. Any invalid
1892 protocol or spec violations should result in immediately dropping the
1893 connection without notice to the other end. Exceptions should be
1894 carefully considered, e.g. an exception may be warranted for a
1895 well-understood idiosyncrasy of a widely-deployed implementation. In
1896 cases where the other end of a connection is 100% trusted and known to
1897 be friendly, skipping validation for performance reasons could also make
1898 sense in certain cases.
1902 Generally speaking violations of the "must" requirements in this spec
1903 should be considered possible attempts to exploit security, and violations
1904 of the "should" suggestions should be considered legitimate (though perhaps
1905 they should generate an error in some cases).
1909 The following extension points are built in to D-Bus on purpose and must
1910 not be treated as invalid protocol. The extension points are intended
1911 for use by future versions of this spec, they are not intended for third
1912 parties. At the moment, the only way a third party could extend D-Bus
1913 without breaking interoperability would be to introduce a way to negotiate new
1914 feature support as part of the auth protocol, using EXTENSION_-prefixed
1915 commands. There is not yet a standard way to negotiate features.
1919 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1920 commands result in an ERROR rather than a disconnect. This enables
1921 future extensions to the protocol. Commands starting with EXTENSION_ are
1922 reserved for third parties.
1927 The authentication protocol supports pluggable auth mechanisms.
1932 The address format (see <xref linkend="addresses"/>) supports new
1938 Messages with an unknown type (something other than
1939 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1940 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1941 Unknown-type messages must still be well-formed in the same way
1942 as the known messages, however. They still have the normal
1948 Header fields with an unknown or unexpected field code must be ignored,
1949 though again they must still be well-formed.
1954 New standard interfaces (with new methods and signals) can of course be added.
1964 <sect1 id="auth-protocol">
1965 <title>Authentication Protocol</title>
1967 Before the flow of messages begins, two applications must
1968 authenticate. A simple plain-text protocol is used for
1969 authentication; this protocol is a SASL profile, and maps fairly
1970 directly from the SASL specification. The message encoding is
1971 NOT used here, only plain text messages.
1974 In examples, "C:" and "S:" indicate lines sent by the client and
1975 server respectively.
1977 <sect2 id="auth-protocol-overview">
1978 <title>Protocol Overview</title>
1980 The protocol is a line-based protocol, where each line ends with
1981 \r\n. Each line begins with an all-caps ASCII command name containing
1982 only the character range [A-Z_], a space, then any arguments for the
1983 command, then the \r\n ending the line. The protocol is
1984 case-sensitive. All bytes must be in the ASCII character set.
1986 Commands from the client to the server are as follows:
1989 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1990 <listitem><para>CANCEL</para></listitem>
1991 <listitem><para>BEGIN</para></listitem>
1992 <listitem><para>DATA <data in hex encoding></para></listitem>
1993 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1994 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1997 From server to client are as follows:
2000 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
2001 <listitem><para>OK <GUID in hex></para></listitem>
2002 <listitem><para>DATA <data in hex encoding></para></listitem>
2003 <listitem><para>ERROR</para></listitem>
2004 <listitem><para>AGREE_UNIX_FD</para></listitem>
2008 Unofficial extensions to the command set must begin with the letters
2009 "EXTENSION_", to avoid conflicts with future official commands.
2010 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2013 <sect2 id="auth-nul-byte">
2014 <title>Special credentials-passing nul byte</title>
2016 Immediately after connecting to the server, the client must send a
2017 single nul byte. This byte may be accompanied by credentials
2018 information on some operating systems that use sendmsg() with
2019 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2020 sockets. However, the nul byte must be sent even on other kinds of
2021 socket, and even on operating systems that do not require a byte to be
2022 sent in order to transmit credentials. The text protocol described in
2023 this document begins after the single nul byte. If the first byte
2024 received from the client is not a nul byte, the server may disconnect
2028 A nul byte in any context other than the initial byte is an error;
2029 the protocol is ASCII-only.
2032 The credentials sent along with the nul byte may be used with the
2033 SASL mechanism EXTERNAL.
2036 <sect2 id="auth-command-auth">
2037 <title>AUTH command</title>
2039 If an AUTH command has no arguments, it is a request to list
2040 available mechanisms. The server must respond with a REJECTED
2041 command listing the mechanisms it understands, or with an error.
2044 If an AUTH command specifies a mechanism, and the server supports
2045 said mechanism, the server should begin exchanging SASL
2046 challenge-response data with the client using DATA commands.
2049 If the server does not support the mechanism given in the AUTH
2050 command, it must send either a REJECTED command listing the mechanisms
2051 it does support, or an error.
2054 If the [initial-response] argument is provided, it is intended for use
2055 with mechanisms that have no initial challenge (or an empty initial
2056 challenge), as if it were the argument to an initial DATA command. If
2057 the selected mechanism has an initial challenge and [initial-response]
2058 was provided, the server should reject authentication by sending
2062 If authentication succeeds after exchanging DATA commands,
2063 an OK command must be sent to the client.
2066 The first octet received by the server after the \r\n of the BEGIN
2067 command from the client must be the first octet of the
2068 authenticated/encrypted stream of D-Bus messages.
2071 If BEGIN is received by the server, the first octet received
2072 by the client after the \r\n of the OK command must be the
2073 first octet of the authenticated/encrypted stream of D-Bus
2077 <sect2 id="auth-command-cancel">
2078 <title>CANCEL Command</title>
2080 At any time up to sending the BEGIN command, the client may send a
2081 CANCEL command. On receiving the CANCEL command, the server must
2082 send a REJECTED command and abort the current authentication
2086 <sect2 id="auth-command-data">
2087 <title>DATA Command</title>
2089 The DATA command may come from either client or server, and simply
2090 contains a hex-encoded block of data to be interpreted
2091 according to the SASL mechanism in use.
2094 Some SASL mechanisms support sending an "empty string";
2095 FIXME we need some way to do this.
2098 <sect2 id="auth-command-begin">
2099 <title>BEGIN Command</title>
2101 The BEGIN command acknowledges that the client has received an
2102 OK command from the server, and that the stream of messages
2106 The first octet received by the server after the \r\n of the BEGIN
2107 command from the client must be the first octet of the
2108 authenticated/encrypted stream of D-Bus messages.
2111 <sect2 id="auth-command-rejected">
2112 <title>REJECTED Command</title>
2114 The REJECTED command indicates that the current authentication
2115 exchange has failed, and further exchange of DATA is inappropriate.
2116 The client would normally try another mechanism, or try providing
2117 different responses to challenges.
2119 Optionally, the REJECTED command has a space-separated list of
2120 available auth mechanisms as arguments. If a server ever provides
2121 a list of supported mechanisms, it must provide the same list
2122 each time it sends a REJECTED message. Clients are free to
2123 ignore all lists received after the first.
2126 <sect2 id="auth-command-ok">
2127 <title>OK Command</title>
2129 The OK command indicates that the client has been
2130 authenticated. The client may now proceed with negotiating
2131 Unix file descriptor passing. To do that it shall send
2132 NEGOTIATE_UNIX_FD to the server.
2135 Otherwise, the client must respond to the OK command by
2136 sending a BEGIN command, followed by its stream of messages,
2137 or by disconnecting. The server must not accept additional
2138 commands using this protocol after the BEGIN command has been
2139 received. Further communication will be a stream of D-Bus
2140 messages (optionally encrypted, as negotiated) rather than
2144 If a client sends BEGIN the first octet received by the client
2145 after the \r\n of the OK command must be the first octet of
2146 the authenticated/encrypted stream of D-Bus messages.
2149 The OK command has one argument, which is the GUID of the server.
2150 See <xref linkend="addresses"/> for more on server GUIDs.
2153 <sect2 id="auth-command-error">
2154 <title>ERROR Command</title>
2156 The ERROR command indicates that either server or client did not
2157 know a command, does not accept the given command in the current
2158 context, or did not understand the arguments to the command. This
2159 allows the protocol to be extended; a client or server can send a
2160 command present or permitted only in new protocol versions, and if
2161 an ERROR is received instead of an appropriate response, fall back
2162 to using some other technique.
2165 If an ERROR is sent, the server or client that sent the
2166 error must continue as if the command causing the ERROR had never been
2167 received. However, the the server or client receiving the error
2168 should try something other than whatever caused the error;
2169 if only canceling/rejecting the authentication.
2172 If the D-Bus protocol changes incompatibly at some future time,
2173 applications implementing the new protocol would probably be able to
2174 check for support of the new protocol by sending a new command and
2175 receiving an ERROR from applications that don't understand it. Thus the
2176 ERROR feature of the auth protocol is an escape hatch that lets us
2177 negotiate extensions or changes to the D-Bus protocol in the future.
2180 <sect2 id="auth-command-negotiate-unix-fd">
2181 <title>NEGOTIATE_UNIX_FD Command</title>
2183 The NEGOTIATE_UNIX_FD command indicates that the client
2184 supports Unix file descriptor passing. This command may only
2185 be sent after the connection is authenticated, i.e. after OK
2186 was received by the client. This command may only be sent on
2187 transports that support Unix file descriptor passing.
2190 On receiving NEGOTIATE_UNIX_FD the server must respond with
2191 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2192 the transport chosen supports Unix file descriptor passing and
2193 the server supports this feature. It shall respond the latter
2194 if the transport does not support Unix file descriptor
2195 passing, the server does not support this feature, or the
2196 server decides not to enable file descriptor passing due to
2197 security or other reasons.
2200 <sect2 id="auth-command-agree-unix-fd">
2201 <title>AGREE_UNIX_FD Command</title>
2203 The AGREE_UNIX_FD command indicates that the server supports
2204 Unix file descriptor passing. This command may only be sent
2205 after the connection is authenticated, and the client sent
2206 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2207 command may only be sent on transports that support Unix file
2211 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2212 followed by its stream of messages, or by disconnecting. The
2213 server must not accept additional commands using this protocol
2214 after the BEGIN command has been received. Further
2215 communication will be a stream of D-Bus messages (optionally
2216 encrypted, as negotiated) rather than this protocol.
2219 <sect2 id="auth-command-future">
2220 <title>Future Extensions</title>
2222 Future extensions to the authentication and negotiation
2223 protocol are possible. For that new commands may be
2224 introduced. If a client or server receives an unknown command
2225 it shall respond with ERROR and not consider this fatal. New
2226 commands may be introduced both before, and after
2227 authentication, i.e. both before and after the OK command.
2230 <sect2 id="auth-examples">
2231 <title>Authentication examples</title>
2235 <title>Example of successful magic cookie authentication</title>
2237 (MAGIC_COOKIE is a made up mechanism)
2239 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2245 <title>Example of finding out mechanisms then picking one</title>
2248 S: REJECTED KERBEROS_V4 SKEY
2249 C: AUTH SKEY 7ab83f32ee
2250 S: DATA 8799cabb2ea93e
2251 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2257 <title>Example of client sends unknown command then falls back to regular auth</title>
2261 C: AUTH MAGIC_COOKIE 3736343435313230333039
2267 <title>Example of server doesn't support initial auth mechanism</title>
2269 C: AUTH MAGIC_COOKIE 3736343435313230333039
2270 S: REJECTED KERBEROS_V4 SKEY
2271 C: AUTH SKEY 7ab83f32ee
2272 S: DATA 8799cabb2ea93e
2273 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2279 <title>Example of wrong password or the like followed by successful retry</title>
2281 C: AUTH MAGIC_COOKIE 3736343435313230333039
2282 S: REJECTED KERBEROS_V4 SKEY
2283 C: AUTH SKEY 7ab83f32ee
2284 S: DATA 8799cabb2ea93e
2285 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2287 C: AUTH SKEY 7ab83f32ee
2288 S: DATA 8799cabb2ea93e
2289 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2295 <title>Example of skey cancelled and restarted</title>
2297 C: AUTH MAGIC_COOKIE 3736343435313230333039
2298 S: REJECTED KERBEROS_V4 SKEY
2299 C: AUTH SKEY 7ab83f32ee
2300 S: DATA 8799cabb2ea93e
2303 C: AUTH SKEY 7ab83f32ee
2304 S: DATA 8799cabb2ea93e
2305 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2311 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2313 (MAGIC_COOKIE is a made up mechanism)
2315 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2317 C: NEGOTIATE_UNIX_FD
2323 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2325 (MAGIC_COOKIE is a made up mechanism)
2327 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2329 C: NEGOTIATE_UNIX_FD
2336 <sect2 id="auth-states">
2337 <title>Authentication state diagrams</title>
2340 This section documents the auth protocol in terms of
2341 a state machine for the client and the server. This is
2342 probably the most robust way to implement the protocol.
2345 <sect3 id="auth-states-client">
2346 <title>Client states</title>
2349 To more precisely describe the interaction between the
2350 protocol state machine and the authentication mechanisms the
2351 following notation is used: MECH(CHALL) means that the
2352 server challenge CHALL was fed to the mechanism MECH, which
2358 CONTINUE(RESP) means continue the auth conversation
2359 and send RESP as the response to the server;
2365 OK(RESP) means that after sending RESP to the server
2366 the client side of the auth conversation is finished
2367 and the server should return "OK";
2373 ERROR means that CHALL was invalid and could not be
2379 Both RESP and CHALL may be empty.
2383 The Client starts by getting an initial response from the
2384 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2385 the mechanism did not provide an initial response. If the
2386 mechanism returns CONTINUE, the client starts in state
2387 <emphasis>WaitingForData</emphasis>, if the mechanism
2388 returns OK the client starts in state
2389 <emphasis>WaitingForOK</emphasis>.
2393 The client should keep track of available mechanisms and
2394 which it mechanisms it has already attempted. This list is
2395 used to decide which AUTH command to send. When the list is
2396 exhausted, the client should give up and close the
2401 <title><emphasis>WaitingForData</emphasis></title>
2409 MECH(CHALL) returns CONTINUE(RESP) → send
2411 <emphasis>WaitingForData</emphasis>
2415 MECH(CHALL) returns OK(RESP) → send DATA
2416 RESP, goto <emphasis>WaitingForOK</emphasis>
2420 MECH(CHALL) returns ERROR → send ERROR
2421 [msg], goto <emphasis>WaitingForData</emphasis>
2429 Receive REJECTED [mechs] →
2430 send AUTH [next mech], goto
2431 WaitingForData or <emphasis>WaitingForOK</emphasis>
2436 Receive ERROR → send
2438 <emphasis>WaitingForReject</emphasis>
2443 Receive OK → send
2444 BEGIN, terminate auth
2445 conversation, authenticated
2450 Receive anything else → send
2452 <emphasis>WaitingForData</emphasis>
2460 <title><emphasis>WaitingForOK</emphasis></title>
2465 Receive OK → send BEGIN, terminate auth
2466 conversation, <emphasis>authenticated</emphasis>
2471 Receive REJECTED [mechs] → send AUTH [next mech],
2472 goto <emphasis>WaitingForData</emphasis> or
2473 <emphasis>WaitingForOK</emphasis>
2479 Receive DATA → send CANCEL, goto
2480 <emphasis>WaitingForReject</emphasis>
2486 Receive ERROR → send CANCEL, goto
2487 <emphasis>WaitingForReject</emphasis>
2493 Receive anything else → send ERROR, goto
2494 <emphasis>WaitingForOK</emphasis>
2502 <title><emphasis>WaitingForReject</emphasis></title>
2507 Receive REJECTED [mechs] → send AUTH [next mech],
2508 goto <emphasis>WaitingForData</emphasis> or
2509 <emphasis>WaitingForOK</emphasis>
2515 Receive anything else → terminate auth
2516 conversation, disconnect
2525 <sect3 id="auth-states-server">
2526 <title>Server states</title>
2529 For the server MECH(RESP) means that the client response
2530 RESP was fed to the the mechanism MECH, which returns one of
2535 CONTINUE(CHALL) means continue the auth conversation and
2536 send CHALL as the challenge to the client;
2542 OK means that the client has been successfully
2549 REJECTED means that the client failed to authenticate or
2550 there was an error in RESP.
2555 The server starts out in state
2556 <emphasis>WaitingForAuth</emphasis>. If the client is
2557 rejected too many times the server must disconnect the
2562 <title><emphasis>WaitingForAuth</emphasis></title>
2568 Receive AUTH → send REJECTED [mechs], goto
2569 <emphasis>WaitingForAuth</emphasis>
2575 Receive AUTH MECH RESP
2579 MECH not valid mechanism → send REJECTED
2581 <emphasis>WaitingForAuth</emphasis>
2585 MECH(RESP) returns CONTINUE(CHALL) → send
2587 <emphasis>WaitingForData</emphasis>
2591 MECH(RESP) returns OK → send OK, goto
2592 <emphasis>WaitingForBegin</emphasis>
2596 MECH(RESP) returns REJECTED → send REJECTED
2598 <emphasis>WaitingForAuth</emphasis>
2606 Receive BEGIN → terminate
2607 auth conversation, disconnect
2613 Receive ERROR → send REJECTED [mechs], goto
2614 <emphasis>WaitingForAuth</emphasis>
2620 Receive anything else → send
2622 <emphasis>WaitingForAuth</emphasis>
2631 <title><emphasis>WaitingForData</emphasis></title>
2639 MECH(RESP) returns CONTINUE(CHALL) → send
2641 <emphasis>WaitingForData</emphasis>
2645 MECH(RESP) returns OK → send OK, goto
2646 <emphasis>WaitingForBegin</emphasis>
2650 MECH(RESP) returns REJECTED → send REJECTED
2652 <emphasis>WaitingForAuth</emphasis>
2660 Receive BEGIN → terminate auth conversation,
2667 Receive CANCEL → send REJECTED [mechs], goto
2668 <emphasis>WaitingForAuth</emphasis>
2674 Receive ERROR → send REJECTED [mechs], goto
2675 <emphasis>WaitingForAuth</emphasis>
2681 Receive anything else → send ERROR, goto
2682 <emphasis>WaitingForData</emphasis>
2690 <title><emphasis>WaitingForBegin</emphasis></title>
2695 Receive BEGIN → terminate auth conversation,
2696 client authenticated
2702 Receive CANCEL → send REJECTED [mechs], goto
2703 <emphasis>WaitingForAuth</emphasis>
2709 Receive ERROR → send REJECTED [mechs], goto
2710 <emphasis>WaitingForAuth</emphasis>
2716 Receive anything else → send ERROR, goto
2717 <emphasis>WaitingForBegin</emphasis>
2727 <sect2 id="auth-mechanisms">
2728 <title>Authentication mechanisms</title>
2730 This section describes some new authentication mechanisms.
2731 D-Bus also allows any standard SASL mechanism of course.
2733 <sect3 id="auth-mechanisms-sha">
2734 <title>DBUS_COOKIE_SHA1</title>
2736 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2737 has the ability to read a private file owned by the user being
2738 authenticated. If the client can prove that it has access to a secret
2739 cookie stored in this file, then the client is authenticated.
2740 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2744 Throughout this description, "hex encoding" must output the digits
2745 from a to f in lower-case; the digits A to F must not be used
2746 in the DBUS_COOKIE_SHA1 mechanism.
2749 Authentication proceeds as follows:
2753 The client sends the username it would like to authenticate
2759 The server sends the name of its "cookie context" (see below); a
2760 space character; the integer ID of the secret cookie the client
2761 must demonstrate knowledge of; a space character; then a
2762 randomly-generated challenge string, all of this hex-encoded into
2768 The client locates the cookie and generates its own
2769 randomly-generated challenge string. The client then concatenates
2770 the server's decoded challenge, a ":" character, its own challenge,
2771 another ":" character, and the cookie. It computes the SHA-1 hash
2772 of this composite string as a hex digest. It concatenates the
2773 client's challenge string, a space character, and the SHA-1 hex
2774 digest, hex-encodes the result and sends it back to the server.
2779 The server generates the same concatenated string used by the
2780 client and computes its SHA-1 hash. It compares the hash with
2781 the hash received from the client; if the two hashes match, the
2782 client is authenticated.
2788 Each server has a "cookie context," which is a name that identifies a
2789 set of cookies that apply to that server. A sample context might be
2790 "org_freedesktop_session_bus". Context names must be valid ASCII,
2791 nonzero length, and may not contain the characters slash ("/"),
2792 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2793 tab ("\t"), or period ("."). There is a default context,
2794 "org_freedesktop_general" that's used by servers that do not specify
2798 Cookies are stored in a user's home directory, in the directory
2799 <filename>~/.dbus-keyrings/</filename>. This directory must
2800 not be readable or writable by other users. If it is,
2801 clients and servers must ignore it. The directory
2802 contains cookie files named after the cookie context.
2805 A cookie file contains one cookie per line. Each line
2806 has three space-separated fields:
2810 The cookie ID number, which must be a non-negative integer and
2811 may not be used twice in the same file.
2816 The cookie's creation time, in UNIX seconds-since-the-epoch
2822 The cookie itself, a hex-encoded random block of bytes. The cookie
2823 may be of any length, though obviously security increases
2824 as the length increases.
2830 Only server processes modify the cookie file.
2831 They must do so with this procedure:
2835 Create a lockfile name by appending ".lock" to the name of the
2836 cookie file. The server should attempt to create this file
2837 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2838 fails, the lock fails. Servers should retry for a reasonable
2839 period of time, then they may choose to delete an existing lock
2840 to keep users from having to manually delete a stale
2841 lock. <footnote><para>Lockfiles are used instead of real file
2842 locking <literal>fcntl()</literal> because real locking
2843 implementations are still flaky on network
2844 filesystems.</para></footnote>
2849 Once the lockfile has been created, the server loads the cookie
2850 file. It should then delete any cookies that are old (the
2851 timeout can be fairly short), or more than a reasonable
2852 time in the future (so that cookies never accidentally
2853 become permanent, if the clock was set far into the future
2854 at some point). If no recent keys remain, the
2855 server may generate a new key.
2860 The pruned and possibly added-to cookie file
2861 must be resaved atomically (using a temporary
2862 file which is rename()'d).
2867 The lock must be dropped by deleting the lockfile.
2873 Clients need not lock the file in order to load it,
2874 because servers are required to save the file atomically.
2879 <sect1 id="addresses">
2880 <title>Server Addresses</title>
2882 Server addresses consist of a transport name followed by a colon, and
2883 then an optional, comma-separated list of keys and values in the form key=value.
2884 Each value is escaped.
2888 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2889 Which is the address to a unix socket with the path /tmp/dbus-test.
2892 Value escaping is similar to URI escaping but simpler.
2896 The set of optionally-escaped bytes is:
2897 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2898 <emphasis>byte</emphasis> (note, not character) which is not in the
2899 set of optionally-escaped bytes must be replaced with an ASCII
2900 percent (<literal>%</literal>) and the value of the byte in hex.
2901 The hex value must always be two digits, even if the first digit is
2902 zero. The optionally-escaped bytes may be escaped if desired.
2907 To unescape, append each byte in the value; if a byte is an ASCII
2908 percent (<literal>%</literal>) character then append the following
2909 hex value instead. It is an error if a <literal>%</literal> byte
2910 does not have two hex digits following. It is an error if a
2911 non-optionally-escaped byte is seen unescaped.
2915 The set of optionally-escaped bytes is intended to preserve address
2916 readability and convenience.
2920 A server may specify a key-value pair with the key <literal>guid</literal>
2921 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2922 describes the format of the <literal>guid</literal> field. If present,
2923 this UUID may be used to distinguish one server address from another. A
2924 server should use a different UUID for each address it listens on. For
2925 example, if a message bus daemon offers both UNIX domain socket and TCP
2926 connections, but treats clients the same regardless of how they connect,
2927 those two connections are equivalent post-connection but should have
2928 distinct UUIDs to distinguish the kinds of connection.
2932 The intent of the address UUID feature is to allow a client to avoid
2933 opening multiple identical connections to the same server, by allowing the
2934 client to check whether an address corresponds to an already-existing
2935 connection. Comparing two addresses is insufficient, because addresses
2936 can be recycled by distinct servers, and equivalent addresses may look
2937 different if simply compared as strings (for example, the host in a TCP
2938 address can be given as an IP address or as a hostname).
2942 Note that the address key is <literal>guid</literal> even though the
2943 rest of the API and documentation says "UUID," for historical reasons.
2947 [FIXME clarify if attempting to connect to each is a requirement
2948 or just a suggestion]
2949 When connecting to a server, multiple server addresses can be
2950 separated by a semi-colon. The library will then try to connect
2951 to the first address and if that fails, it'll try to connect to
2952 the next one specified, and so forth. For example
2953 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2957 Some addresses are <firstterm>connectable</firstterm>. A connectable
2958 address is one containing enough information for a client to connect
2959 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
2960 is a connectable address. It is not necessarily possible to listen
2961 on every connectable address: for instance, it is not possible to
2962 listen on a <literal>unixexec:</literal> address.
2966 Some addresses are <firstterm>listenable</firstterm>. A listenable
2967 address is one containing enough information for a server to listen on
2968 it, producing a connectable address (which may differ from the
2969 original address). Many listenable addresses are not connectable:
2970 for instance, <literal>tcp:host=127.0.0.1</literal>
2971 is listenable, but not connectable (because it does not specify
2976 Listening on an address that is not connectable will result in a
2977 connectable address that is not the same as the listenable address.
2978 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
2979 might result in the connectable address
2980 <literal>tcp:host=127.0.0.1,port=30958</literal>,
2981 or listening on <literal>unix:tmpdir=/tmp</literal>
2982 might result in the connectable address
2983 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>.
2987 <sect1 id="transports">
2988 <title>Transports</title>
2990 [FIXME we need to specify in detail each transport and its possible arguments]
2992 Current transports include: unix domain sockets (including
2993 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
2994 using in-process pipes. Future possible transports include one that
2995 tunnels over X11 protocol.
2998 <sect2 id="transports-unix-domain-sockets">
2999 <title>Unix Domain Sockets</title>
3001 Unix domain sockets can be either paths in the file system or on Linux
3002 kernels, they can be abstract which are similar to paths but
3003 do not show up in the file system.
3007 When a socket is opened by the D-Bus library it truncates the path
3008 name right before the first trailing Nul byte. This is true for both
3009 normal paths and abstract paths. Note that this is a departure from
3010 previous versions of D-Bus that would create sockets with a fixed
3011 length path name. Names which were shorter than the fixed length
3012 would be padded by Nul bytes.
3015 Unix domain sockets are not available on Windows.
3018 Unix addresses that specify <literal>path</literal> or
3019 <literal>abstract</literal> are both listenable and connectable.
3020 Unix addresses that specify <literal>tmpdir</literal> are only
3021 listenable: the corresponding connectable address will specify
3022 either <literal>path</literal> or <literal>abstract</literal>.
3024 <sect3 id="transports-unix-domain-sockets-addresses">
3025 <title>Server Address Format</title>
3027 Unix domain socket addresses are identified by the "unix:" prefix
3028 and support the following key/value pairs:
3035 <entry>Values</entry>
3036 <entry>Description</entry>
3042 <entry>(path)</entry>
3043 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3046 <entry>tmpdir</entry>
3047 <entry>(path)</entry>
3048 <entry>temporary directory in which a socket file with a random file name starting with 'dbus-' will be created by the server. This key can only be used in server addresses, not in client addresses. If set, the "path" and "abstract" key must not be set.</entry>
3051 <entry>abstract</entry>
3052 <entry>(string)</entry>
3053 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tmpdir" key must not be set. This key is only supported on platforms with "abstract Unix sockets", of which Linux is the only known example.</entry>
3059 Exactly one of the keys <literal>path</literal>,
3060 <literal>abstract</literal> or
3061 <literal>tmpdir</literal> must be provided.
3065 <sect2 id="transports-launchd">
3066 <title>launchd</title>
3068 launchd is an open-source server management system that replaces init, inetd
3069 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3070 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3074 launchd allocates a socket and provides it with the unix path through the
3075 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3076 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3077 it through its environment.
3078 Other processes can query for the launchd socket by executing:
3079 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3080 This is normally done by the D-Bus client library so doesn't have to be done
3084 launchd is not available on Microsoft Windows.
3087 launchd addresses are listenable and connectable.
3089 <sect3 id="transports-launchd-addresses">
3090 <title>Server Address Format</title>
3092 launchd addresses are identified by the "launchd:" prefix
3093 and support the following key/value pairs:
3100 <entry>Values</entry>
3101 <entry>Description</entry>
3107 <entry>(environment variable)</entry>
3108 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3114 The <literal>env</literal> key is required.
3118 <sect2 id="transports-systemd">
3119 <title>systemd</title>
3121 systemd is an open-source server management system that
3122 replaces init and inetd on newer Linux systems. It supports
3123 socket activation. The D-Bus systemd transport is used to acquire
3124 socket activation file descriptors from systemd and use them
3125 as D-Bus transport when the current process is spawned by
3126 socket activation from it.
3129 The systemd transport accepts only one or more Unix domain or
3130 TCP streams sockets passed in via socket activation.
3133 The systemd transport is not available on non-Linux operating systems.
3136 The systemd transport defines no parameter keys.
3139 systemd addresses are listenable, but not connectable. The
3140 corresponding connectable address is the <literal>unix</literal>
3141 or <literal>tcp</literal> address of the socket.
3144 <sect2 id="transports-tcp-sockets">
3145 <title>TCP Sockets</title>
3147 The tcp transport provides TCP/IP based connections between clients
3148 located on the same or different hosts.
3151 Using tcp transport without any additional secure authentification mechanismus
3152 over a network is unsecure.
3155 On Windows and most Unix platforms, the TCP stack is unable to transfer
3156 credentials over a TCP connection, so the EXTERNAL authentication
3157 mechanism does not work for this transport.
3160 All <literal>tcp</literal> addresses are listenable.
3161 <literal>tcp</literal> addresses in which both
3162 <literal>host</literal> and <literal>port</literal> are
3163 specified, and <literal>port</literal> is non-zero,
3164 are also connectable.
3166 <sect3 id="transports-tcp-sockets-addresses">
3167 <title>Server Address Format</title>
3169 TCP/IP socket addresses are identified by the "tcp:" prefix
3170 and support the following key/value pairs:
3177 <entry>Values</entry>
3178 <entry>Description</entry>
3184 <entry>(string)</entry>
3185 <entry>DNS name or IP address</entry>
3189 <entry>(string)</entry>
3190 <entry>Used in a listenable address to configure the interface
3191 on which the server will listen: either the IP address of one of
3192 the local machine's interfaces (most commonly <literal>127.0.0.1
3193 </literal>), or a DNS name that resolves to one of those IP
3194 addresses, or '*' to listen on all interfaces simultaneously.
3195 If not specified, the default is the same value as "host".
3200 <entry>(number)</entry>
3201 <entry>The tcp port the server will open. A zero value let the server
3202 choose a free port provided from the underlaying operating system.
3203 libdbus is able to retrieve the real used port from the server.
3207 <entry>family</entry>
3208 <entry>(string)</entry>
3209 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3216 <sect2 id="transports-nonce-tcp-sockets">
3217 <title>Nonce-secured TCP Sockets</title>
3219 The nonce-tcp transport provides a secured TCP transport, using a
3220 simple authentication mechanism to ensure that only clients with read
3221 access to a certain location in the filesystem can connect to the server.
3222 The server writes a secret, the nonce, to a file and an incoming client
3223 connection is only accepted if the client sends the nonce right after
3224 the connect. The nonce mechanism requires no setup and is orthogonal to
3225 the higher-level authentication mechanisms described in the
3226 Authentication section.
3230 On start, the server generates a random 16 byte nonce and writes it
3231 to a file in the user's temporary directory. The nonce file location
3232 is published as part of the server's D-Bus address using the
3233 "noncefile" key-value pair.
3235 After an accept, the server reads 16 bytes from the socket. If the
3236 read bytes do not match the nonce stored in the nonce file, the
3237 server MUST immediately drop the connection.
3238 If the nonce match the received byte sequence, the client is accepted
3239 and the transport behaves like an unsecured tcp transport.
3242 After a successful connect to the server socket, the client MUST read
3243 the nonce from the file published by the server via the noncefile=
3244 key-value pair and send it over the socket. After that, the
3245 transport behaves like an unsecured tcp transport.
3248 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3249 <literal>host</literal>, <literal>port</literal> and
3250 <literal>noncefile</literal> are all specified,
3251 and <literal>port</literal> is nonzero, are also connectable.
3253 <sect3 id="transports-nonce-tcp-sockets-addresses">
3254 <title>Server Address Format</title>
3256 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3257 and support the following key/value pairs:
3264 <entry>Values</entry>
3265 <entry>Description</entry>
3271 <entry>(string)</entry>
3272 <entry>DNS name or IP address</entry>
3276 <entry>(string)</entry>
3277 <entry>The same as for tcp: addresses
3282 <entry>(number)</entry>
3283 <entry>The tcp port the server will open. A zero value let the server
3284 choose a free port provided from the underlaying operating system.
3285 libdbus is able to retrieve the real used port from the server.
3289 <entry>family</entry>
3290 <entry>(string)</entry>
3291 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3294 <entry>noncefile</entry>
3295 <entry>(path)</entry>
3296 <entry>File location containing the secret.
3297 This is only meaningful in connectable addresses:
3298 a listening D-Bus server that offers this transport
3299 will always create a new nonce file.</entry>
3306 <sect2 id="transports-exec">
3307 <title>Executed Subprocesses on Unix</title>
3309 This transport forks off a process and connects its standard
3310 input and standard output with an anonymous Unix domain
3311 socket. This socket is then used for communication by the
3312 transport. This transport may be used to use out-of-process
3313 forwarder programs as basis for the D-Bus protocol.
3316 The forked process will inherit the standard error output and
3317 process group from the parent process.
3320 Executed subprocesses are not available on Windows.
3323 <literal>unixexec</literal> addresses are connectable, but are not
3326 <sect3 id="transports-exec-addresses">
3327 <title>Server Address Format</title>
3329 Executed subprocess addresses are identified by the "unixexec:" prefix
3330 and support the following key/value pairs:
3337 <entry>Values</entry>
3338 <entry>Description</entry>
3344 <entry>(path)</entry>
3345 <entry>Path of the binary to execute, either an absolute
3346 path or a binary name that is searched for in the default
3347 search path of the OS. This corresponds to the first
3348 argument of execlp(). This key is mandatory.</entry>
3351 <entry>argv0</entry>
3352 <entry>(string)</entry>
3353 <entry>The program name to use when executing the
3354 binary. If omitted the same value as specified for path=
3355 will be used. This corresponds to the second argument of
3359 <entry>argv1, argv2, ...</entry>
3360 <entry>(string)</entry>
3361 <entry>Arguments to pass to the binary. This corresponds
3362 to the third and later arguments of execlp(). If a
3363 specific argvX is not specified no further argvY for Y > X
3364 are taken into account.</entry>
3372 <sect1 id="meta-transports">
3373 <title>Meta Transports</title>
3375 Meta transports are a kind of transport with special enhancements or
3376 behavior. Currently available meta transports include: autolaunch
3379 <sect2 id="meta-transports-autolaunch">
3380 <title>Autolaunch</title>
3381 <para>The autolaunch transport provides a way for dbus clients to autodetect
3382 a running dbus session bus and to autolaunch a session bus if not present.
3385 On Unix, <literal>autolaunch</literal> addresses are connectable,
3389 On Windows, <literal>autolaunch</literal> addresses are both
3390 connectable and listenable.
3393 <sect3 id="meta-transports-autolaunch-addresses">
3394 <title>Server Address Format</title>
3396 Autolaunch addresses uses the "autolaunch:" prefix and support the
3397 following key/value pairs:
3404 <entry>Values</entry>
3405 <entry>Description</entry>
3410 <entry>scope</entry>
3411 <entry>(string)</entry>
3412 <entry>scope of autolaunch (Windows only)
3416 "*install-path" - limit session bus to dbus installation path.
3417 The dbus installation path is determined from the location of
3418 the shared dbus library. If the library is located in a 'bin'
3419 subdirectory the installation root is the directory above,
3420 otherwise the directory where the library lives is taken as
3423 <install-root>/bin/[lib]dbus-1.dll
3424 <install-root>/[lib]dbus-1.dll
3430 "*user" - limit session bus to the recent user.
3435 other values - specify dedicated session bus like "release",
3447 <sect3 id="meta-transports-autolaunch-windows-implementation">
3448 <title>Windows implementation</title>
3450 On start, the server opens a platform specific transport, creates a mutex
3451 and a shared memory section containing the related session bus address.
3452 This mutex will be inspected by the dbus client library to detect a
3453 running dbus session bus. The access to the mutex and the shared memory
3454 section are protected by global locks.
3457 In the recent implementation the autolaunch transport uses a tcp transport
3458 on localhost with a port choosen from the operating system. This detail may
3459 change in the future.
3462 Disclaimer: The recent implementation is in an early state and may not
3463 work in all cirumstances and/or may have security issues. Because of this
3464 the implementation is not documentated yet.
3471 <title>UUIDs</title>
3473 A working D-Bus implementation uses universally-unique IDs in two places.
3474 First, each server address has a UUID identifying the address,
3475 as described in <xref linkend="addresses"/>. Second, each operating
3476 system kernel instance running a D-Bus client or server has a UUID
3477 identifying that kernel, retrieved by invoking the method
3478 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3479 linkend="standard-interfaces-peer"/>).
3482 The term "UUID" in this document is intended literally, i.e. an
3483 identifier that is universally unique. It is not intended to refer to
3484 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3487 The UUID must contain 128 bits of data and be hex-encoded. The
3488 hex-encoded string may not contain hyphens or other non-hex-digit
3489 characters, and it must be exactly 32 characters long. To generate a
3490 UUID, the current reference implementation concatenates 96 bits of random
3491 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3495 It would also be acceptable and probably better to simply generate 128
3496 bits of random data, as long as the random number generator is of high
3497 quality. The timestamp could conceivably help if the random bits are not
3498 very random. With a quality random number generator, collisions are
3499 extremely unlikely even with only 96 bits, so it's somewhat academic.
3502 Implementations should, however, stick to random data for the first 96 bits
3507 <sect1 id="standard-interfaces">
3508 <title>Standard Interfaces</title>
3510 See <xref linkend="message-protocol-types-notation"/> for details on
3511 the notation used in this section. There are some standard interfaces
3512 that may be useful across various D-Bus applications.
3514 <sect2 id="standard-interfaces-peer">
3515 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3517 The <literal>org.freedesktop.DBus.Peer</literal> interface
3520 org.freedesktop.DBus.Peer.Ping ()
3521 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3525 On receipt of the <literal>METHOD_CALL</literal> message
3526 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3527 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3528 usual. It does not matter which object path a ping is sent to. The
3529 reference implementation handles this method automatically.
3532 On receipt of the <literal>METHOD_CALL</literal> message
3533 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3534 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3535 UUID representing the identity of the machine the process is running on.
3536 This UUID must be the same for all processes on a single system at least
3537 until that system next reboots. It should be the same across reboots
3538 if possible, but this is not always possible to implement and is not
3540 It does not matter which object path a GetMachineId is sent to. The
3541 reference implementation handles this method automatically.
3544 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3545 a virtual machine running on a hypervisor, rather than a physical machine.
3546 Basically if two processes see the same UUID, they should also see the same
3547 shared memory, UNIX domain sockets, process IDs, and other features that require
3548 a running OS kernel in common between the processes.
3551 The UUID is often used where other programs might use a hostname. Hostnames
3552 can change without rebooting, however, or just be "localhost" - so the UUID
3556 <xref linkend="uuids"/> explains the format of the UUID.
3560 <sect2 id="standard-interfaces-introspectable">
3561 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3563 This interface has one method:
3565 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3569 Objects instances may implement
3570 <literal>Introspect</literal> which returns an XML description of
3571 the object, including its interfaces (with signals and methods), objects
3572 below it in the object path tree, and its properties.
3575 <xref linkend="introspection-format"/> describes the format of this XML string.
3578 <sect2 id="standard-interfaces-properties">
3579 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3581 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3582 or <firstterm>attributes</firstterm>. These can be exposed via the
3583 <literal>org.freedesktop.DBus.Properties</literal> interface.
3587 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3588 in STRING property_name,
3590 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3591 in STRING property_name,
3593 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3594 out DICT<STRING,VARIANT> props);
3598 It is conventional to give D-Bus properties names consisting of
3599 capitalized words without punctuation ("CamelCase"), like
3600 <link linkend="message-protocol-names-member">member names</link>.
3601 For instance, the GObject property
3602 <literal>connection-status</literal> or the Qt property
3603 <literal>connectionStatus</literal> could be represented on D-Bus
3604 as <literal>ConnectionStatus</literal>.
3607 Strictly speaking, D-Bus property names are not required to follow
3608 the same naming restrictions as member names, but D-Bus property
3609 names that would not be valid member names (in particular,
3610 GObject-style dash-separated property names) can cause interoperability
3611 problems and should be avoided.
3614 The available properties and whether they are writable can be determined
3615 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3616 see <xref linkend="standard-interfaces-introspectable"/>.
3619 An empty string may be provided for the interface name; in this case,
3620 if there are multiple properties on an object with the same name,
3621 the results are undefined (picking one by according to an arbitrary
3622 deterministic rule, or returning an error, are the reasonable
3626 If one or more properties change on an object, the
3627 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3628 signal may be emitted (this signal was added in 0.14):
3632 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3633 DICT<STRING,VARIANT> changed_properties,
3634 ARRAY<STRING> invalidated_properties);
3638 where <literal>changed_properties</literal> is a dictionary
3639 containing the changed properties with the new values and
3640 <literal>invalidated_properties</literal> is an array of
3641 properties that changed but the value is not conveyed.
3644 Whether the <literal>PropertiesChanged</literal> signal is
3645 supported can be determined by calling
3646 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3647 that the signal may be supported for an object but it may
3648 differ how whether and how it is used on a per-property basis
3649 (for e.g. performance or security reasons). Each property (or
3650 the parent interface) must be annotated with the
3651 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3652 annotation to convey this (usually the default value
3653 <literal>true</literal> is sufficient meaning that the
3654 annotation does not need to be used). See <xref
3655 linkend="introspection-format"/> for details on this
3660 <sect2 id="standard-interfaces-objectmanager">
3661 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3663 An API can optionally make use of this interface for one or
3664 more sub-trees of objects. The root of each sub-tree implements
3665 this interface so other applications can get all objects,
3666 interfaces and properties in a single method call. It is
3667 appropriate to use this interface if users of the tree of
3668 objects are expected to be interested in all interfaces of all
3669 objects in the tree; a more granular API should be used if
3670 users of the objects are expected to be interested in a small
3671 subset of the objects, a small subset of their interfaces, or
3675 The method that applications can use to get all objects and
3676 properties is <literal>GetManagedObjects</literal>:
3680 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3684 The return value of this method is a dict whose keys are
3685 object paths. All returned object paths are children of the
3686 object path implementing this interface, i.e. their object
3687 paths start with the ObjectManager's object path plus '/'.
3690 Each value is a dict whose keys are interfaces names. Each
3691 value in this inner dict is the same dict that would be
3692 returned by the <link
3693 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3694 method for that combination of object path and interface. If
3695 an interface has no properties, the empty dict is returned.
3698 Changes are emitted using the following two signals:
3702 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3703 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3704 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3705 ARRAY<STRING> interfaces);
3709 The <literal>InterfacesAdded</literal> signal is emitted when
3710 either a new object is added or when an existing object gains
3711 one or more interfaces. The
3712 <literal>InterfacesRemoved</literal> signal is emitted
3713 whenever an object is removed or it loses one or more
3714 interfaces. The second parameter of the
3715 <literal>InterfacesAdded</literal> signal contains a dict with
3716 the interfaces and properties (if any) that have been added to
3717 the given object path. Similarly, the second parameter of the
3718 <literal>InterfacesRemoved</literal> signal contains an array
3719 of the interfaces that were removed. Note that changes on
3720 properties on existing interfaces are not reported using this
3721 interface - an application should also monitor the existing <link
3722 linkend="standard-interfaces-properties">PropertiesChanged</link>
3723 signal on each object.
3726 Applications SHOULD NOT export objects that are children of an
3727 object (directly or otherwise) implementing this interface but
3728 which are not returned in the reply from the
3729 <literal>GetManagedObjects()</literal> method of this
3730 interface on the given object.
3733 The intent of the <literal>ObjectManager</literal> interface
3734 is to make it easy to write a robust client
3735 implementation. The trivial client implementation only needs
3736 to make two method calls:
3740 org.freedesktop.DBus.AddMatch (bus_proxy,
3741 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3742 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3746 on the message bus and the remote application's
3747 <literal>ObjectManager</literal>, respectively. Whenever a new
3748 remote object is created (or an existing object gains a new
3749 interface), the <literal>InterfacesAdded</literal> signal is
3750 emitted, and since this signal contains all properties for the
3751 interfaces, no calls to the
3752 <literal>org.freedesktop.Properties</literal> interface on the
3753 remote object are needed. Additionally, since the initial
3754 <literal>AddMatch()</literal> rule already includes signal
3755 messages from the newly created child object, no new
3756 <literal>AddMatch()</literal> call is needed.
3761 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3762 interface was added in version 0.17 of the D-Bus
3769 <sect1 id="introspection-format">
3770 <title>Introspection Data Format</title>
3772 As described in <xref linkend="standard-interfaces-introspectable"/>,
3773 objects may be introspected at runtime, returning an XML string
3774 that describes the object. The same XML format may be used in
3775 other contexts as well, for example as an "IDL" for generating
3776 static language bindings.
3779 Here is an example of introspection data:
3781 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3782 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3783 <node name="/com/example/sample_object">
3784 <interface name="com.example.SampleInterface">
3785 <method name="Frobate">
3786 <arg name="foo" type="i" direction="in"/>
3787 <arg name="bar" type="s" direction="out"/>
3788 <arg name="baz" type="a{us}" direction="out"/>
3789 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3791 <method name="Bazify">
3792 <arg name="bar" type="(iiu)" direction="in"/>
3793 <arg name="bar" type="v" direction="out"/>
3795 <method name="Mogrify">
3796 <arg name="bar" type="(iiav)" direction="in"/>
3798 <signal name="Changed">
3799 <arg name="new_value" type="b"/>
3801 <property name="Bar" type="y" access="readwrite"/>
3803 <node name="child_of_sample_object"/>
3804 <node name="another_child_of_sample_object"/>
3809 A more formal DTD and spec needs writing, but here are some quick notes.
3813 Only the root <node> element can omit the node name, as it's
3814 known to be the object that was introspected. If the root
3815 <node> does have a name attribute, it must be an absolute
3816 object path. If child <node> have object paths, they must be
3822 If a child <node> has any sub-elements, then they
3823 must represent a complete introspection of the child.
3824 If a child <node> is empty, then it may or may
3825 not have sub-elements; the child must be introspected
3826 in order to find out. The intent is that if an object
3827 knows that its children are "fast" to introspect
3828 it can go ahead and return their information, but
3829 otherwise it can omit it.
3834 The direction element on <arg> may be omitted,
3835 in which case it defaults to "in" for method calls
3836 and "out" for signals. Signals only allow "out"
3837 so while direction may be specified, it's pointless.
3842 The possible directions are "in" and "out",
3843 unlike CORBA there is no "inout"
3848 The possible property access flags are
3849 "readwrite", "read", and "write"
3854 Multiple interfaces can of course be listed for
3860 The "name" attribute on arguments is optional.
3866 Method, interface, property, and signal elements may have
3867 "annotations", which are generic key/value pairs of metadata.
3868 They are similar conceptually to Java's annotations and C# attributes.
3869 Well-known annotations:
3876 <entry>Values (separated by ,)</entry>
3877 <entry>Description</entry>
3882 <entry>org.freedesktop.DBus.Deprecated</entry>
3883 <entry>true,false</entry>
3884 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3887 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3888 <entry>(string)</entry>
3889 <entry>The C symbol; may be used for methods and interfaces</entry>
3892 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3893 <entry>true,false</entry>
3894 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3897 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3898 <entry>true,invalidates,false</entry>
3901 If set to <literal>false</literal>, the
3902 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3904 linkend="standard-interfaces-properties"/> is not
3905 guaranteed to be emitted if the property changes.
3908 If set to <literal>invalidates</literal> the signal
3909 is emitted but the value is not included in the
3913 If set to <literal>true</literal> the signal is
3914 emitted with the value included.
3917 The value for the annotation defaults to
3918 <literal>true</literal> if the enclosing interface
3919 element does not specify the annotation. Otherwise it
3920 defaults to the value specified in the enclosing
3929 <sect1 id="message-bus">
3930 <title>Message Bus Specification</title>
3931 <sect2 id="message-bus-overview">
3932 <title>Message Bus Overview</title>
3934 The message bus accepts connections from one or more applications.
3935 Once connected, applications can exchange messages with other
3936 applications that are also connected to the bus.
3939 In order to route messages among connections, the message bus keeps a
3940 mapping from names to connections. Each connection has one
3941 unique-for-the-lifetime-of-the-bus name automatically assigned.
3942 Applications may request additional names for a connection. Additional
3943 names are usually "well-known names" such as
3944 "com.example.TextEditor". When a name is bound to a connection,
3945 that connection is said to <firstterm>own</firstterm> the name.
3948 The bus itself owns a special name,
3949 <literal>org.freedesktop.DBus</literal>, with an object
3950 located at <literal>/org/freedesktop/DBus</literal> that
3951 implements the <literal>org.freedesktop.DBus</literal>
3952 interface. This service allows applications to make
3953 administrative requests of the bus itself. For example,
3954 applications can ask the bus to assign a name to a connection.
3957 Each name may have <firstterm>queued owners</firstterm>. When an
3958 application requests a name for a connection and the name is already in
3959 use, the bus will optionally add the connection to a queue waiting for
3960 the name. If the current owner of the name disconnects or releases
3961 the name, the next connection in the queue will become the new owner.
3965 This feature causes the right thing to happen if you start two text
3966 editors for example; the first one may request "com.example.TextEditor",
3967 and the second will be queued as a possible owner of that name. When
3968 the first exits, the second will take over.
3972 Applications may send <firstterm>unicast messages</firstterm> to
3973 a specific recipient or to the message bus itself, or
3974 <firstterm>broadcast messages</firstterm> to all interested recipients.
3975 See <xref linkend="message-bus-routing"/> for details.
3979 <sect2 id="message-bus-names">
3980 <title>Message Bus Names</title>
3982 Each connection has at least one name, assigned at connection time and
3983 returned in response to the
3984 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3985 automatically-assigned name is called the connection's <firstterm>unique
3986 name</firstterm>. Unique names are never reused for two different
3987 connections to the same bus.
3990 Ownership of a unique name is a prerequisite for interaction with
3991 the message bus. It logically follows that the unique name is always
3992 the first name that an application comes to own, and the last
3993 one that it loses ownership of.
3996 Unique connection names must begin with the character ':' (ASCII colon
3997 character); bus names that are not unique names must not begin
3998 with this character. (The bus must reject any attempt by an application
3999 to manually request a name beginning with ':'.) This restriction
4000 categorically prevents "spoofing"; messages sent to a unique name
4001 will always go to the expected connection.
4004 When a connection is closed, all the names that it owns are deleted (or
4005 transferred to the next connection in the queue if any).
4008 A connection can request additional names to be associated with it using
4009 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4010 linkend="message-protocol-names-bus"/> describes the format of a valid
4011 name. These names can be released again using the
4012 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4015 <sect3 id="bus-messages-request-name">
4016 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4020 UINT32 RequestName (in STRING name, in UINT32 flags)
4027 <entry>Argument</entry>
4029 <entry>Description</entry>
4035 <entry>STRING</entry>
4036 <entry>Name to request</entry>
4040 <entry>UINT32</entry>
4041 <entry>Flags</entry>
4051 <entry>Argument</entry>
4053 <entry>Description</entry>
4059 <entry>UINT32</entry>
4060 <entry>Return value</entry>
4067 This method call should be sent to
4068 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4069 assign the given name to the method caller. Each name maintains a
4070 queue of possible owners, where the head of the queue is the primary
4071 or current owner of the name. Each potential owner in the queue
4072 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4073 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4074 call. When RequestName is invoked the following occurs:
4078 If the method caller is currently the primary owner of the name,
4079 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4080 values are updated with the values from the new RequestName call,
4081 and nothing further happens.
4087 If the current primary owner (head of the queue) has
4088 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4089 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4090 the caller of RequestName replaces the current primary owner at
4091 the head of the queue and the current primary owner moves to the
4092 second position in the queue. If the caller of RequestName was
4093 in the queue previously its flags are updated with the values from
4094 the new RequestName in addition to moving it to the head of the queue.
4100 If replacement is not possible, and the method caller is
4101 currently in the queue but not the primary owner, its flags are
4102 updated with the values from the new RequestName call.
4108 If replacement is not possible, and the method caller is
4109 currently not in the queue, the method caller is appended to the
4116 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4117 set and is not the primary owner, it is removed from the
4118 queue. This can apply to the previous primary owner (if it
4119 was replaced) or the method caller (if it updated the
4120 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4121 queue, or if it was just added to the queue with that flag set).
4127 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4128 queue," even if another application already in the queue had specified
4129 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4130 that does not allow replacement goes away, and the next primary owner
4131 does allow replacement. In this case, queued items that specified
4132 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4133 automatically replace the new primary owner. In other words,
4134 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4135 time RequestName is called. This is deliberate to avoid an infinite loop
4136 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4137 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4140 The flags argument contains any of the following values logically ORed
4147 <entry>Conventional Name</entry>
4148 <entry>Value</entry>
4149 <entry>Description</entry>
4154 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4158 If an application A specifies this flag and succeeds in
4159 becoming the owner of the name, and another application B
4160 later calls RequestName with the
4161 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4162 will lose ownership and receive a
4163 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4164 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4165 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4166 is not specified by application B, then application B will not replace
4167 application A as the owner.
4172 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4176 Try to replace the current owner if there is one. If this
4177 flag is not set the application will only become the owner of
4178 the name if there is no current owner. If this flag is set,
4179 the application will replace the current owner if
4180 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4185 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4189 Without this flag, if an application requests a name that is
4190 already owned, the application will be placed in a queue to
4191 own the name when the current owner gives it up. If this
4192 flag is given, the application will not be placed in the
4193 queue, the request for the name will simply fail. This flag
4194 also affects behavior when an application is replaced as
4195 name owner; by default the application moves back into the
4196 waiting queue, unless this flag was provided when the application
4197 became the name owner.
4205 The return code can be one of the following values:
4211 <entry>Conventional Name</entry>
4212 <entry>Value</entry>
4213 <entry>Description</entry>
4218 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4219 <entry>1</entry> <entry>The caller is now the primary owner of
4220 the name, replacing any previous owner. Either the name had no
4221 owner before, or the caller specified
4222 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4223 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4226 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4229 <entry>The name already had an owner,
4230 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4231 the current owner did not specify
4232 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4233 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4237 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4238 <entry>The name already has an owner,
4239 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4240 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4241 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4242 specified by the requesting application.</entry>
4245 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4247 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4255 <sect3 id="bus-messages-release-name">
4256 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4260 UINT32 ReleaseName (in STRING name)
4267 <entry>Argument</entry>
4269 <entry>Description</entry>
4275 <entry>STRING</entry>
4276 <entry>Name to release</entry>
4286 <entry>Argument</entry>
4288 <entry>Description</entry>
4294 <entry>UINT32</entry>
4295 <entry>Return value</entry>
4302 This method call should be sent to
4303 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4304 release the method caller's claim to the given name. If the caller is
4305 the primary owner, a new primary owner will be selected from the
4306 queue if any other owners are waiting. If the caller is waiting in
4307 the queue for the name, the caller will removed from the queue and
4308 will not be made an owner of the name if it later becomes available.
4309 If there are no other owners in the queue for the name, it will be
4310 removed from the bus entirely.
4312 The return code can be one of the following values:
4318 <entry>Conventional Name</entry>
4319 <entry>Value</entry>
4320 <entry>Description</entry>
4325 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4326 <entry>1</entry> <entry>The caller has released his claim on
4327 the given name. Either the caller was the primary owner of
4328 the name, and the name is now unused or taken by somebody
4329 waiting in the queue for the name, or the caller was waiting
4330 in the queue for the name and has now been removed from the
4334 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4336 <entry>The given name does not exist on this bus.</entry>
4339 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4341 <entry>The caller was not the primary owner of this name,
4342 and was also not waiting in the queue to own this name.</entry>
4350 <sect3 id="bus-messages-list-queued-owners">
4351 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4355 ARRAY of STRING ListQueuedOwners (in STRING name)
4362 <entry>Argument</entry>
4364 <entry>Description</entry>
4370 <entry>STRING</entry>
4371 <entry>The well-known bus name to query, such as
4372 <literal>com.example.cappuccino</literal></entry>
4382 <entry>Argument</entry>
4384 <entry>Description</entry>
4390 <entry>ARRAY of STRING</entry>
4391 <entry>The unique bus names of connections currently queued
4392 for the name</entry>
4399 This method call should be sent to
4400 <literal>org.freedesktop.DBus</literal> and lists the connections
4401 currently queued for a bus name (see
4402 <xref linkend="term-queued-owner"/>).
4407 <sect2 id="message-bus-routing">
4408 <title>Message Bus Message Routing</title>
4411 Messages may have a <literal>DESTINATION</literal> field (see <xref
4412 linkend="message-protocol-header-fields"/>), resulting in a
4413 <firstterm>unicast message</firstterm>. If the
4414 <literal>DESTINATION</literal> field is present, it specifies a message
4415 recipient by name. Method calls and replies normally specify this field.
4416 The message bus must send messages (of any type) with the
4417 <literal>DESTINATION</literal> field set to the specified recipient,
4418 regardless of whether the recipient has set up a match rule matching
4423 When the message bus receives a signal, if the
4424 <literal>DESTINATION</literal> field is absent, it is considered to
4425 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4426 applications with <firstterm>message matching rules</firstterm> that
4427 match the message. Most signal messages are broadcasts.
4431 Unicast signal messages (those with a <literal>DESTINATION</literal>
4432 field) are not commonly used, but they are treated like any unicast
4433 message: they are delivered to the specified receipient,
4434 regardless of its match rules. One use for unicast signals is to
4435 avoid a race condition in which a signal is emitted before the intended
4436 recipient can call <xref linkend="bus-messages-add-match"/> to
4437 receive that signal: if the signal is sent directly to that recipient
4438 using a unicast message, it does not need to add a match rule at all,
4439 and there is no race condition. Another use for unicast signals,
4440 on message buses whose security policy prevents eavesdropping, is to
4441 send sensitive information which should only be visible to one
4446 When the message bus receives a method call, if the
4447 <literal>DESTINATION</literal> field is absent, the call is taken to be
4448 a standard one-to-one message and interpreted by the message bus
4449 itself. For example, sending an
4450 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4451 <literal>DESTINATION</literal> will cause the message bus itself to
4452 reply to the ping immediately; the message bus will not make this
4453 message visible to other applications.
4457 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4458 the ping message were sent with a <literal>DESTINATION</literal> name of
4459 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4460 forwarded, and the Yoyodyne Corporation screensaver application would be
4461 expected to reply to the ping.
4465 Message bus implementations may impose a security policy which
4466 prevents certain messages from being sent or received.
4467 When a message cannot be sent or received due to a security
4468 policy, the message bus should send an error reply, unless the
4469 original message had the <literal>NO_REPLY</literal> flag.
4472 <sect3 id="message-bus-routing-eavesdropping">
4473 <title>Eavesdropping</title>
4475 Receiving a unicast message whose <literal>DESTINATION</literal>
4476 indicates a different recipient is called
4477 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4478 a security boundary (like the standard system bus), the security
4479 policy should usually prevent eavesdropping, since unicast messages
4480 are normally kept private and may contain security-sensitive
4485 Eavesdropping is mainly useful for debugging tools, such as
4486 the <literal>dbus-monitor</literal> tool in the reference
4487 implementation of D-Bus. Tools which eavesdrop on the message bus
4488 should be careful to avoid sending a reply or error in response to
4489 messages intended for a different client.
4493 Clients may attempt to eavesdrop by adding match rules
4494 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4495 the <literal>eavesdrop='true'</literal> match. If the message bus'
4496 security policy does not allow eavesdropping, the match rule can
4497 still be added, but will not have any practical effect. For
4498 compatibility with older message bus implementations, if adding such
4499 a match rule results in an error reply, the client may fall back to
4500 adding the same rule with the <literal>eavesdrop</literal> match
4505 <sect3 id="message-bus-routing-match-rules">
4506 <title>Match Rules</title>
4508 An important part of the message bus routing protocol is match
4509 rules. Match rules describe the messages that should be sent to a
4510 client, based on the contents of the message. Broadcast signals
4511 are only sent to clients which have a suitable match rule: this
4512 avoids waking up client processes to deal with signals that are
4513 not relevant to that client.
4516 Messages that list a client as their <literal>DESTINATION</literal>
4517 do not need to match the client's match rules, and are sent to that
4518 client regardless. As a result, match rules are mainly used to
4519 receive a subset of broadcast signals.
4522 Match rules can also be used for eavesdropping
4523 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4524 if the security policy of the message bus allows it.
4527 Match rules are added using the AddMatch bus method
4528 (see <xref linkend="bus-messages-add-match"/>). Rules are
4529 specified as a string of comma separated key/value pairs.
4530 Excluding a key from the rule indicates a wildcard match.
4531 For instance excluding the the member from a match rule but
4532 adding a sender would let all messages from that sender through.
4533 An example of a complete rule would be
4534 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4537 The following table describes the keys that can be used to create
4544 <entry>Possible Values</entry>
4545 <entry>Description</entry>
4550 <entry><literal>type</literal></entry>
4551 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4552 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4555 <entry><literal>sender</literal></entry>
4556 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4557 and <xref linkend="term-unique-name"/> respectively)
4559 <entry>Match messages sent by a particular sender. An example of a sender match
4560 is sender='org.freedesktop.Hal'</entry>
4563 <entry><literal>interface</literal></entry>
4564 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4565 <entry>Match messages sent over or to a particular interface. An example of an
4566 interface match is interface='org.freedesktop.Hal.Manager'.
4567 If a message omits the interface header, it must not match any rule
4568 that specifies this key.</entry>
4571 <entry><literal>member</literal></entry>
4572 <entry>Any valid method or signal name</entry>
4573 <entry>Matches messages which have the give method or signal name. An example of
4574 a member match is member='NameOwnerChanged'</entry>
4577 <entry><literal>path</literal></entry>
4578 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4579 <entry>Matches messages which are sent from or to the given object. An example of a
4580 path match is path='/org/freedesktop/Hal/Manager'</entry>
4583 <entry><literal>path_namespace</literal></entry>
4584 <entry>An object path</entry>
4587 Matches messages which are sent from or to an
4588 object for which the object path is either the
4589 given value, or that value followed by one or
4590 more path components.
4595 <literal>path_namespace='/com/example/foo'</literal>
4596 would match signals sent by
4597 <literal>/com/example/foo</literal>
4599 <literal>/com/example/foo/bar</literal>,
4601 <literal>/com/example/foobar</literal>.
4605 Using both <literal>path</literal> and
4606 <literal>path_namespace</literal> in the same match
4607 rule is not allowed.
4612 This match key was added in version 0.16 of the
4613 D-Bus specification and implemented by the bus
4614 daemon in dbus 1.5.0 and later.
4620 <entry><literal>destination</literal></entry>
4621 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4622 <entry>Matches messages which are being sent to the given unique name. An
4623 example of a destination match is destination=':1.0'</entry>
4626 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4627 <entry>Any string</entry>
4628 <entry>Arg matches are special and are used for further restricting the
4629 match based on the arguments in the body of a message. Only arguments of type
4630 STRING can be matched in this way. An example of an argument match
4631 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4635 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4636 <entry>Any string</entry>
4638 <para>Argument path matches provide a specialised form of wildcard matching for
4639 path-like namespaces. They can match arguments whose type is either STRING or
4640 OBJECT_PATH. As with normal argument matches,
4641 if the argument is exactly equal to the string given in the match
4642 rule then the rule is satisfied. Additionally, there is also a
4643 match when either the string given in the match rule or the
4644 appropriate message argument ends with '/' and is a prefix of the
4645 other. An example argument path match is arg0path='/aa/bb/'. This
4646 would match messages with first arguments of '/', '/aa/',
4647 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4648 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4650 <para>This is intended for monitoring “directories” in file system-like
4651 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4652 system. An application interested in all nodes in a particular hierarchy would
4653 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4654 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4655 represent a modification to the “bar” property, or a signal with zeroth
4656 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4657 many properties within that directory, and the interested application would be
4658 notified in both cases.</para>
4661 This match key was added in version 0.12 of the
4662 D-Bus specification, implemented for STRING
4663 arguments by the bus daemon in dbus 1.2.0 and later,
4664 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4671 <entry><literal>arg0namespace</literal></entry>
4672 <entry>Like a bus name, except that the string is not
4673 required to contain a '.' (period)</entry>
4675 <para>Match messages whose first argument is of type STRING, and is a bus name
4676 or interface name within the specified namespace. This is primarily intended
4677 for watching name owner changes for a group of related bus names, rather than
4678 for a single name or all name changes.</para>
4680 <para>Because every valid interface name is also a valid
4681 bus name, this can also be used for messages whose
4682 first argument is an interface name.</para>
4684 <para>For example, the match rule
4685 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4686 matches name owner changes for bus names such as
4687 <literal>com.example.backend.foo</literal>,
4688 <literal>com.example.backend.foo.bar</literal>, and
4689 <literal>com.example.backend</literal> itself.</para>
4691 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4694 This match key was added in version 0.16 of the
4695 D-Bus specification and implemented by the bus
4696 daemon in dbus 1.5.0 and later.
4702 <entry><literal>eavesdrop</literal></entry>
4703 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4704 <entry>Since D-Bus 1.5.6, match rules do not
4705 match messages which have a <literal>DESTINATION</literal>
4706 field unless the match rule specifically
4708 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4709 by specifying <literal>eavesdrop='true'</literal>
4710 in the match rule. <literal>eavesdrop='false'</literal>
4711 restores the default behaviour. Messages are
4712 delivered to their <literal>DESTINATION</literal>
4713 regardless of match rules, so this match does not
4714 affect normal delivery of unicast messages.
4715 If the message bus has a security policy which forbids
4716 eavesdropping, this match may still be used without error,
4717 but will not have any practical effect.
4718 In older versions of D-Bus, this match was not allowed
4719 in match rules, and all match rules behaved as if
4720 <literal>eavesdrop='true'</literal> had been used.
4729 <sect2 id="message-bus-starting-services">
4730 <title>Message Bus Starting Services</title>
4732 The message bus can start applications on behalf of other applications.
4733 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4734 An application that can be started in this way is called a
4735 <firstterm>service</firstterm>.
4738 With D-Bus, starting a service is normally done by name. That is,
4739 applications ask the message bus to start some program that will own a
4740 well-known name, such as <literal>com.example.TextEditor</literal>.
4741 This implies a contract documented along with the name
4742 <literal>com.example.TextEditor</literal> for which object
4743 the owner of that name will provide, and what interfaces those
4747 To find an executable corresponding to a particular name, the bus daemon
4748 looks for <firstterm>service description files</firstterm>. Service
4749 description files define a mapping from names to executables. Different
4750 kinds of message bus will look for these files in different places, see
4751 <xref linkend="message-bus-types"/>.
4754 Service description files have the ".service" file
4755 extension. The message bus will only load service description files
4756 ending with .service; all other files will be ignored. The file format
4757 is similar to that of <ulink
4758 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4759 entries</ulink>. All service description files must be in UTF-8
4760 encoding. To ensure that there will be no name collisions, service files
4761 must be namespaced using the same mechanism as messages and service
4766 On the well-known system bus, the name of a service description file
4767 must be its well-known name plus <literal>.service</literal>,
4769 <literal>com.example.ConfigurationDatabase.service</literal>.
4773 On the well-known session bus, services should follow the same
4774 service description file naming convention as on the system bus,
4775 but for backwards compatibility they are not required to do so.
4779 [FIXME the file format should be much better specified than "similar to
4780 .desktop entries" esp. since desktop entries are already
4781 badly-specified. ;-)]
4782 These sections from the specification apply to service files as well:
4785 <listitem><para>General syntax</para></listitem>
4786 <listitem><para>Comment format</para></listitem>
4789 Service description files must contain a
4790 <literal>D-BUS Service</literal> group with at least the keys
4791 <literal>Name</literal> (the well-known name of the service)
4792 and <literal>Exec</literal> (the command to be executed).
4795 <title>Example service description file</title>
4797 # Sample service description file
4799 Name=com.example.ConfigurationDatabase
4800 Exec=/usr/bin/sample-configd
4806 Additionally, service description files for the well-known system
4807 bus on Unix must contain a <literal>User</literal> key, whose value
4808 is the name of a user account (e.g. <literal>root</literal>).
4809 The system service will be run as that user.
4813 When an application asks to start a service by name, the bus daemon tries to
4814 find a service that will own that name. It then tries to spawn the
4815 executable associated with it. If this fails, it will report an
4820 On the well-known system bus, it is not possible for two .service files
4821 in the same directory to offer the same service, because they are
4822 constrained to have names that match the service name.
4826 On the well-known session bus, if two .service files in the same
4827 directory offer the same service name, the result is undefined.
4828 Distributors should avoid this situation, for instance by naming
4829 session services' .service files according to their service name.
4833 If two .service files in different directories offer the same
4834 service name, the one in the higher-priority directory is used:
4835 for instance, on the system bus, .service files in
4836 /usr/local/share/dbus-1/system-services take precedence over those
4837 in /usr/share/dbus-1/system-services.
4840 The executable launched will have the environment variable
4841 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4842 message bus so it can connect and request the appropriate names.
4845 The executable being launched may want to know whether the message bus
4846 starting it is one of the well-known message buses (see <xref
4847 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4848 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4849 of the well-known buses. The currently-defined values for this variable
4850 are <literal>system</literal> for the systemwide message bus,
4851 and <literal>session</literal> for the per-login-session message
4852 bus. The new executable must still connect to the address given
4853 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4854 resulting connection is to the well-known bus.
4857 [FIXME there should be a timeout somewhere, either specified
4858 in the .service file, by the client, or just a global value
4859 and if the client being activated fails to connect within that
4860 timeout, an error should be sent back.]
4863 <sect3 id="message-bus-starting-services-scope">
4864 <title>Message Bus Service Scope</title>
4866 The "scope" of a service is its "per-", such as per-session,
4867 per-machine, per-home-directory, or per-display. The reference
4868 implementation doesn't yet support starting services in a different
4869 scope from the message bus itself. So e.g. if you start a service
4870 on the session bus its scope is per-session.
4873 We could add an optional scope to a bus name. For example, for
4874 per-(display,session pair), we could have a unique ID for each display
4875 generated automatically at login and set on screen 0 by executing a
4876 special "set display ID" binary. The ID would be stored in a
4877 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4878 random bytes. This ID would then be used to scope names.
4879 Starting/locating a service could be done by ID-name pair rather than
4883 Contrast this with a per-display scope. To achieve that, we would
4884 want a single bus spanning all sessions using a given display.
4885 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4886 property on screen 0 of the display, pointing to this bus.
4891 <sect2 id="message-bus-types">
4892 <title>Well-known Message Bus Instances</title>
4894 Two standard message bus instances are defined here, along with how
4895 to locate them and where their service files live.
4897 <sect3 id="message-bus-types-login">
4898 <title>Login session message bus</title>
4900 Each time a user logs in, a <firstterm>login session message
4901 bus</firstterm> may be started. All applications in the user's login
4902 session may interact with one another using this message bus.
4905 The address of the login session message bus is given
4906 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4907 variable. If that variable is not set, applications may
4908 also try to read the address from the X Window System root
4909 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4910 The root window property must have type <literal>STRING</literal>.
4911 The environment variable should have precedence over the
4912 root window property.
4914 <para>The address of the login session message bus is given in the
4915 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4916 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4917 "autolaunch:", the system should use platform-specific methods of
4918 locating a running D-Bus session server, or starting one if a running
4919 instance cannot be found. Note that this mechanism is not recommended
4920 for attempting to determine if a daemon is running. It is inherently
4921 racy to attempt to make this determination, since the bus daemon may
4922 be started just before or just after the determination is made.
4923 Therefore, it is recommended that applications do not try to make this
4924 determination for their functionality purposes, and instead they
4925 should attempt to start the server.</para>
4927 <sect4 id="message-bus-types-login-x-windows">
4928 <title>X Windowing System</title>
4930 For the X Windowing System, the application must locate the
4931 window owner of the selection represented by the atom formed by
4935 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4939 <para>the current user's username</para>
4943 <para>the literal character '_' (underscore)</para>
4947 <para>the machine's ID</para>
4953 The following properties are defined for the window that owns
4955 <informaltable frame="all">
4964 <para>meaning</para>
4970 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4974 <para>the actual address of the server socket</para>
4980 <para>_DBUS_SESSION_BUS_PID</para>
4984 <para>the PID of the server process</para>
4993 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4994 present in this window.
4998 If the X selection cannot be located or if reading the
4999 properties from the window fails, the implementation MUST conclude
5000 that there is no D-Bus server running and proceed to start a new
5001 server. (See below on concurrency issues)
5005 Failure to connect to the D-Bus server address thus obtained
5006 MUST be treated as a fatal connection error and should be reported
5011 As an alternative, an implementation MAY find the information
5012 in the following file located in the current user's home directory,
5013 in subdirectory .dbus/session-bus/:
5016 <para>the machine's ID</para>
5020 <para>the literal character '-' (dash)</para>
5024 <para>the X display without the screen number, with the
5025 following prefixes removed, if present: ":", "localhost:"
5026 ."localhost.localdomain:". That is, a display of
5027 "localhost:10.0" produces just the number "10"</para>
5033 The contents of this file NAME=value assignment pairs and
5034 lines starting with # are comments (no comments are allowed
5035 otherwise). The following variable names are defined:
5042 <para>Variable</para>
5046 <para>meaning</para>
5052 <para>DBUS_SESSION_BUS_ADDRESS</para>
5056 <para>the actual address of the server socket</para>
5062 <para>DBUS_SESSION_BUS_PID</para>
5066 <para>the PID of the server process</para>
5072 <para>DBUS_SESSION_BUS_WINDOWID</para>
5076 <para>the window ID</para>
5085 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5090 Failure to open this file MUST be interpreted as absence of a
5091 running server. Therefore, the implementation MUST proceed to
5092 attempting to launch a new bus server if the file cannot be
5097 However, success in opening this file MUST NOT lead to the
5098 conclusion that the server is running. Thus, a failure to connect to
5099 the bus address obtained by the alternative method MUST NOT be
5100 considered a fatal error. If the connection cannot be established,
5101 the implementation MUST proceed to check the X selection settings or
5102 to start the server on its own.
5106 If the implementation concludes that the D-Bus server is not
5107 running it MUST attempt to start a new server and it MUST also
5108 ensure that the daemon started as an effect of the "autolaunch"
5109 mechanism provides the lookup mechanisms described above, so
5110 subsequent calls can locate the newly started server. The
5111 implementation MUST also ensure that if two or more concurrent
5112 initiations happen, only one server remains running and all other
5113 initiations are able to obtain the address of this server and
5114 connect to it. In other words, the implementation MUST ensure that
5115 the X selection is not present when it attempts to set it, without
5116 allowing another process to set the selection between the
5117 verification and the setting (e.g., by using XGrabServer /
5124 On Unix systems, the session bus should search for .service files
5125 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5127 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5128 Implementations may also search additional locations, which
5129 should be searched with lower priority than anything in
5130 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5131 for example, the reference implementation also
5132 looks in <literal>${datadir}/dbus-1/services</literal> as
5133 set at compile time.
5136 As described in the XDG Base Directory Specification, software
5137 packages should install their session .service files to their
5138 configured <literal>${datadir}/dbus-1/services</literal>,
5139 where <literal>${datadir}</literal> is as defined by the GNU
5140 coding standards. System administrators or users can arrange
5141 for these service files to be read by setting XDG_DATA_DIRS or by
5142 symlinking them into the default locations.
5146 <sect3 id="message-bus-types-system">
5147 <title>System message bus</title>
5149 A computer may have a <firstterm>system message bus</firstterm>,
5150 accessible to all applications on the system. This message bus may be
5151 used to broadcast system events, such as adding new hardware devices,
5152 changes in the printer queue, and so forth.
5155 The address of the system message bus is given
5156 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5157 variable. If that variable is not set, applications should try
5158 to connect to the well-known address
5159 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5162 The D-Bus reference implementation actually honors the
5163 <literal>$(localstatedir)</literal> configure option
5164 for this address, on both client and server side.
5169 On Unix systems, the system bus should default to searching
5170 for .service files in
5171 <literal>/usr/local/share/dbus-1/system-services</literal>,
5172 <literal>/usr/share/dbus-1/system-services</literal> and
5173 <literal>/lib/dbus-1/system-services</literal>, with that order
5174 of precedence. It may also search other implementation-specific
5175 locations, but should not vary these locations based on environment
5179 The system bus is security-sensitive and is typically executed
5180 by an init system with a clean environment. Its launch helper
5181 process is particularly security-sensitive, and specifically
5182 clears its own environment.
5187 Software packages should install their system .service
5188 files to their configured
5189 <literal>${datadir}/dbus-1/system-services</literal>,
5190 where <literal>${datadir}</literal> is as defined by the GNU
5191 coding standards. System administrators can arrange
5192 for these service files to be read by editing the system bus'
5193 configuration file or by symlinking them into the default
5199 <sect2 id="message-bus-messages">
5200 <title>Message Bus Messages</title>
5202 The special message bus name <literal>org.freedesktop.DBus</literal>
5203 responds to a number of additional messages.
5206 <sect3 id="bus-messages-hello">
5207 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5218 <entry>Argument</entry>
5220 <entry>Description</entry>
5226 <entry>STRING</entry>
5227 <entry>Unique name assigned to the connection</entry>
5234 Before an application is able to send messages to other applications
5235 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5236 to the message bus to obtain a unique name. If an application without
5237 a unique name tries to send a message to another application, or a
5238 message to the message bus itself that isn't the
5239 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5240 disconnected from the bus.
5243 There is no corresponding "disconnect" request; if a client wishes to
5244 disconnect from the bus, it simply closes the socket (or other
5245 communication channel).
5248 <sect3 id="bus-messages-list-names">
5249 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5253 ARRAY of STRING ListNames ()
5260 <entry>Argument</entry>
5262 <entry>Description</entry>
5268 <entry>ARRAY of STRING</entry>
5269 <entry>Array of strings where each string is a bus name</entry>
5276 Returns a list of all currently-owned names on the bus.
5279 <sect3 id="bus-messages-list-activatable-names">
5280 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5284 ARRAY of STRING ListActivatableNames ()
5291 <entry>Argument</entry>
5293 <entry>Description</entry>
5299 <entry>ARRAY of STRING</entry>
5300 <entry>Array of strings where each string is a bus name</entry>
5307 Returns a list of all names that can be activated on the bus.
5310 <sect3 id="bus-messages-name-exists">
5311 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5315 BOOLEAN NameHasOwner (in STRING name)
5322 <entry>Argument</entry>
5324 <entry>Description</entry>
5330 <entry>STRING</entry>
5331 <entry>Name to check</entry>
5341 <entry>Argument</entry>
5343 <entry>Description</entry>
5349 <entry>BOOLEAN</entry>
5350 <entry>Return value, true if the name exists</entry>
5357 Checks if the specified name exists (currently has an owner).
5361 <sect3 id="bus-messages-name-owner-changed">
5362 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5366 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5373 <entry>Argument</entry>
5375 <entry>Description</entry>
5381 <entry>STRING</entry>
5382 <entry>Name with a new owner</entry>
5386 <entry>STRING</entry>
5387 <entry>Old owner or empty string if none</entry>
5391 <entry>STRING</entry>
5392 <entry>New owner or empty string if none</entry>
5399 This signal indicates that the owner of a name has changed.
5400 It's also the signal to use to detect the appearance of
5401 new names on the bus.
5404 <sect3 id="bus-messages-name-lost">
5405 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5409 NameLost (STRING name)
5416 <entry>Argument</entry>
5418 <entry>Description</entry>
5424 <entry>STRING</entry>
5425 <entry>Name which was lost</entry>
5432 This signal is sent to a specific application when it loses
5433 ownership of a name.
5437 <sect3 id="bus-messages-name-acquired">
5438 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5442 NameAcquired (STRING name)
5449 <entry>Argument</entry>
5451 <entry>Description</entry>
5457 <entry>STRING</entry>
5458 <entry>Name which was acquired</entry>
5465 This signal is sent to a specific application when it gains
5466 ownership of a name.
5470 <sect3 id="bus-messages-start-service-by-name">
5471 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5475 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5482 <entry>Argument</entry>
5484 <entry>Description</entry>
5490 <entry>STRING</entry>
5491 <entry>Name of the service to start</entry>
5495 <entry>UINT32</entry>
5496 <entry>Flags (currently not used)</entry>
5506 <entry>Argument</entry>
5508 <entry>Description</entry>
5514 <entry>UINT32</entry>
5515 <entry>Return value</entry>
5520 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5524 The return value can be one of the following values:
5529 <entry>Identifier</entry>
5530 <entry>Value</entry>
5531 <entry>Description</entry>
5536 <entry>DBUS_START_REPLY_SUCCESS</entry>
5538 <entry>The service was successfully started.</entry>
5541 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5543 <entry>A connection already owns the given name.</entry>
5552 <sect3 id="bus-messages-update-activation-environment">
5553 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5557 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5564 <entry>Argument</entry>
5566 <entry>Description</entry>
5572 <entry>ARRAY of DICT<STRING,STRING></entry>
5573 <entry>Environment to add or update</entry>
5578 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5581 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5584 Note, both the environment variable names and values must be valid UTF-8. There's no way to update the activation environment with data that is invalid UTF-8.
5589 <sect3 id="bus-messages-get-name-owner">
5590 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5594 STRING GetNameOwner (in STRING name)
5601 <entry>Argument</entry>
5603 <entry>Description</entry>
5609 <entry>STRING</entry>
5610 <entry>Name to get the owner of</entry>
5620 <entry>Argument</entry>
5622 <entry>Description</entry>
5628 <entry>STRING</entry>
5629 <entry>Return value, a unique connection name</entry>
5634 Returns the unique connection name of the primary owner of the name
5635 given. If the requested name doesn't have an owner, returns a
5636 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5640 <sect3 id="bus-messages-get-connection-unix-user">
5641 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5645 UINT32 GetConnectionUnixUser (in STRING bus_name)
5652 <entry>Argument</entry>
5654 <entry>Description</entry>
5660 <entry>STRING</entry>
5661 <entry>Unique or well-known bus name of the connection to
5662 query, such as <literal>:12.34</literal> or
5663 <literal>com.example.tea</literal></entry>
5673 <entry>Argument</entry>
5675 <entry>Description</entry>
5681 <entry>UINT32</entry>
5682 <entry>Unix user ID</entry>
5687 Returns the Unix user ID of the process connected to the server. If
5688 unable to determine it (for instance, because the process is not on the
5689 same machine as the bus daemon), an error is returned.
5693 <sect3 id="bus-messages-get-connection-unix-process-id">
5694 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5698 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5705 <entry>Argument</entry>
5707 <entry>Description</entry>
5713 <entry>STRING</entry>
5714 <entry>Unique or well-known bus name of the connection to
5715 query, such as <literal>:12.34</literal> or
5716 <literal>com.example.tea</literal></entry>
5726 <entry>Argument</entry>
5728 <entry>Description</entry>
5734 <entry>UINT32</entry>
5735 <entry>Unix process id</entry>
5740 Returns the Unix process ID of the process connected to the server. If
5741 unable to determine it (for instance, because the process is not on the
5742 same machine as the bus daemon), an error is returned.
5746 <sect3 id="bus-messages-get-connection-credentials">
5747 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5751 DICT<STRING,VARIANT> GetConnectionCredentials (in STRING bus_name)
5758 <entry>Argument</entry>
5760 <entry>Description</entry>
5766 <entry>STRING</entry>
5767 <entry>Unique or well-known bus name of the connection to
5768 query, such as <literal>:12.34</literal> or
5769 <literal>com.example.tea</literal></entry>
5779 <entry>Argument</entry>
5781 <entry>Description</entry>
5787 <entry>DICT<STRING,VARIANT></entry>
5788 <entry>Credentials</entry>
5796 Returns as many credentials as possible for the process connected to
5797 the server. If unable to determine certain credentials (for instance,
5798 because the process is not on the same machine as the bus daemon,
5799 or because this version of the bus daemon does not support a
5800 particular security framework), or if the values of those credentials
5801 cannot be represented as documented here, then those credentials
5806 Keys in the returned dictionary not containing "." are defined
5807 by this specification. Bus daemon implementors supporting
5808 credentials frameworks not mentioned in this document should either
5809 contribute patches to this specification, or use keys containing
5810 "." and starting with a reversed domain name.
5816 <entry>Value type</entry>
5817 <entry>Value</entry>
5822 <entry>UnixUserID</entry>
5823 <entry>UINT32</entry>
5824 <entry>The numeric Unix user ID, as defined by POSIX</entry>
5827 <entry>ProcessID</entry>
5828 <entry>UINT32</entry>
5829 <entry>The numeric process ID, on platforms that have
5830 this concept. On Unix, this is the process ID defined by
5839 This method was added in D-Bus 1.7 to reduce the round-trips
5840 required to list a process's credentials. In older versions, calling
5841 this method will fail: applications should recover by using the
5842 separate methods such as
5843 <xref linkend="bus-messages-get-connection-unix-user"/>
5848 <sect3 id="bus-messages-get-adt-audit-session-data">
5849 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
5853 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
5860 <entry>Argument</entry>
5862 <entry>Description</entry>
5868 <entry>STRING</entry>
5869 <entry>Unique or well-known bus name of the connection to
5870 query, such as <literal>:12.34</literal> or
5871 <literal>com.example.tea</literal></entry>
5881 <entry>Argument</entry>
5883 <entry>Description</entry>
5889 <entry>ARRAY of BYTE</entry>
5890 <entry>auditing data as returned by
5891 adt_export_session_data()</entry>
5896 Returns auditing data used by Solaris ADT, in an unspecified
5897 binary format. If you know what this means, please contribute
5898 documentation via the D-Bus bug tracking system.
5899 This method is on the core DBus interface for historical reasons;
5900 the same information should be made available via
5901 <xref linkend="bus-messages-get-connection-credentials"/>
5906 <sect3 id="bus-messages-get-connection-selinux-security-context">
5907 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
5911 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
5918 <entry>Argument</entry>
5920 <entry>Description</entry>
5926 <entry>STRING</entry>
5927 <entry>Unique or well-known bus name of the connection to
5928 query, such as <literal>:12.34</literal> or
5929 <literal>com.example.tea</literal></entry>
5939 <entry>Argument</entry>
5941 <entry>Description</entry>
5947 <entry>ARRAY of BYTE</entry>
5948 <entry>some sort of string of bytes, not necessarily UTF-8,
5949 not including '\0'</entry>
5954 Returns the security context used by SELinux, in an unspecified
5955 format. If you know what this means, please contribute
5956 documentation via the D-Bus bug tracking system.
5957 This method is on the core DBus interface for historical reasons;
5958 the same information should be made available via
5959 <xref linkend="bus-messages-get-connection-credentials"/>
5965 <sect3 id="bus-messages-add-match">
5966 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5970 AddMatch (in STRING rule)
5977 <entry>Argument</entry>
5979 <entry>Description</entry>
5985 <entry>STRING</entry>
5986 <entry>Match rule to add to the connection</entry>
5991 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5992 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5996 <sect3 id="bus-messages-remove-match">
5997 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
6001 RemoveMatch (in STRING rule)
6008 <entry>Argument</entry>
6010 <entry>Description</entry>
6016 <entry>STRING</entry>
6017 <entry>Match rule to remove from the connection</entry>
6022 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6023 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6028 <sect3 id="bus-messages-get-id">
6029 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6033 GetId (out STRING id)
6040 <entry>Argument</entry>
6042 <entry>Description</entry>
6048 <entry>STRING</entry>
6049 <entry>Unique ID identifying the bus daemon</entry>
6054 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6055 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6056 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6057 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6058 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6059 by org.freedesktop.DBus.Peer.GetMachineId().
6060 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6068 <appendix id="implementation-notes">
6069 <title>Implementation notes</title>
6070 <sect1 id="implementation-notes-subsection">
6078 <glossary><title>Glossary</title>
6080 This glossary defines some of the terms used in this specification.
6083 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6086 The message bus maintains an association between names and
6087 connections. (Normally, there's one connection per application.) A
6088 bus name is simply an identifier used to locate connections. For
6089 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6090 name might be used to send a message to a screensaver from Yoyodyne
6091 Corporation. An application is said to <firstterm>own</firstterm> a
6092 name if the message bus has associated the application's connection
6093 with the name. Names may also have <firstterm>queued
6094 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6095 The bus assigns a unique name to each connection,
6096 see <xref linkend="term-unique-name"/>. Other names
6097 can be thought of as "well-known names" and are
6098 used to find applications that offer specific functionality.
6102 See <xref linkend="message-protocol-names-bus"/> for details of
6103 the syntax and naming conventions for bus names.
6108 <glossentry id="term-message"><glossterm>Message</glossterm>
6111 A message is the atomic unit of communication via the D-Bus
6112 protocol. It consists of a <firstterm>header</firstterm> and a
6113 <firstterm>body</firstterm>; the body is made up of
6114 <firstterm>arguments</firstterm>.
6119 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6122 The message bus is a special application that forwards
6123 or routes messages between a group of applications
6124 connected to the message bus. It also manages
6125 <firstterm>names</firstterm> used for routing
6131 <glossentry id="term-name"><glossterm>Name</glossterm>
6134 See <xref linkend="term-bus-name"/>. "Name" may
6135 also be used to refer to some of the other names
6136 in D-Bus, such as interface names.
6141 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6144 Used to prevent collisions when defining new interfaces, bus names
6145 etc. The convention used is the same one Java uses for defining
6146 classes: a reversed domain name.
6147 See <xref linkend="message-protocol-names-bus"/>,
6148 <xref linkend="message-protocol-names-interface"/>,
6149 <xref linkend="message-protocol-names-error"/>,
6150 <xref linkend="message-protocol-marshaling-object-path"/>.
6155 <glossentry id="term-object"><glossterm>Object</glossterm>
6158 Each application contains <firstterm>objects</firstterm>, which have
6159 <firstterm>interfaces</firstterm> and
6160 <firstterm>methods</firstterm>. Objects are referred to by a name,
6161 called a <firstterm>path</firstterm>.
6166 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6169 An application talking directly to another application, without going
6170 through a message bus. One-to-one connections may be "peer to peer" or
6171 "client to server." The D-Bus protocol has no concept of client
6172 vs. server after a connection has authenticated; the flow of messages
6173 is symmetrical (full duplex).
6178 <glossentry id="term-path"><glossterm>Path</glossterm>
6181 Object references (object names) in D-Bus are organized into a
6182 filesystem-style hierarchy, so each object is named by a path. As in
6183 LDAP, there's no difference between "files" and "directories"; a path
6184 can refer to an object, while still having child objects below it.
6189 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6192 Each bus name has a primary owner; messages sent to the name go to the
6193 primary owner. However, certain names also maintain a queue of
6194 secondary owners "waiting in the wings." If the primary owner releases
6195 the name, then the first secondary owner in the queue automatically
6196 becomes the new owner of the name.
6201 <glossentry id="term-service"><glossterm>Service</glossterm>
6204 A service is an executable that can be launched by the bus daemon.
6205 Services normally guarantee some particular features, for example they
6206 may guarantee that they will request a specific name such as
6207 "com.example.Screensaver", have a singleton object
6208 "/com/example/Application", and that object will implement the
6209 interface "com.example.Screensaver.Control".
6214 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6217 ".service files" tell the bus about service applications that can be
6218 launched (see <xref linkend="term-service"/>). Most importantly they
6219 provide a mapping from bus names to services that will request those
6220 names when they start up.
6225 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6228 The special name automatically assigned to each connection by the
6229 message bus. This name will never change owner, and will be unique
6230 (never reused during the lifetime of the message bus).
6231 It will begin with a ':' character.
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