1 <?xml version="1.0" standalone="no" ?> 2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" 3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" 4 [ 5 ]> 6 <article id="index"> 7 <articleinfo> 8 <title>D-Bus Specification</title> 9 <releaseinfo>Version 0.19</releaseinfo> 10 <date>2012-02-21</date> 11 <authorgroup> 12 <author> 13 <firstname>Havoc</firstname> 14 <surname>Pennington</surname> 15 <affiliation> 16 <orgname>Red Hat, Inc.</orgname> 17 <address> 18 <email>hp (a] pobox.com</email> 19 </address> 20 </affiliation> 21 </author> 22 <author> 23 <firstname>Anders</firstname> 24 <surname>Carlsson</surname> 25 <affiliation> 26 <orgname>CodeFactory AB</orgname> 27 <address> 28 <email>andersca (a] codefactory.se</email> 29 </address> 30 </affiliation> 31 </author> 32 <author> 33 <firstname>Alexander</firstname> 34 <surname>Larsson</surname> 35 <affiliation> 36 <orgname>Red Hat, Inc.</orgname> 37 <address> 38 <email>alexl (a] redhat.com</email> 39 </address> 40 </affiliation> 41 </author> 42 <author> 43 <firstname>Sven</firstname> 44 <surname>Herzberg</surname> 45 <affiliation> 46 <orgname>Imendio AB</orgname> 47 <address> 48 <email>sven (a] imendio.com</email> 49 </address> 50 </affiliation> 51 </author> 52 <author> 53 <firstname>Simon</firstname> 54 <surname>McVittie</surname> 55 <affiliation> 56 <orgname>Collabora Ltd.</orgname> 57 <address> 58 <email>simon.mcvittie (a] collabora.co.uk</email> 59 </address> 60 </affiliation> 61 </author> 62 <author> 63 <firstname>David</firstname> 64 <surname>Zeuthen</surname> 65 <affiliation> 66 <orgname>Red Hat, Inc.</orgname> 67 <address> 68 <email>davidz (a] redhat.com</email> 69 </address> 70 </affiliation> 71 </author> 72 </authorgroup> 73 <revhistory> 74 <revision> 75 <revnumber>current</revnumber> 76 <date><ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink></date> 77 <authorinitials></authorinitials> 78 <revremark></revremark> 79 </revision> 80 <revision> 81 <revnumber>0.19</revnumber> 82 <date>20 February 2012</date> 83 <authorinitials>smcv/lp</authorinitials> 84 <revremark>formally define unique connection names and well-known 85 bus names; document best practices for interface, bus, member and 86 error names, and object paths; document the search path for session 87 and system services on Unix; document the systemd transport</revremark> 88 </revision> 89 <revision> 90 <revnumber>0.18</revnumber> 91 <date>29 July 2011</date> 92 <authorinitials>smcv</authorinitials> 93 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop 94 match keyword; promote type system to a top-level section</revremark> 95 </revision> 96 <revision> 97 <revnumber>0.17</revnumber> 98 <date>1 June 2011</date> 99 <authorinitials>smcv/davidz</authorinitials> 100 <revremark>define ObjectManager; reserve extra pseudo-type-codes used 101 by GVariant</revremark> 102 </revision> 103 <revision> 104 <revnumber>0.16</revnumber> 105 <date>11 April 2011</date> 106 <authorinitials></authorinitials> 107 <revremark>add path_namespace, arg0namespace; argNpath matches object 108 paths</revremark> 109 </revision> 110 <revision> 111 <revnumber>0.15</revnumber> 112 <date>3 November 2010</date> 113 <authorinitials></authorinitials> 114 <revremark></revremark> 115 </revision> 116 <revision> 117 <revnumber>0.14</revnumber> 118 <date>12 May 2010</date> 119 <authorinitials></authorinitials> 120 <revremark></revremark> 121 </revision> 122 <revision> 123 <revnumber>0.13</revnumber> 124 <date>23 Dezember 2009</date> 125 <authorinitials></authorinitials> 126 <revremark></revremark> 127 </revision> 128 <revision> 129 <revnumber>0.12</revnumber> 130 <date>7 November, 2006</date> 131 <authorinitials></authorinitials> 132 <revremark></revremark> 133 </revision> 134 <revision> 135 <revnumber>0.11</revnumber> 136 <date>6 February 2005</date> 137 <authorinitials></authorinitials> 138 <revremark></revremark> 139 </revision> 140 <revision> 141 <revnumber>0.10</revnumber> 142 <date>28 January 2005</date> 143 <authorinitials></authorinitials> 144 <revremark></revremark> 145 </revision> 146 <revision> 147 <revnumber>0.9</revnumber> 148 <date>7 Januar 2005</date> 149 <authorinitials></authorinitials> 150 <revremark></revremark> 151 </revision> 152 <revision> 153 <revnumber>0.8</revnumber> 154 <date>06 September 2003</date> 155 <authorinitials></authorinitials> 156 <revremark>First released document.</revremark> 157 </revision> 158 </revhistory> 159 </articleinfo> 160 161 <sect1 id="introduction"> 162 <title>Introduction</title> 163 <para> 164 D-Bus is a system for low-latency, low-overhead, easy to use 165 interprocess communication (IPC). In more detail: 166 <itemizedlist> 167 <listitem> 168 <para> 169 D-Bus is <emphasis>low-latency</emphasis> because it is designed 170 to avoid round trips and allow asynchronous operation, much like 171 the X protocol. 172 </para> 173 </listitem> 174 <listitem> 175 <para> 176 D-Bus is <emphasis>low-overhead</emphasis> because it uses a 177 binary protocol, and does not have to convert to and from a text 178 format such as XML. Because D-Bus is intended for potentially 179 high-resolution same-machine IPC, not primarily for Internet IPC, 180 this is an interesting optimization. 181 </para> 182 </listitem> 183 <listitem> 184 <para> 185 D-Bus is <emphasis>easy to use</emphasis> because it works in terms 186 of <firstterm>messages</firstterm> rather than byte streams, and 187 automatically handles a lot of the hard IPC issues. Also, the D-Bus 188 library is designed to be wrapped in a way that lets developers use 189 their framework's existing object/type system, rather than learning 190 a new one specifically for IPC. 191 </para> 192 </listitem> 193 </itemizedlist> 194 </para> 195 196 <para> 197 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server) 198 protocol, specified in <xref linkend="message-protocol"/>. That is, it is 199 a system for one application to talk to a single other 200 application. However, the primary intended application of the protocol is the 201 D-Bus <firstterm>message bus</firstterm>, specified in <xref 202 linkend="message-bus"/>. The message bus is a special application that 203 accepts connections from multiple other applications, and forwards 204 messages among them. 205 </para> 206 207 <para> 208 Uses of D-Bus include notification of system changes (notification of when 209 a camera is plugged in to a computer, or a new version of some software 210 has been installed), or desktop interoperability, for example a file 211 monitoring service or a configuration service. 212 </para> 213 214 <para> 215 D-Bus is designed for two specific use cases: 216 <itemizedlist> 217 <listitem> 218 <para> 219 A "system bus" for notifications from the system to user sessions, 220 and to allow the system to request input from user sessions. 221 </para> 222 </listitem> 223 <listitem> 224 <para> 225 A "session bus" used to implement desktop environments such as 226 GNOME and KDE. 227 </para> 228 </listitem> 229 </itemizedlist> 230 D-Bus is not intended to be a generic IPC system for any possible 231 application, and intentionally omits many features found in other 232 IPC systems for this reason. 233 </para> 234 235 <para> 236 At the same time, the bus daemons offer a number of features not found in 237 other IPC systems, such as single-owner "bus names" (similar to X 238 selections), on-demand startup of services, and security policies. 239 In many ways, these features are the primary motivation for developing 240 D-Bus; other systems would have sufficed if IPC were the only goal. 241 </para> 242 243 <para> 244 D-Bus may turn out to be useful in unanticipated applications, but future 245 versions of this spec and the reference implementation probably will not 246 incorporate features that interfere with the core use cases. 247 </para> 248 249 <para> 250 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 251 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 252 document are to be interpreted as described in RFC 2119. However, the 253 document could use a serious audit to be sure it makes sense to do 254 so. Also, they are not capitalized. 255 </para> 256 257 <sect2 id="stability"> 258 <title>Protocol and Specification Stability</title> 259 <para> 260 The D-Bus protocol is frozen (only compatible extensions are allowed) as 261 of November 8, 2006. However, this specification could still use a fair 262 bit of work to make interoperable reimplementation possible without 263 reference to the D-Bus reference implementation. Thus, this 264 specification is not marked 1.0. To mark it 1.0, we'd like to see 265 someone invest significant effort in clarifying the specification 266 language, and growing the specification to cover more aspects of the 267 reference implementation's behavior. 268 </para> 269 <para> 270 Until this work is complete, any attempt to reimplement D-Bus will 271 probably require looking at the reference implementation and/or asking 272 questions on the D-Bus mailing list about intended behavior. 273 Questions on the list are very welcome. 274 </para> 275 <para> 276 Nonetheless, this document should be a useful starting point and is 277 to our knowledge accurate, though incomplete. 278 </para> 279 </sect2> 280 281 </sect1> 282 283 <sect1 id="type-system"> 284 <title>Type System</title> 285 286 <para> 287 D-Bus has a type system, in which values of various types can be 288 serialized into a sequence of bytes referred to as the 289 <firstterm>wire format</firstterm> in a standard way. 290 Converting a value from some other representation into the wire 291 format is called <firstterm>marshaling</firstterm> and converting 292 it back from the wire format is <firstterm>unmarshaling</firstterm>. 293 </para> 294 295 <sect2 id="message-protocol-signatures"> 296 <title>Type Signatures</title> 297 298 <para> 299 The D-Bus protocol does not include type tags in the marshaled data; a 300 block of marshaled values must have a known <firstterm>type 301 signature</firstterm>. The type signature is made up of <firstterm>type 302 codes</firstterm>. A type code is an ASCII character representing the 303 type of a value. Because ASCII characters are used, the type signature 304 will always form a valid ASCII string. A simple string compare 305 determines whether two type signatures are equivalent. 306 </para> 307 308 <para> 309 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is 310 the ASCII character 'i'. So the signature for a block of values 311 containing a single <literal>INT32</literal> would be: 312 <programlisting> 313 "i" 314 </programlisting> 315 A block of values containing two <literal>INT32</literal> would have this signature: 316 <programlisting> 317 "ii" 318 </programlisting> 319 </para> 320 321 <para> 322 All <firstterm>basic</firstterm> types work like 323 <literal>INT32</literal> in this example. To marshal and unmarshal 324 basic types, you simply read one value from the data 325 block corresponding to each type code in the signature. 326 In addition to basic types, there are four <firstterm>container</firstterm> 327 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>, 328 and <literal>DICT_ENTRY</literal>. 329 </para> 330 331 <para> 332 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type 333 code does not appear in signatures. Instead, ASCII characters 334 '(' and ')' are used to mark the beginning and end of the struct. 335 So for example, a struct containing two integers would have this 336 signature: 337 <programlisting> 338 "(ii)" 339 </programlisting> 340 Structs can be nested, so for example a struct containing 341 an integer and another struct: 342 <programlisting> 343 "(i(ii))" 344 </programlisting> 345 The value block storing that struct would contain three integers; the 346 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or 347 "(iii)" or "iii". 348 </para> 349 350 <para> 351 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol, 352 but is useful in code that implements the protocol. This type code 353 is specified to allow such code to interoperate in non-protocol contexts. 354 </para> 355 356 <para> 357 Empty structures are not allowed; there must be at least one 358 type code between the parentheses. 359 </para> 360 361 <para> 362 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be 363 followed by a <firstterm>single complete type</firstterm>. The single 364 complete type following the array is the type of each array element. So 365 the simple example is: 366 <programlisting> 367 "ai" 368 </programlisting> 369 which is an array of 32-bit integers. But an array can be of any type, 370 such as this array-of-struct-with-two-int32-fields: 371 <programlisting> 372 "a(ii)" 373 </programlisting> 374 Or this array of array of integer: 375 <programlisting> 376 "aai" 377 </programlisting> 378 </para> 379 380 <para> 381 The phrase <firstterm>single complete type</firstterm> deserves some 382 definition. A single complete type is a basic type code, a variant type code, 383 an array with its element type, or a struct with its fields. 384 So the following signatures are not single complete types: 385 <programlisting> 386 "aa" 387 </programlisting> 388 <programlisting> 389 "(ii" 390 </programlisting> 391 <programlisting> 392 "ii)" 393 </programlisting> 394 And the following signatures contain multiple complete types: 395 <programlisting> 396 "ii" 397 </programlisting> 398 <programlisting> 399 "aiai" 400 </programlisting> 401 <programlisting> 402 "(ii)(ii)" 403 </programlisting> 404 Note however that a single complete type may <emphasis>contain</emphasis> 405 multiple other single complete types. 406 </para> 407 408 <para> 409 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of 410 type <literal>VARIANT</literal> will have the signature of a single complete type as part 411 of the <emphasis>value</emphasis>. This signature will be followed by a 412 marshaled value of that type. 413 </para> 414 415 <para> 416 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather 417 than parentheses it uses curly braces, and it has more restrictions. 418 The restrictions are: it occurs only as an array element type; it has 419 exactly two single complete types inside the curly braces; the first 420 single complete type (the "key") must be a basic type rather than a 421 container type. Implementations must not accept dict entries outside of 422 arrays, must not accept dict entries with zero, one, or more than two 423 fields, and must not accept dict entries with non-basic-typed keys. A 424 dict entry is always a key-value pair. 425 </para> 426 427 <para> 428 The first field in the <literal>DICT_ENTRY</literal> is always the key. 429 A message is considered corrupt if the same key occurs twice in the same 430 array of <literal>DICT_ENTRY</literal>. However, for performance reasons 431 implementations are not required to reject dicts with duplicate keys. 432 </para> 433 434 <para> 435 In most languages, an array of dict entry would be represented as a 436 map, hash table, or dict object. 437 </para> 438 439 <para> 440 The following table summarizes the D-Bus types. 441 <informaltable> 442 <tgroup cols="3"> 443 <thead> 444 <row> 445 <entry>Conventional Name</entry> 446 <entry>Code</entry> 447 <entry>Description</entry> 448 </row> 449 </thead> 450 <tbody> 451 <row> 452 <entry><literal>INVALID</literal></entry> 453 <entry>0 (ASCII NUL)</entry> 454 <entry>Not a valid type code, used to terminate signatures</entry> 455 </row><row> 456 <entry><literal>BYTE</literal></entry> 457 <entry>121 (ASCII 'y')</entry> 458 <entry>8-bit unsigned integer</entry> 459 </row><row> 460 <entry><literal>BOOLEAN</literal></entry> 461 <entry>98 (ASCII 'b')</entry> 462 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry> 463 </row><row> 464 <entry><literal>INT16</literal></entry> 465 <entry>110 (ASCII 'n')</entry> 466 <entry>16-bit signed integer</entry> 467 </row><row> 468 <entry><literal>UINT16</literal></entry> 469 <entry>113 (ASCII 'q')</entry> 470 <entry>16-bit unsigned integer</entry> 471 </row><row> 472 <entry><literal>INT32</literal></entry> 473 <entry>105 (ASCII 'i')</entry> 474 <entry>32-bit signed integer</entry> 475 </row><row> 476 <entry><literal>UINT32</literal></entry> 477 <entry>117 (ASCII 'u')</entry> 478 <entry>32-bit unsigned integer</entry> 479 </row><row> 480 <entry><literal>INT64</literal></entry> 481 <entry>120 (ASCII 'x')</entry> 482 <entry>64-bit signed integer</entry> 483 </row><row> 484 <entry><literal>UINT64</literal></entry> 485 <entry>116 (ASCII 't')</entry> 486 <entry>64-bit unsigned integer</entry> 487 </row><row> 488 <entry><literal>DOUBLE</literal></entry> 489 <entry>100 (ASCII 'd')</entry> 490 <entry>IEEE 754 double</entry> 491 </row><row> 492 <entry><literal>STRING</literal></entry> 493 <entry>115 (ASCII 's')</entry> 494 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry> 495 </row><row> 496 <entry><literal>OBJECT_PATH</literal></entry> 497 <entry>111 (ASCII 'o')</entry> 498 <entry>Name of an object instance</entry> 499 </row><row> 500 <entry><literal>SIGNATURE</literal></entry> 501 <entry>103 (ASCII 'g')</entry> 502 <entry>A type signature</entry> 503 </row><row> 504 <entry><literal>ARRAY</literal></entry> 505 <entry>97 (ASCII 'a')</entry> 506 <entry>Array</entry> 507 </row><row> 508 <entry><literal>STRUCT</literal></entry> 509 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry> 510 <entry>Struct; type code 114 'r' is reserved for use in 511 bindings and implementations to represent the general 512 concept of a struct, and must not appear in signatures 513 used on D-Bus.</entry> 514 </row><row> 515 <entry><literal>VARIANT</literal></entry> 516 <entry>118 (ASCII 'v') </entry> 517 <entry>Variant type (the type of the value is part of the value itself)</entry> 518 </row><row> 519 <entry><literal>DICT_ENTRY</literal></entry> 520 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry> 521 <entry>Entry in a dict or map (array of key-value pairs). 522 Type code 101 'e' is reserved for use in bindings and 523 implementations to represent the general concept of a 524 dict or dict-entry, and must not appear in signatures 525 used on D-Bus.</entry> 526 </row><row> 527 <entry><literal>UNIX_FD</literal></entry> 528 <entry>104 (ASCII 'h')</entry> 529 <entry>Unix file descriptor</entry> 530 </row> 531 <row> 532 <entry>(reserved)</entry> 533 <entry>109 (ASCII 'm')</entry> 534 <entry>Reserved for <ulink 535 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a 536 'maybe' type compatible with the one in GVariant</ulink>, 537 and must not appear in signatures used on D-Bus until 538 specified here</entry> 539 </row> 540 <row> 541 <entry>(reserved)</entry> 542 <entry>42 (ASCII '*')</entry> 543 <entry>Reserved for use in bindings/implementations to 544 represent any <firstterm>single complete type</firstterm>, 545 and must not appear in signatures used on D-Bus.</entry> 546 </row> 547 <row> 548 <entry>(reserved)</entry> 549 <entry>63 (ASCII '?')</entry> 550 <entry>Reserved for use in bindings/implementations to 551 represent any <firstterm>basic type</firstterm>, and must 552 not appear in signatures used on D-Bus.</entry> 553 </row> 554 <row> 555 <entry>(reserved)</entry> 556 <entry>64 (ASCII '@'), 38 (ASCII '&'), 557 94 (ASCII '^')</entry> 558 <entry>Reserved for internal use by bindings/implementations, 559 and must not appear in signatures used on D-Bus. 560 GVariant uses these type-codes to encode calling 561 conventions.</entry> 562 </row> 563 </tbody> 564 </tgroup> 565 </informaltable> 566 </para> 567 568 </sect2> 569 570 <sect2 id="message-protocol-marshaling"> 571 <title>Marshaling (Wire Format)</title> 572 573 <para> 574 Given a type signature, a block of bytes can be converted into typed 575 values. This section describes the format of the block of bytes. Byte 576 order and alignment issues are handled uniformly for all D-Bus types. 577 </para> 578 579 <para> 580 A block of bytes has an associated byte order. The byte order 581 has to be discovered in some way; for D-Bus messages, the 582 byte order is part of the message header as described in 583 <xref linkend="message-protocol-messages"/>. For now, assume 584 that the byte order is known to be either little endian or big 585 endian. 586 </para> 587 588 <para> 589 Each value in a block of bytes is aligned "naturally," for example 590 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an 591 8-byte boundary. To properly align a value, <firstterm>alignment 592 padding</firstterm> may be necessary. The alignment padding must always 593 be the minimum required padding to properly align the following value; 594 and it must always be made up of nul bytes. The alignment padding must 595 not be left uninitialized (it can't contain garbage), and more padding 596 than required must not be used. 597 </para> 598 599 <para> 600 Given all this, the types are marshaled on the wire as follows: 601 <informaltable> 602 <tgroup cols="3"> 603 <thead> 604 <row> 605 <entry>Conventional Name</entry> 606 <entry>Encoding</entry> 607 <entry>Alignment</entry> 608 </row> 609 </thead> 610 <tbody> 611 <row> 612 <entry><literal>INVALID</literal></entry> 613 <entry>Not applicable; cannot be marshaled.</entry> 614 <entry>N/A</entry> 615 </row><row> 616 <entry><literal>BYTE</literal></entry> 617 <entry>A single 8-bit byte.</entry> 618 <entry>1</entry> 619 </row><row> 620 <entry><literal>BOOLEAN</literal></entry> 621 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry> 622 <entry>4</entry> 623 </row><row> 624 <entry><literal>INT16</literal></entry> 625 <entry>16-bit signed integer in the message's byte order.</entry> 626 <entry>2</entry> 627 </row><row> 628 <entry><literal>UINT16</literal></entry> 629 <entry>16-bit unsigned integer in the message's byte order.</entry> 630 <entry>2</entry> 631 </row><row> 632 <entry><literal>INT32</literal></entry> 633 <entry>32-bit signed integer in the message's byte order.</entry> 634 <entry>4</entry> 635 </row><row> 636 <entry><literal>UINT32</literal></entry> 637 <entry>32-bit unsigned integer in the message's byte order.</entry> 638 <entry>4</entry> 639 </row><row> 640 <entry><literal>INT64</literal></entry> 641 <entry>64-bit signed integer in the message's byte order.</entry> 642 <entry>8</entry> 643 </row><row> 644 <entry><literal>UINT64</literal></entry> 645 <entry>64-bit unsigned integer in the message's byte order.</entry> 646 <entry>8</entry> 647 </row><row> 648 <entry><literal>DOUBLE</literal></entry> 649 <entry>64-bit IEEE 754 double in the message's byte order.</entry> 650 <entry>8</entry> 651 </row><row> 652 <entry><literal>STRING</literal></entry> 653 <entry>A <literal>UINT32</literal> indicating the string's 654 length in bytes excluding its terminating nul, followed by 655 non-nul string data of the given length, followed by a terminating nul 656 byte. 657 </entry> 658 <entry> 659 4 (for the length) 660 </entry> 661 </row><row> 662 <entry><literal>OBJECT_PATH</literal></entry> 663 <entry>Exactly the same as <literal>STRING</literal> except the 664 content must be a valid object path (see below). 665 </entry> 666 <entry> 667 4 (for the length) 668 </entry> 669 </row><row> 670 <entry><literal>SIGNATURE</literal></entry> 671 <entry>The same as <literal>STRING</literal> except the length is a single 672 byte (thus signatures have a maximum length of 255) 673 and the content must be a valid signature (see below). 674 </entry> 675 <entry> 676 1 677 </entry> 678 </row><row> 679 <entry><literal>ARRAY</literal></entry> 680 <entry> 681 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by 682 alignment padding to the alignment boundary of the array element type, 683 followed by each array element. The array length is from the 684 end of the alignment padding to the end of the last element, 685 i.e. it does not include the padding after the length, 686 or any padding after the last element. 687 Arrays have a maximum length defined to be 2 to the 26th power or 688 67108864. Implementations must not send or accept arrays exceeding this 689 length. 690 </entry> 691 <entry> 692 4 (for the length) 693 </entry> 694 </row><row> 695 <entry><literal>STRUCT</literal></entry> 696 <entry> 697 A struct must start on an 8-byte boundary regardless of the 698 type of the struct fields. The struct value consists of each 699 field marshaled in sequence starting from that 8-byte 700 alignment boundary. 701 </entry> 702 <entry> 703 8 704 </entry> 705 </row><row> 706 <entry><literal>VARIANT</literal></entry> 707 <entry> 708 A variant type has a marshaled 709 <literal>SIGNATURE</literal> followed by a marshaled 710 value with the type given in the signature. Unlike 711 a message signature, the variant signature can 712 contain only a single complete type. So "i", "ai" 713 or "(ii)" is OK, but "ii" is not. Use of variants may not 714 cause a total message depth to be larger than 64, including 715 other container types such as structures. 716 </entry> 717 <entry> 718 1 (alignment of the signature) 719 </entry> 720 </row><row> 721 <entry><literal>DICT_ENTRY</literal></entry> 722 <entry> 723 Identical to STRUCT. 724 </entry> 725 <entry> 726 8 727 </entry> 728 </row><row> 729 <entry><literal>UNIX_FD</literal></entry> 730 <entry>32-bit unsigned integer in the message's byte 731 order. The actual file descriptors need to be 732 transferred out-of-band via some platform specific 733 mechanism. On the wire, values of this type store the index to the 734 file descriptor in the array of file descriptors that 735 accompany the message.</entry> 736 <entry>4</entry> 737 </row> 738 </tbody> 739 </tgroup> 740 </informaltable> 741 </para> 742 743 <sect3 id="message-protocol-marshaling-object-path"> 744 <title>Valid Object Paths</title> 745 746 <para> 747 An object path is a name used to refer to an object instance. 748 Conceptually, each participant in a D-Bus message exchange may have 749 any number of object instances (think of C++ or Java objects) and each 750 such instance will have a path. Like a filesystem, the object 751 instances in an application form a hierarchical tree. 752 </para> 753 754 <para> 755 The following rules define a valid object path. Implementations must 756 not send or accept messages with invalid object paths. 757 <itemizedlist> 758 <listitem> 759 <para> 760 The path may be of any length. 761 </para> 762 </listitem> 763 <listitem> 764 <para> 765 The path must begin with an ASCII '/' (integer 47) character, 766 and must consist of elements separated by slash characters. 767 </para> 768 </listitem> 769 <listitem> 770 <para> 771 Each element must only contain the ASCII characters 772 "[A-Z][a-z][0-9]_" 773 </para> 774 </listitem> 775 <listitem> 776 <para> 777 No element may be the empty string. 778 </para> 779 </listitem> 780 <listitem> 781 <para> 782 Multiple '/' characters cannot occur in sequence. 783 </para> 784 </listitem> 785 <listitem> 786 <para> 787 A trailing '/' character is not allowed unless the 788 path is the root path (a single '/' character). 789 </para> 790 </listitem> 791 </itemizedlist> 792 </para> 793 794 <para> 795 Object paths are often namespaced by starting with a reversed 796 domain name and containing an interface version number, in the 797 same way as 798 <link linkend="message-protocol-names-interface">interface 799 names</link> and 800 <link linkend="message-protocol-names-bus">well-known 801 bus names</link>. 802 This makes it possible to implement more than one service, or 803 more than one version of a service, in the same process, 804 even if the services share a connection but cannot otherwise 805 co-operate (for instance, if they are implemented by different 806 plugins). 807 </para> 808 809 <para> 810 For instance, if the owner of <literal>example.com</literal> is 811 developing a D-Bus API for a music player, they might use the 812 hierarchy of object paths that start with 813 <literal>/com/example/MusicPlayer1</literal> for its objects. 814 </para> 815 </sect3> 816 817 <sect3 id="message-protocol-marshaling-signature"> 818 <title>Valid Signatures</title> 819 <para> 820 An implementation must not send or accept invalid signatures. 821 Valid signatures will conform to the following rules: 822 <itemizedlist> 823 <listitem> 824 <para> 825 The signature ends with a nul byte. 826 </para> 827 </listitem> 828 <listitem> 829 <para> 830 The signature is a list of single complete types. 831 Arrays must have element types, and structs must 832 have both open and close parentheses. 833 </para> 834 </listitem> 835 <listitem> 836 <para> 837 Only type codes and open and close parentheses are 838 allowed in the signature. The <literal>STRUCT</literal> type code 839 is not allowed in signatures, because parentheses 840 are used instead. 841 </para> 842 </listitem> 843 <listitem> 844 <para> 845 The maximum depth of container type nesting is 32 array type 846 codes and 32 open parentheses. This implies that the maximum 847 total depth of recursion is 64, for an "array of array of array 848 of ... struct of struct of struct of ..." where there are 32 849 array and 32 struct. 850 </para> 851 </listitem> 852 <listitem> 853 <para> 854 The maximum length of a signature is 255. 855 </para> 856 </listitem> 857 <listitem> 858 <para> 859 Signatures must be nul-terminated. 860 </para> 861 </listitem> 862 </itemizedlist> 863 </para> 864 </sect3> 865 866 </sect2> 867 868 </sect1> 869 870 <sect1 id="message-protocol"> 871 <title>Message Protocol</title> 872 873 <para> 874 A <firstterm>message</firstterm> consists of a 875 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you 876 think of a message as a package, the header is the address, and the body 877 contains the package contents. The message delivery system uses the header 878 information to figure out where to send the message and how to interpret 879 it; the recipient interprets the body of the message. 880 </para> 881 882 <para> 883 The body of the message is made up of zero or more 884 <firstterm>arguments</firstterm>, which are typed values, such as an 885 integer or a byte array. 886 </para> 887 888 <para> 889 Both header and body use the D-Bus <link linkend="type-system">type 890 system</link> and format for serializing data. 891 </para> 892 893 <sect2 id="message-protocol-messages"> 894 <title>Message Format</title> 895 896 <para> 897 A message consists of a header and a body. The header is a block of 898 values with a fixed signature and meaning. The body is a separate block 899 of values, with a signature specified in the header. 900 </para> 901 902 <para> 903 The length of the header must be a multiple of 8, allowing the body to 904 begin on an 8-byte boundary when storing the entire message in a single 905 buffer. If the header does not naturally end on an 8-byte boundary 906 up to 7 bytes of nul-initialized alignment padding must be added. 907 </para> 908 909 <para> 910 The message body need not end on an 8-byte boundary. 911 </para> 912 913 <para> 914 The maximum length of a message, including header, header alignment padding, 915 and body is 2 to the 27th power or 134217728. Implementations must not 916 send or accept messages exceeding this size. 917 </para> 918 919 <para> 920 The signature of the header is: 921 <programlisting> 922 "yyyyuua(yv)" 923 </programlisting> 924 Written out more readably, this is: 925 <programlisting> 926 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT) 927 </programlisting> 928 </para> 929 930 <para> 931 These values have the following meanings: 932 <informaltable> 933 <tgroup cols="2"> 934 <thead> 935 <row> 936 <entry>Value</entry> 937 <entry>Description</entry> 938 </row> 939 </thead> 940 <tbody> 941 <row> 942 <entry>1st <literal>BYTE</literal></entry> 943 <entry>Endianness flag; ASCII 'l' for little-endian 944 or ASCII 'B' for big-endian. Both header and body are 945 in this endianness.</entry> 946 </row> 947 <row> 948 <entry>2nd <literal>BYTE</literal></entry> 949 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored. 950 Currently-defined types are described below. 951 </entry> 952 </row> 953 <row> 954 <entry>3rd <literal>BYTE</literal></entry> 955 <entry>Bitwise OR of flags. Unknown flags 956 must be ignored. Currently-defined flags are described below. 957 </entry> 958 </row> 959 <row> 960 <entry>4th <literal>BYTE</literal></entry> 961 <entry>Major protocol version of the sending application. If 962 the major protocol version of the receiving application does not 963 match, the applications will not be able to communicate and the 964 D-Bus connection must be disconnected. The major protocol 965 version for this version of the specification is 1. 966 </entry> 967 </row> 968 <row> 969 <entry>1st <literal>UINT32</literal></entry> 970 <entry>Length in bytes of the message body, starting 971 from the end of the header. The header ends after 972 its alignment padding to an 8-boundary. 973 </entry> 974 </row> 975 <row> 976 <entry>2nd <literal>UINT32</literal></entry> 977 <entry>The serial of this message, used as a cookie 978 by the sender to identify the reply corresponding 979 to this request. This must not be zero. 980 </entry> 981 </row> 982 <row> 983 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry> 984 <entry>An array of zero or more <firstterm>header 985 fields</firstterm> where the byte is the field code, and the 986 variant is the field value. The message type determines 987 which fields are required. 988 </entry> 989 </row> 990 </tbody> 991 </tgroup> 992 </informaltable> 993 </para> 994 <para> 995 <firstterm>Message types</firstterm> that can appear in the second byte 996 of the header are: 997 <informaltable> 998 <tgroup cols="3"> 999 <thead> 1000 <row> 1001 <entry>Conventional name</entry> 1002 <entry>Decimal value</entry> 1003 <entry>Description</entry> 1004 </row> 1005 </thead> 1006 <tbody> 1007 <row> 1008 <entry><literal>INVALID</literal></entry> 1009 <entry>0</entry> 1010 <entry>This is an invalid type.</entry> 1011 </row> 1012 <row> 1013 <entry><literal>METHOD_CALL</literal></entry> 1014 <entry>1</entry> 1015 <entry>Method call.</entry> 1016 </row> 1017 <row> 1018 <entry><literal>METHOD_RETURN</literal></entry> 1019 <entry>2</entry> 1020 <entry>Method reply with returned data.</entry> 1021 </row> 1022 <row> 1023 <entry><literal>ERROR</literal></entry> 1024 <entry>3</entry> 1025 <entry>Error reply. If the first argument exists and is a 1026 string, it is an error message.</entry> 1027 </row> 1028 <row> 1029 <entry><literal>SIGNAL</literal></entry> 1030 <entry>4</entry> 1031 <entry>Signal emission.</entry> 1032 </row> 1033 </tbody> 1034 </tgroup> 1035 </informaltable> 1036 </para> 1037 <para> 1038 Flags that can appear in the third byte of the header: 1039 <informaltable> 1040 <tgroup cols="3"> 1041 <thead> 1042 <row> 1043 <entry>Conventional name</entry> 1044 <entry>Hex value</entry> 1045 <entry>Description</entry> 1046 </row> 1047 </thead> 1048 <tbody> 1049 <row> 1050 <entry><literal>NO_REPLY_EXPECTED</literal></entry> 1051 <entry>0x1</entry> 1052 <entry>This message does not expect method return replies or 1053 error replies; the reply can be omitted as an 1054 optimization. However, it is compliant with this specification 1055 to return the reply despite this flag and the only harm 1056 from doing so is extra network traffic. 1057 </entry> 1058 </row> 1059 <row> 1060 <entry><literal>NO_AUTO_START</literal></entry> 1061 <entry>0x2</entry> 1062 <entry>The bus must not launch an owner 1063 for the destination name in response to this message. 1064 </entry> 1065 </row> 1066 </tbody> 1067 </tgroup> 1068 </informaltable> 1069 </para> 1070 1071 <sect3 id="message-protocol-header-fields"> 1072 <title>Header Fields</title> 1073 1074 <para> 1075 The array at the end of the header contains <firstterm>header 1076 fields</firstterm>, where each field is a 1-byte field code followed 1077 by a field value. A header must contain the required header fields for 1078 its message type, and zero or more of any optional header 1079 fields. Future versions of this protocol specification may add new 1080 fields. Implementations must ignore fields they do not 1081 understand. Implementations must not invent their own header fields; 1082 only changes to this specification may introduce new header fields. 1083 </para> 1084 1085 <para> 1086 Again, if an implementation sees a header field code that it does not 1087 expect, it must ignore that field, as it will be part of a new 1088 (but compatible) version of this specification. This also applies 1089 to known header fields appearing in unexpected messages, for 1090 example: if a signal has a reply serial it must be ignored 1091 even though it has no meaning as of this version of the spec. 1092 </para> 1093 1094 <para> 1095 However, implementations must not send or accept known header fields 1096 with the wrong type stored in the field value. So for example a 1097 message with an <literal>INTERFACE</literal> field of type 1098 <literal>UINT32</literal> would be considered corrupt. 1099 </para> 1100 1101 <para> 1102 Here are the currently-defined header fields: 1103 <informaltable> 1104 <tgroup cols="5"> 1105 <thead> 1106 <row> 1107 <entry>Conventional Name</entry> 1108 <entry>Decimal Code</entry> 1109 <entry>Type</entry> 1110 <entry>Required In</entry> 1111 <entry>Description</entry> 1112 </row> 1113 </thead> 1114 <tbody> 1115 <row> 1116 <entry><literal>INVALID</literal></entry> 1117 <entry>0</entry> 1118 <entry>N/A</entry> 1119 <entry>not allowed</entry> 1120 <entry>Not a valid field name (error if it appears in a message)</entry> 1121 </row> 1122 <row> 1123 <entry><literal>PATH</literal></entry> 1124 <entry>1</entry> 1125 <entry><literal>OBJECT_PATH</literal></entry> 1126 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry> 1127 <entry>The object to send a call to, 1128 or the object a signal is emitted from. 1129 The special path 1130 <literal>/org/freedesktop/DBus/Local</literal> is reserved; 1131 implementations should not send messages with this path, 1132 and the reference implementation of the bus daemon will 1133 disconnect any application that attempts to do so. 1134 </entry> 1135 </row> 1136 <row> 1137 <entry><literal>INTERFACE</literal></entry> 1138 <entry>2</entry> 1139 <entry><literal>STRING</literal></entry> 1140 <entry><literal>SIGNAL</literal></entry> 1141 <entry> 1142 The interface to invoke a method call on, or 1143 that a signal is emitted from. Optional for 1144 method calls, required for signals. 1145 The special interface 1146 <literal>org.freedesktop.DBus.Local</literal> is reserved; 1147 implementations should not send messages with this 1148 interface, and the reference implementation of the bus 1149 daemon will disconnect any application that attempts to 1150 do so. 1151 </entry> 1152 </row> 1153 <row> 1154 <entry><literal>MEMBER</literal></entry> 1155 <entry>3</entry> 1156 <entry><literal>STRING</literal></entry> 1157 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry> 1158 <entry>The member, either the method name or signal name.</entry> 1159 </row> 1160 <row> 1161 <entry><literal>ERROR_NAME</literal></entry> 1162 <entry>4</entry> 1163 <entry><literal>STRING</literal></entry> 1164 <entry><literal>ERROR</literal></entry> 1165 <entry>The name of the error that occurred, for errors</entry> 1166 </row> 1167 <row> 1168 <entry><literal>REPLY_SERIAL</literal></entry> 1169 <entry>5</entry> 1170 <entry><literal>UINT32</literal></entry> 1171 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry> 1172 <entry>The serial number of the message this message is a reply 1173 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry> 1174 </row> 1175 <row> 1176 <entry><literal>DESTINATION</literal></entry> 1177 <entry>6</entry> 1178 <entry><literal>STRING</literal></entry> 1179 <entry>optional</entry> 1180 <entry>The name of the connection this message is intended for. 1181 Only used in combination with the message bus, see 1182 <xref linkend="message-bus"/>.</entry> 1183 </row> 1184 <row> 1185 <entry><literal>SENDER</literal></entry> 1186 <entry>7</entry> 1187 <entry><literal>STRING</literal></entry> 1188 <entry>optional</entry> 1189 <entry>Unique name of the sending connection. 1190 The message bus fills in this field so it is reliable; the field is 1191 only meaningful in combination with the message bus.</entry> 1192 </row> 1193 <row> 1194 <entry><literal>SIGNATURE</literal></entry> 1195 <entry>8</entry> 1196 <entry><literal>SIGNATURE</literal></entry> 1197 <entry>optional</entry> 1198 <entry>The signature of the message body. 1199 If omitted, it is assumed to be the 1200 empty signature "" (i.e. the body must be 0-length).</entry> 1201 </row> 1202 <row> 1203 <entry><literal>UNIX_FDS</literal></entry> 1204 <entry>9</entry> 1205 <entry><literal>UINT32</literal></entry> 1206 <entry>optional</entry> 1207 <entry>The number of Unix file descriptors that 1208 accompany the message. If omitted, it is assumed 1209 that no Unix file descriptors accompany the 1210 message. The actual file descriptors need to be 1211 transferred via platform specific mechanism 1212 out-of-band. They must be sent at the same time as 1213 part of the message itself. They may not be sent 1214 before the first byte of the message itself is 1215 transferred or after the last byte of the message 1216 itself.</entry> 1217 </row> 1218 </tbody> 1219 </tgroup> 1220 </informaltable> 1221 </para> 1222 </sect3> 1223 </sect2> 1224 1225 <sect2 id="message-protocol-names"> 1226 <title>Valid Names</title> 1227 <para> 1228 The various names in D-Bus messages have some restrictions. 1229 </para> 1230 <para> 1231 There is a <firstterm>maximum name length</firstterm> 1232 of 255 which applies to bus names, interfaces, and members. 1233 </para> 1234 <sect3 id="message-protocol-names-interface"> 1235 <title>Interface names</title> 1236 <para> 1237 Interfaces have names with type <literal>STRING</literal>, meaning that 1238 they must be valid UTF-8. However, there are also some 1239 additional restrictions that apply to interface names 1240 specifically: 1241 <itemizedlist> 1242 <listitem><para>Interface names are composed of 1 or more elements separated by 1243 a period ('.') character. All elements must contain at least 1244 one character. 1245 </para> 1246 </listitem> 1247 <listitem><para>Each element must only contain the ASCII characters 1248 "[A-Z][a-z][0-9]_" and must not begin with a digit. 1249 </para> 1250 </listitem> 1251 1252 <listitem><para>Interface names must contain at least one '.' (period) 1253 character (and thus at least two elements). 1254 </para></listitem> 1255 1256 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem> 1257 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem> 1258 </itemizedlist> 1259 </para> 1260 1261 <para> 1262 Interface names should start with the reversed DNS domain name of 1263 the author of the interface (in lower-case), like interface names 1264 in Java. It is conventional for the rest of the interface name 1265 to consist of words run together, with initial capital letters 1266 on all words ("CamelCase"). Several levels of hierarchy can be used. 1267 It is also a good idea to include the major version of the interface 1268 in the name, and increment it if incompatible changes are made; 1269 this way, a single object can implement several versions of an 1270 interface in parallel, if necessary. 1271 </para> 1272 1273 <para> 1274 For instance, if the owner of <literal>example.com</literal> is 1275 developing a D-Bus API for a music player, they might define 1276 interfaces called <literal>com.example.MusicPlayer1</literal>, 1277 <literal>com.example.MusicPlayer1.Track</literal> and 1278 <literal>com.example.MusicPlayer1.Seekable</literal>. 1279 </para> 1280 1281 <para> 1282 D-Bus does not distinguish between the concepts that would be 1283 called classes and interfaces in Java: either can be identified on 1284 D-Bus by an interface name. 1285 </para> 1286 </sect3> 1287 <sect3 id="message-protocol-names-bus"> 1288 <title>Bus names</title> 1289 <para> 1290 Connections have one or more bus names associated with them. 1291 A connection has exactly one bus name that is a <firstterm>unique 1292 connection name</firstterm>. The unique connection name remains 1293 with the connection for its entire lifetime. 1294 A bus name is of type <literal>STRING</literal>, 1295 meaning that it must be valid UTF-8. However, there are also 1296 some additional restrictions that apply to bus names 1297 specifically: 1298 <itemizedlist> 1299 <listitem><para>Bus names that start with a colon (':') 1300 character are unique connection names. Other bus names 1301 are called <firstterm>well-known bus names</firstterm>. 1302 </para> 1303 </listitem> 1304 <listitem><para>Bus names are composed of 1 or more elements separated by 1305 a period ('.') character. All elements must contain at least 1306 one character. 1307 </para> 1308 </listitem> 1309 <listitem><para>Each element must only contain the ASCII characters 1310 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique 1311 connection name may begin with a digit, elements in 1312 other bus names must not begin with a digit. 1313 </para> 1314 </listitem> 1315 1316 <listitem><para>Bus names must contain at least one '.' (period) 1317 character (and thus at least two elements). 1318 </para></listitem> 1319 1320 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem> 1321 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem> 1322 </itemizedlist> 1323 </para> 1324 <para> 1325 Note that the hyphen ('-') character is allowed in bus names but 1326 not in interface names. 1327 </para> 1328 1329 <para> 1330 Like <link linkend="message-protocol-names-interface">interface 1331 names</link>, well-known bus names should start with the 1332 reversed DNS domain name of the author of the interface (in 1333 lower-case), and it is conventional for the rest of the well-known 1334 bus name to consist of words run together, with initial 1335 capital letters. As with interface names, including a version 1336 number in well-known bus names is a good idea; it's possible to 1337 have the well-known bus name for more than one version 1338 simultaneously if backwards compatibility is required. 1339 </para> 1340 1341 <para> 1342 If a well-known bus name implies the presence of a "main" interface, 1343 that "main" interface is often given the same name as 1344 the well-known bus name, and situated at the corresponding object 1345 path. For instance, if the owner of <literal>example.com</literal> 1346 is developing a D-Bus API for a music player, they might define 1347 that any application that takes the well-known name 1348 <literal>com.example.MusicPlayer1</literal> should have an object 1349 at the object path <literal>/com/example/MusicPlayer1</literal> 1350 which implements the interface 1351 <literal>com.example.MusicPlayer1</literal>. 1352 </para> 1353 </sect3> 1354 <sect3 id="message-protocol-names-member"> 1355 <title>Member names</title> 1356 <para> 1357 Member (i.e. method or signal) names: 1358 <itemizedlist> 1359 <listitem><para>Must only contain the ASCII characters 1360 "[A-Z][a-z][0-9]_" and may not begin with a 1361 digit.</para></listitem> 1362 <listitem><para>Must not contain the '.' (period) character.</para></listitem> 1363 <listitem><para>Must not exceed the maximum name length.</para></listitem> 1364 <listitem><para>Must be at least 1 byte in length.</para></listitem> 1365 </itemizedlist> 1366 </para> 1367 1368 <para> 1369 It is conventional for member names on D-Bus to consist of 1370 capitalized words with no punctuation ("camel-case"). 1371 Method names should usually be verbs, such as 1372 <literal>GetItems</literal>, and signal names should usually be 1373 a description of an event, such as <literal>ItemsChanged</literal>. 1374 </para> 1375 </sect3> 1376 <sect3 id="message-protocol-names-error"> 1377 <title>Error names</title> 1378 <para> 1379 Error names have the same restrictions as interface names. 1380 </para> 1381 1382 <para> 1383 Error names have the same naming conventions as interface 1384 names, and often contain <literal>.Error.</literal>; for instance, 1385 the owner of <literal>example.com</literal> might define the 1386 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal> 1387 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>. 1388 The errors defined by D-Bus itself, such as 1389 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a 1390 similar pattern. 1391 </para> 1392 </sect3> 1393 </sect2> 1394 1395 <sect2 id="message-protocol-types"> 1396 <title>Message Types</title> 1397 <para> 1398 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and 1399 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields. 1400 This section describes these conventions. 1401 </para> 1402 <sect3 id="message-protocol-types-method"> 1403 <title>Method Calls</title> 1404 <para> 1405 Some messages invoke an operation on a remote object. These are 1406 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such 1407 messages map naturally to methods on objects in a typical program. 1408 </para> 1409 <para> 1410 A method call message is required to have a <literal>MEMBER</literal> header field 1411 indicating the name of the method. Optionally, the message has an 1412 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the 1413 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have 1414 a method with the same name, it is undefined which of the two methods 1415 will be invoked. Implementations may also choose to return an error in 1416 this ambiguous case. However, if a method name is unique 1417 implementations must not require an interface field. 1418 </para> 1419 <para> 1420 Method call messages also include a <literal>PATH</literal> field 1421 indicating the object to invoke the method on. If the call is passing 1422 through a message bus, the message will also have a 1423 <literal>DESTINATION</literal> field giving the name of the connection 1424 to receive the message. 1425 </para> 1426 <para> 1427 When an application handles a method call message, it is required to 1428 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field 1429 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The 1430 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>. 1431 </para> 1432 <para> 1433 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message 1434 are the return value(s) or "out parameters" of the method call. 1435 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown, 1436 and the call fails; no return value will be provided. It makes 1437 no sense to send multiple replies to the same method call. 1438 </para> 1439 <para> 1440 Even if a method call has no return values, a <literal>METHOD_RETURN</literal> 1441 reply is required, so the caller will know the method 1442 was successfully processed. 1443 </para> 1444 <para> 1445 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal> 1446 header field. 1447 </para> 1448 <para> 1449 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>, 1450 then as an optimization the application receiving the method 1451 call may choose to omit the reply message (regardless of 1452 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>). 1453 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal> 1454 flag and reply anyway. 1455 </para> 1456 <para> 1457 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the 1458 destination name does not exist then a program to own the destination 1459 name will be started before the message is delivered. The message 1460 will be held until the new program is successfully started or has 1461 failed to start; in case of failure, an error will be returned. This 1462 flag is only relevant in the context of a message bus, it is ignored 1463 during one-to-one communication with no intermediate bus. 1464 </para> 1465 <sect4 id="message-protocol-types-method-apis"> 1466 <title>Mapping method calls to native APIs</title> 1467 <para> 1468 APIs for D-Bus may map method calls to a method call in a specific 1469 programming language, such as C++, or may map a method call written 1470 in an IDL to a D-Bus message. 1471 </para> 1472 <para> 1473 In APIs of this nature, arguments to a method are often termed "in" 1474 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies 1475 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have 1476 "inout" arguments, which are both sent and received, i.e. the caller 1477 passes in a value which is modified. Mapped to D-Bus, an "inout" 1478 argument is equivalent to an "in" argument, followed by an "out" 1479 argument. You can't pass things "by reference" over the wire, so 1480 "inout" is purely an illusion of the in-process API. 1481 </para> 1482 <para> 1483 Given a method with zero or one return values, followed by zero or more 1484 arguments, where each argument may be "in", "out", or "inout", the 1485 caller constructs a message by appending each "in" or "inout" argument, 1486 in order. "out" arguments are not represented in the caller's message. 1487 </para> 1488 <para> 1489 The recipient constructs a reply by appending first the return value 1490 if any, then each "out" or "inout" argument, in order. 1491 "in" arguments are not represented in the reply message. 1492 </para> 1493 <para> 1494 Error replies are normally mapped to exceptions in languages that have 1495 exceptions. 1496 </para> 1497 <para> 1498 In converting from native APIs to D-Bus, it is perhaps nice to 1499 map D-Bus naming conventions ("FooBar") to native conventions 1500 such as "fooBar" or "foo_bar" automatically. This is OK 1501 as long as you can say that the native API is one that 1502 was specifically written for D-Bus. It makes the most sense 1503 when writing object implementations that will be exported 1504 over the bus. Object proxies used to invoke remote D-Bus 1505 objects probably need the ability to call any D-Bus method, 1506 and thus a magic name mapping like this could be a problem. 1507 </para> 1508 <para> 1509 This specification doesn't require anything of native API bindings; 1510 the preceding is only a suggested convention for consistency 1511 among bindings. 1512 </para> 1513 </sect4> 1514 </sect3> 1515 1516 <sect3 id="message-protocol-types-signal"> 1517 <title>Signal Emission</title> 1518 <para> 1519 Unlike method calls, signal emissions have no replies. 1520 A signal emission is simply a single message of type <literal>SIGNAL</literal>. 1521 It must have three header fields: <literal>PATH</literal> giving the object 1522 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving 1523 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required 1524 for signals, though it is optional for method calls. 1525 </para> 1526 </sect3> 1527 1528 <sect3 id="message-protocol-types-errors"> 1529 <title>Errors</title> 1530 <para> 1531 Messages of type <literal>ERROR</literal> are most commonly replies 1532 to a <literal>METHOD_CALL</literal>, but may be returned in reply 1533 to any kind of message. The message bus for example 1534 will return an <literal>ERROR</literal> in reply to a signal emission if 1535 the bus does not have enough memory to send the signal. 1536 </para> 1537 <para> 1538 An <literal>ERROR</literal> may have any arguments, but if the first 1539 argument is a <literal>STRING</literal>, it must be an error message. 1540 The error message may be logged or shown to the user 1541 in some way. 1542 </para> 1543 </sect3> 1544 1545 <sect3 id="message-protocol-types-notation"> 1546 <title>Notation in this document</title> 1547 <para> 1548 This document uses a simple pseudo-IDL to describe particular method 1549 calls and signals. Here is an example of a method call: 1550 <programlisting> 1551 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags, 1552 out UINT32 resultcode) 1553 </programlisting> 1554 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName, 1555 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument 1556 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period) 1557 characters so it's known that the last part of the name in 1558 the "IDL" is the member name. 1559 </para> 1560 <para> 1561 In C++ that might end up looking like this: 1562 <programlisting> 1563 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name, 1564 unsigned int flags); 1565 </programlisting> 1566 or equally valid, the return value could be done as an argument: 1567 <programlisting> 1568 void org::freedesktop::DBus::StartServiceByName (const char *name, 1569 unsigned int flags, 1570 unsigned int *resultcode); 1571 </programlisting> 1572 It's really up to the API designer how they want to make 1573 this look. You could design an API where the namespace wasn't used 1574 in C++, using STL or Qt, using varargs, or whatever you wanted. 1575 </para> 1576 <para> 1577 Signals are written as follows: 1578 <programlisting> 1579 org.freedesktop.DBus.NameLost (STRING name) 1580 </programlisting> 1581 Signals don't specify "in" vs. "out" because only 1582 a single direction is possible. 1583 </para> 1584 <para> 1585 It isn't especially encouraged to use this lame pseudo-IDL in actual 1586 API implementations; you might use the native notation for the 1587 language you're using, or you might use COM or CORBA IDL, for example. 1588 </para> 1589 </sect3> 1590 </sect2> 1591 1592 <sect2 id="message-protocol-handling-invalid"> 1593 <title>Invalid Protocol and Spec Extensions</title> 1594 1595 <para> 1596 For security reasons, the D-Bus protocol should be strictly parsed and 1597 validated, with the exception of defined extension points. Any invalid 1598 protocol or spec violations should result in immediately dropping the 1599 connection without notice to the other end. Exceptions should be 1600 carefully considered, e.g. an exception may be warranted for a 1601 well-understood idiosyncrasy of a widely-deployed implementation. In 1602 cases where the other end of a connection is 100% trusted and known to 1603 be friendly, skipping validation for performance reasons could also make 1604 sense in certain cases. 1605 </para> 1606 1607 <para> 1608 Generally speaking violations of the "must" requirements in this spec 1609 should be considered possible attempts to exploit security, and violations 1610 of the "should" suggestions should be considered legitimate (though perhaps 1611 they should generate an error in some cases). 1612 </para> 1613 1614 <para> 1615 The following extension points are built in to D-Bus on purpose and must 1616 not be treated as invalid protocol. The extension points are intended 1617 for use by future versions of this spec, they are not intended for third 1618 parties. At the moment, the only way a third party could extend D-Bus 1619 without breaking interoperability would be to introduce a way to negotiate new 1620 feature support as part of the auth protocol, using EXTENSION_-prefixed 1621 commands. There is not yet a standard way to negotiate features. 1622 <itemizedlist> 1623 <listitem> 1624 <para> 1625 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown 1626 commands result in an ERROR rather than a disconnect. This enables 1627 future extensions to the protocol. Commands starting with EXTENSION_ are 1628 reserved for third parties. 1629 </para> 1630 </listitem> 1631 <listitem> 1632 <para> 1633 The authentication protocol supports pluggable auth mechanisms. 1634 </para> 1635 </listitem> 1636 <listitem> 1637 <para> 1638 The address format (see <xref linkend="addresses"/>) supports new 1639 kinds of transport. 1640 </para> 1641 </listitem> 1642 <listitem> 1643 <para> 1644 Messages with an unknown type (something other than 1645 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, 1646 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored. 1647 Unknown-type messages must still be well-formed in the same way 1648 as the known messages, however. They still have the normal 1649 header and body. 1650 </para> 1651 </listitem> 1652 <listitem> 1653 <para> 1654 Header fields with an unknown or unexpected field code must be ignored, 1655 though again they must still be well-formed. 1656 </para> 1657 </listitem> 1658 <listitem> 1659 <para> 1660 New standard interfaces (with new methods and signals) can of course be added. 1661 </para> 1662 </listitem> 1663 </itemizedlist> 1664 </para> 1665 1666 </sect2> 1667 1668 </sect1> 1669 1670 <sect1 id="auth-protocol"> 1671 <title>Authentication Protocol</title> 1672 <para> 1673 Before the flow of messages begins, two applications must 1674 authenticate. A simple plain-text protocol is used for 1675 authentication; this protocol is a SASL profile, and maps fairly 1676 directly from the SASL specification. The message encoding is 1677 NOT used here, only plain text messages. 1678 </para> 1679 <para> 1680 In examples, "C:" and "S:" indicate lines sent by the client and 1681 server respectively. 1682 </para> 1683 <sect2 id="auth-protocol-overview"> 1684 <title>Protocol Overview</title> 1685 <para> 1686 The protocol is a line-based protocol, where each line ends with 1687 \r\n. Each line begins with an all-caps ASCII command name containing 1688 only the character range [A-Z_], a space, then any arguments for the 1689 command, then the \r\n ending the line. The protocol is 1690 case-sensitive. All bytes must be in the ASCII character set. 1691 1692 Commands from the client to the server are as follows: 1693 1694 <itemizedlist> 1695 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem> 1696 <listitem><para>CANCEL</para></listitem> 1697 <listitem><para>BEGIN</para></listitem> 1698 <listitem><para>DATA <data in hex encoding></para></listitem> 1699 <listitem><para>ERROR [human-readable error explanation]</para></listitem> 1700 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem> 1701 </itemizedlist> 1702 1703 From server to client are as follows: 1704 1705 <itemizedlist> 1706 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem> 1707 <listitem><para>OK <GUID in hex></para></listitem> 1708 <listitem><para>DATA <data in hex encoding></para></listitem> 1709 <listitem><para>ERROR</para></listitem> 1710 <listitem><para>AGREE_UNIX_FD</para></listitem> 1711 </itemizedlist> 1712 </para> 1713 <para> 1714 Unofficial extensions to the command set must begin with the letters 1715 "EXTENSION_", to avoid conflicts with future official commands. 1716 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF". 1717 </para> 1718 </sect2> 1719 <sect2 id="auth-nul-byte"> 1720 <title>Special credentials-passing nul byte</title> 1721 <para> 1722 Immediately after connecting to the server, the client must send a 1723 single nul byte. This byte may be accompanied by credentials 1724 information on some operating systems that use sendmsg() with 1725 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain 1726 sockets. However, the nul byte must be sent even on other kinds of 1727 socket, and even on operating systems that do not require a byte to be 1728 sent in order to transmit credentials. The text protocol described in 1729 this document begins after the single nul byte. If the first byte 1730 received from the client is not a nul byte, the server may disconnect 1731 that client. 1732 </para> 1733 <para> 1734 A nul byte in any context other than the initial byte is an error; 1735 the protocol is ASCII-only. 1736 </para> 1737 <para> 1738 The credentials sent along with the nul byte may be used with the 1739 SASL mechanism EXTERNAL. 1740 </para> 1741 </sect2> 1742 <sect2 id="auth-command-auth"> 1743 <title>AUTH command</title> 1744 <para> 1745 If an AUTH command has no arguments, it is a request to list 1746 available mechanisms. The server must respond with a REJECTED 1747 command listing the mechanisms it understands, or with an error. 1748 </para> 1749 <para> 1750 If an AUTH command specifies a mechanism, and the server supports 1751 said mechanism, the server should begin exchanging SASL 1752 challenge-response data with the client using DATA commands. 1753 </para> 1754 <para> 1755 If the server does not support the mechanism given in the AUTH 1756 command, it must send either a REJECTED command listing the mechanisms 1757 it does support, or an error. 1758 </para> 1759 <para> 1760 If the [initial-response] argument is provided, it is intended for use 1761 with mechanisms that have no initial challenge (or an empty initial 1762 challenge), as if it were the argument to an initial DATA command. If 1763 the selected mechanism has an initial challenge and [initial-response] 1764 was provided, the server should reject authentication by sending 1765 REJECTED. 1766 </para> 1767 <para> 1768 If authentication succeeds after exchanging DATA commands, 1769 an OK command must be sent to the client. 1770 </para> 1771 <para> 1772 The first octet received by the server after the \r\n of the BEGIN 1773 command from the client must be the first octet of the 1774 authenticated/encrypted stream of D-Bus messages. 1775 </para> 1776 <para> 1777 If BEGIN is received by the server, the first octet received 1778 by the client after the \r\n of the OK command must be the 1779 first octet of the authenticated/encrypted stream of D-Bus 1780 messages. 1781 </para> 1782 </sect2> 1783 <sect2 id="auth-command-cancel"> 1784 <title>CANCEL Command</title> 1785 <para> 1786 At any time up to sending the BEGIN command, the client may send a 1787 CANCEL command. On receiving the CANCEL command, the server must 1788 send a REJECTED command and abort the current authentication 1789 exchange. 1790 </para> 1791 </sect2> 1792 <sect2 id="auth-command-data"> 1793 <title>DATA Command</title> 1794 <para> 1795 The DATA command may come from either client or server, and simply 1796 contains a hex-encoded block of data to be interpreted 1797 according to the SASL mechanism in use. 1798 </para> 1799 <para> 1800 Some SASL mechanisms support sending an "empty string"; 1801 FIXME we need some way to do this. 1802 </para> 1803 </sect2> 1804 <sect2 id="auth-command-begin"> 1805 <title>BEGIN Command</title> 1806 <para> 1807 The BEGIN command acknowledges that the client has received an 1808 OK command from the server, and that the stream of messages 1809 is about to begin. 1810 </para> 1811 <para> 1812 The first octet received by the server after the \r\n of the BEGIN 1813 command from the client must be the first octet of the 1814 authenticated/encrypted stream of D-Bus messages. 1815 </para> 1816 </sect2> 1817 <sect2 id="auth-command-rejected"> 1818 <title>REJECTED Command</title> 1819 <para> 1820 The REJECTED command indicates that the current authentication 1821 exchange has failed, and further exchange of DATA is inappropriate. 1822 The client would normally try another mechanism, or try providing 1823 different responses to challenges. 1824 </para><para> 1825 Optionally, the REJECTED command has a space-separated list of 1826 available auth mechanisms as arguments. If a server ever provides 1827 a list of supported mechanisms, it must provide the same list 1828 each time it sends a REJECTED message. Clients are free to 1829 ignore all lists received after the first. 1830 </para> 1831 </sect2> 1832 <sect2 id="auth-command-ok"> 1833 <title>OK Command</title> 1834 <para> 1835 The OK command indicates that the client has been 1836 authenticated. The client may now proceed with negotiating 1837 Unix file descriptor passing. To do that it shall send 1838 NEGOTIATE_UNIX_FD to the server. 1839 </para> 1840 <para> 1841 Otherwise, the client must respond to the OK command by 1842 sending a BEGIN command, followed by its stream of messages, 1843 or by disconnecting. The server must not accept additional 1844 commands using this protocol after the BEGIN command has been 1845 received. Further communication will be a stream of D-Bus 1846 messages (optionally encrypted, as negotiated) rather than 1847 this protocol. 1848 </para> 1849 <para> 1850 If a client sends BEGIN the first octet received by the client 1851 after the \r\n of the OK command must be the first octet of 1852 the authenticated/encrypted stream of D-Bus messages. 1853 </para> 1854 <para> 1855 The OK command has one argument, which is the GUID of the server. 1856 See <xref linkend="addresses"/> for more on server GUIDs. 1857 </para> 1858 </sect2> 1859 <sect2 id="auth-command-error"> 1860 <title>ERROR Command</title> 1861 <para> 1862 The ERROR command indicates that either server or client did not 1863 know a command, does not accept the given command in the current 1864 context, or did not understand the arguments to the command. This 1865 allows the protocol to be extended; a client or server can send a 1866 command present or permitted only in new protocol versions, and if 1867 an ERROR is received instead of an appropriate response, fall back 1868 to using some other technique. 1869 </para> 1870 <para> 1871 If an ERROR is sent, the server or client that sent the 1872 error must continue as if the command causing the ERROR had never been 1873 received. However, the the server or client receiving the error 1874 should try something other than whatever caused the error; 1875 if only canceling/rejecting the authentication. 1876 </para> 1877 <para> 1878 If the D-Bus protocol changes incompatibly at some future time, 1879 applications implementing the new protocol would probably be able to 1880 check for support of the new protocol by sending a new command and 1881 receiving an ERROR from applications that don't understand it. Thus the 1882 ERROR feature of the auth protocol is an escape hatch that lets us 1883 negotiate extensions or changes to the D-Bus protocol in the future. 1884 </para> 1885 </sect2> 1886 <sect2 id="auth-command-negotiate-unix-fd"> 1887 <title>NEGOTIATE_UNIX_FD Command</title> 1888 <para> 1889 The NEGOTIATE_UNIX_FD command indicates that the client 1890 supports Unix file descriptor passing. This command may only 1891 be sent after the connection is authenticated, i.e. after OK 1892 was received by the client. This command may only be sent on 1893 transports that support Unix file descriptor passing. 1894 </para> 1895 <para> 1896 On receiving NEGOTIATE_UNIX_FD the server must respond with 1897 either AGREE_UNIX_FD or ERROR. It shall respond the former if 1898 the transport chosen supports Unix file descriptor passing and 1899 the server supports this feature. It shall respond the latter 1900 if the transport does not support Unix file descriptor 1901 passing, the server does not support this feature, or the 1902 server decides not to enable file descriptor passing due to 1903 security or other reasons. 1904 </para> 1905 </sect2> 1906 <sect2 id="auth-command-agree-unix-fd"> 1907 <title>AGREE_UNIX_FD Command</title> 1908 <para> 1909 The AGREE_UNIX_FD command indicates that the server supports 1910 Unix file descriptor passing. This command may only be sent 1911 after the connection is authenticated, and the client sent 1912 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This 1913 command may only be sent on transports that support Unix file 1914 descriptor passing. 1915 </para> 1916 <para> 1917 On receiving AGREE_UNIX_FD the client must respond with BEGIN, 1918 followed by its stream of messages, or by disconnecting. The 1919 server must not accept additional commands using this protocol 1920 after the BEGIN command has been received. Further 1921 communication will be a stream of D-Bus messages (optionally 1922 encrypted, as negotiated) rather than this protocol. 1923 </para> 1924 </sect2> 1925 <sect2 id="auth-command-future"> 1926 <title>Future Extensions</title> 1927 <para> 1928 Future extensions to the authentication and negotiation 1929 protocol are possible. For that new commands may be 1930 introduced. If a client or server receives an unknown command 1931 it shall respond with ERROR and not consider this fatal. New 1932 commands may be introduced both before, and after 1933 authentication, i.e. both before and after the OK command. 1934 </para> 1935 </sect2> 1936 <sect2 id="auth-examples"> 1937 <title>Authentication examples</title> 1938 1939 <para> 1940 <figure> 1941 <title>Example of successful magic cookie authentication</title> 1942 <programlisting> 1943 (MAGIC_COOKIE is a made up mechanism) 1944 1945 C: AUTH MAGIC_COOKIE 3138363935333137393635383634 1946 S: OK 1234deadbeef 1947 C: BEGIN 1948 </programlisting> 1949 </figure> 1950 <figure> 1951 <title>Example of finding out mechanisms then picking one</title> 1952 <programlisting> 1953 C: AUTH 1954 S: REJECTED KERBEROS_V4 SKEY 1955 C: AUTH SKEY 7ab83f32ee 1956 S: DATA 8799cabb2ea93e 1957 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f 1958 S: OK 1234deadbeef 1959 C: BEGIN 1960 </programlisting> 1961 </figure> 1962 <figure> 1963 <title>Example of client sends unknown command then falls back to regular auth</title> 1964 <programlisting> 1965 C: FOOBAR 1966 S: ERROR 1967 C: AUTH MAGIC_COOKIE 3736343435313230333039 1968 S: OK 1234deadbeef 1969 C: BEGIN 1970 </programlisting> 1971 </figure> 1972 <figure> 1973 <title>Example of server doesn't support initial auth mechanism</title> 1974 <programlisting> 1975 C: AUTH MAGIC_COOKIE 3736343435313230333039 1976 S: REJECTED KERBEROS_V4 SKEY 1977 C: AUTH SKEY 7ab83f32ee 1978 S: DATA 8799cabb2ea93e 1979 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f 1980 S: OK 1234deadbeef 1981 C: BEGIN 1982 </programlisting> 1983 </figure> 1984 <figure> 1985 <title>Example of wrong password or the like followed by successful retry</title> 1986 <programlisting> 1987 C: AUTH MAGIC_COOKIE 3736343435313230333039 1988 S: REJECTED KERBEROS_V4 SKEY 1989 C: AUTH SKEY 7ab83f32ee 1990 S: DATA 8799cabb2ea93e 1991 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f 1992 S: REJECTED 1993 C: AUTH SKEY 7ab83f32ee 1994 S: DATA 8799cabb2ea93e 1995 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f 1996 S: OK 1234deadbeef 1997 C: BEGIN 1998 </programlisting> 1999 </figure> 2000 <figure> 2001 <title>Example of skey cancelled and restarted</title> 2002 <programlisting> 2003 C: AUTH MAGIC_COOKIE 3736343435313230333039 2004 S: REJECTED KERBEROS_V4 SKEY 2005 C: AUTH SKEY 7ab83f32ee 2006 S: DATA 8799cabb2ea93e 2007 C: CANCEL 2008 S: REJECTED 2009 C: AUTH SKEY 7ab83f32ee 2010 S: DATA 8799cabb2ea93e 2011 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f 2012 S: OK 1234deadbeef 2013 C: BEGIN 2014 </programlisting> 2015 </figure> 2016 <figure> 2017 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title> 2018 <programlisting> 2019 (MAGIC_COOKIE is a made up mechanism) 2020 2021 C: AUTH MAGIC_COOKIE 3138363935333137393635383634 2022 S: OK 1234deadbeef 2023 C: NEGOTIATE_UNIX_FD 2024 S: AGREE_UNIX_FD 2025 C: BEGIN 2026 </programlisting> 2027 </figure> 2028 <figure> 2029 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title> 2030 <programlisting> 2031 (MAGIC_COOKIE is a made up mechanism) 2032 2033 C: AUTH MAGIC_COOKIE 3138363935333137393635383634 2034 S: OK 1234deadbeef 2035 C: NEGOTIATE_UNIX_FD 2036 S: ERROR 2037 C: BEGIN 2038 </programlisting> 2039 </figure> 2040 </para> 2041 </sect2> 2042 <sect2 id="auth-states"> 2043 <title>Authentication state diagrams</title> 2044 2045 <para> 2046 This section documents the auth protocol in terms of 2047 a state machine for the client and the server. This is 2048 probably the most robust way to implement the protocol. 2049 </para> 2050 2051 <sect3 id="auth-states-client"> 2052 <title>Client states</title> 2053 2054 <para> 2055 To more precisely describe the interaction between the 2056 protocol state machine and the authentication mechanisms the 2057 following notation is used: MECH(CHALL) means that the 2058 server challenge CHALL was fed to the mechanism MECH, which 2059 returns one of 2060 2061 <itemizedlist> 2062 <listitem> 2063 <para> 2064 CONTINUE(RESP) means continue the auth conversation 2065 and send RESP as the response to the server; 2066 </para> 2067 </listitem> 2068 2069 <listitem> 2070 <para> 2071 OK(RESP) means that after sending RESP to the server 2072 the client side of the auth conversation is finished 2073 and the server should return "OK"; 2074 </para> 2075 </listitem> 2076 2077 <listitem> 2078 <para> 2079 ERROR means that CHALL was invalid and could not be 2080 processed. 2081 </para> 2082 </listitem> 2083 </itemizedlist> 2084 2085 Both RESP and CHALL may be empty. 2086 </para> 2087 2088 <para> 2089 The Client starts by getting an initial response from the 2090 default mechanism and sends AUTH MECH RESP, or AUTH MECH if 2091 the mechanism did not provide an initial response. If the 2092 mechanism returns CONTINUE, the client starts in state 2093 <emphasis>WaitingForData</emphasis>, if the mechanism 2094 returns OK the client starts in state 2095 <emphasis>WaitingForOK</emphasis>. 2096 </para> 2097 2098 <para> 2099 The client should keep track of available mechanisms and 2100 which it mechanisms it has already attempted. This list is 2101 used to decide which AUTH command to send. When the list is 2102 exhausted, the client should give up and close the 2103 connection. 2104 </para> 2105 2106 <formalpara> 2107 <title><emphasis>WaitingForData</emphasis></title> 2108 <para> 2109 <itemizedlist> 2110 <listitem> 2111 <para> 2112 Receive DATA CHALL 2113 <simplelist> 2114 <member> 2115 MECH(CHALL) returns CONTINUE(RESP) → send 2116 DATA RESP, goto 2117 <emphasis>WaitingForData</emphasis> 2118 </member> 2119 2120 <member> 2121 MECH(CHALL) returns OK(RESP) → send DATA 2122 RESP, goto <emphasis>WaitingForOK</emphasis> 2123 </member> 2124 2125 <member> 2126 MECH(CHALL) returns ERROR → send ERROR 2127 [msg], goto <emphasis>WaitingForData</emphasis> 2128 </member> 2129 </simplelist> 2130 </para> 2131 </listitem> 2132 2133 <listitem> 2134 <para> 2135 Receive REJECTED [mechs] → 2136 send AUTH [next mech], goto 2137 WaitingForData or <emphasis>WaitingForOK</emphasis> 2138 </para> 2139 </listitem> 2140 <listitem> 2141 <para> 2142 Receive ERROR → send 2143 CANCEL, goto 2144 <emphasis>WaitingForReject</emphasis> 2145 </para> 2146 </listitem> 2147 <listitem> 2148 <para> 2149 Receive OK → send 2150 BEGIN, terminate auth 2151 conversation, authenticated 2152 </para> 2153 </listitem> 2154 <listitem> 2155 <para> 2156 Receive anything else → send 2157 ERROR, goto 2158 <emphasis>WaitingForData</emphasis> 2159 </para> 2160 </listitem> 2161 </itemizedlist> 2162 </para> 2163 </formalpara> 2164 2165 <formalpara> 2166 <title><emphasis>WaitingForOK</emphasis></title> 2167 <para> 2168 <itemizedlist> 2169 <listitem> 2170 <para> 2171 Receive OK → send BEGIN, terminate auth 2172 conversation, <emphasis>authenticated</emphasis> 2173 </para> 2174 </listitem> 2175 <listitem> 2176 <para> 2177 Receive REJECT [mechs] → send AUTH [next mech], 2178 goto <emphasis>WaitingForData</emphasis> or 2179 <emphasis>WaitingForOK</emphasis> 2180 </para> 2181 </listitem> 2182 2183 <listitem> 2184 <para> 2185 Receive DATA → send CANCEL, goto 2186 <emphasis>WaitingForReject</emphasis> 2187 </para> 2188 </listitem> 2189 2190 <listitem> 2191 <para> 2192 Receive ERROR → send CANCEL, goto 2193 <emphasis>WaitingForReject</emphasis> 2194 </para> 2195 </listitem> 2196 2197 <listitem> 2198 <para> 2199 Receive anything else → send ERROR, goto 2200 <emphasis>WaitingForOK</emphasis> 2201 </para> 2202 </listitem> 2203 </itemizedlist> 2204 </para> 2205 </formalpara> 2206 2207 <formalpara> 2208 <title><emphasis>WaitingForReject</emphasis></title> 2209 <para> 2210 <itemizedlist> 2211 <listitem> 2212 <para> 2213 Receive REJECT [mechs] → send AUTH [next mech], 2214 goto <emphasis>WaitingForData</emphasis> or 2215 <emphasis>WaitingForOK</emphasis> 2216 </para> 2217 </listitem> 2218 2219 <listitem> 2220 <para> 2221 Receive anything else → terminate auth 2222 conversation, disconnect 2223 </para> 2224 </listitem> 2225 </itemizedlist> 2226 </para> 2227 </formalpara> 2228 2229 </sect3> 2230 2231 <sect3 id="auth-states-server"> 2232 <title>Server states</title> 2233 2234 <para> 2235 For the server MECH(RESP) means that the client response 2236 RESP was fed to the the mechanism MECH, which returns one of 2237 2238 <itemizedlist> 2239 <listitem> 2240 <para> 2241 CONTINUE(CHALL) means continue the auth conversation and 2242 send CHALL as the challenge to the client; 2243 </para> 2244 </listitem> 2245 2246 <listitem> 2247 <para> 2248 OK means that the client has been successfully 2249 authenticated; 2250 </para> 2251 </listitem> 2252 2253 <listitem> 2254 <para> 2255 REJECT means that the client failed to authenticate or 2256 there was an error in RESP. 2257 </para> 2258 </listitem> 2259 </itemizedlist> 2260 2261 The server starts out in state 2262 <emphasis>WaitingForAuth</emphasis>. If the client is 2263 rejected too many times the server must disconnect the 2264 client. 2265 </para> 2266 2267 <formalpara> 2268 <title><emphasis>WaitingForAuth</emphasis></title> 2269 <para> 2270 <itemizedlist> 2271 2272 <listitem> 2273 <para> 2274 Receive AUTH → send REJECTED [mechs], goto 2275 <emphasis>WaitingForAuth</emphasis> 2276 </para> 2277 </listitem> 2278 2279 <listitem> 2280 <para> 2281 Receive AUTH MECH RESP 2282 2283 <simplelist> 2284 <member> 2285 MECH not valid mechanism → send REJECTED 2286 [mechs], goto 2287 <emphasis>WaitingForAuth</emphasis> 2288 </member> 2289 2290 <member> 2291 MECH(RESP) returns CONTINUE(CHALL) → send 2292 DATA CHALL, goto 2293 <emphasis>WaitingForData</emphasis> 2294 </member> 2295 2296 <member> 2297 MECH(RESP) returns OK → send OK, goto 2298 <emphasis>WaitingForBegin</emphasis> 2299 </member> 2300 2301 <member> 2302 MECH(RESP) returns REJECT → send REJECTED 2303 [mechs], goto 2304 <emphasis>WaitingForAuth</emphasis> 2305 </member> 2306 </simplelist> 2307 </para> 2308 </listitem> 2309 2310 <listitem> 2311 <para> 2312 Receive BEGIN → terminate 2313 auth conversation, disconnect 2314 </para> 2315 </listitem> 2316 2317 <listitem> 2318 <para> 2319 Receive ERROR → send REJECTED [mechs], goto 2320 <emphasis>WaitingForAuth</emphasis> 2321 </para> 2322 </listitem> 2323 2324 <listitem> 2325 <para> 2326 Receive anything else → send 2327 ERROR, goto 2328 <emphasis>WaitingForAuth</emphasis> 2329 </para> 2330 </listitem> 2331 </itemizedlist> 2332 </para> 2333 </formalpara> 2334 2335 2336 <formalpara> 2337 <title><emphasis>WaitingForData</emphasis></title> 2338 <para> 2339 <itemizedlist> 2340 <listitem> 2341 <para> 2342 Receive DATA RESP 2343 <simplelist> 2344 <member> 2345 MECH(RESP) returns CONTINUE(CHALL) → send 2346 DATA CHALL, goto 2347 <emphasis>WaitingForData</emphasis> 2348 </member> 2349 2350 <member> 2351 MECH(RESP) returns OK → send OK, goto 2352 <emphasis>WaitingForBegin</emphasis> 2353 </member> 2354 2355 <member> 2356 MECH(RESP) returns REJECT → send REJECTED 2357 [mechs], goto 2358 <emphasis>WaitingForAuth</emphasis> 2359 </member> 2360 </simplelist> 2361 </para> 2362 </listitem> 2363 2364 <listitem> 2365 <para> 2366 Receive BEGIN → terminate auth conversation, 2367 disconnect 2368 </para> 2369 </listitem> 2370 2371 <listitem> 2372 <para> 2373 Receive CANCEL → send REJECTED [mechs], goto 2374 <emphasis>WaitingForAuth</emphasis> 2375 </para> 2376 </listitem> 2377 2378 <listitem> 2379 <para> 2380 Receive ERROR → send REJECTED [mechs], goto 2381 <emphasis>WaitingForAuth</emphasis> 2382 </para> 2383 </listitem> 2384 2385 <listitem> 2386 <para> 2387 Receive anything else → send ERROR, goto 2388 <emphasis>WaitingForData</emphasis> 2389 </para> 2390 </listitem> 2391 </itemizedlist> 2392 </para> 2393 </formalpara> 2394 2395 <formalpara> 2396 <title><emphasis>WaitingForBegin</emphasis></title> 2397 <para> 2398 <itemizedlist> 2399 <listitem> 2400 <para> 2401 Receive BEGIN → terminate auth conversation, 2402 client authenticated 2403 </para> 2404 </listitem> 2405 2406 <listitem> 2407 <para> 2408 Receive CANCEL → send REJECTED [mechs], goto 2409 <emphasis>WaitingForAuth</emphasis> 2410 </para> 2411 </listitem> 2412 2413 <listitem> 2414 <para> 2415 Receive ERROR → send REJECTED [mechs], goto 2416 <emphasis>WaitingForAuth</emphasis> 2417 </para> 2418 </listitem> 2419 2420 <listitem> 2421 <para> 2422 Receive anything else → send ERROR, goto 2423 <emphasis>WaitingForBegin</emphasis> 2424 </para> 2425 </listitem> 2426 </itemizedlist> 2427 </para> 2428 </formalpara> 2429 2430 </sect3> 2431 2432 </sect2> 2433 <sect2 id="auth-mechanisms"> 2434 <title>Authentication mechanisms</title> 2435 <para> 2436 This section describes some new authentication mechanisms. 2437 D-Bus also allows any standard SASL mechanism of course. 2438 </para> 2439 <sect3 id="auth-mechanisms-sha"> 2440 <title>DBUS_COOKIE_SHA1</title> 2441 <para> 2442 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client 2443 has the ability to read a private file owned by the user being 2444 authenticated. If the client can prove that it has access to a secret 2445 cookie stored in this file, then the client is authenticated. 2446 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home 2447 directory. 2448 </para> 2449 <para> 2450 Throughout this description, "hex encoding" must output the digits 2451 from a to f in lower-case; the digits A to F must not be used 2452 in the DBUS_COOKIE_SHA1 mechanism. 2453 </para> 2454 <para> 2455 Authentication proceeds as follows: 2456 <itemizedlist> 2457 <listitem> 2458 <para> 2459 The client sends the username it would like to authenticate 2460 as, hex-encoded. 2461 </para> 2462 </listitem> 2463 <listitem> 2464 <para> 2465 The server sends the name of its "cookie context" (see below); a 2466 space character; the integer ID of the secret cookie the client 2467 must demonstrate knowledge of; a space character; then a 2468 randomly-generated challenge string, all of this hex-encoded into 2469 one, single string. 2470 </para> 2471 </listitem> 2472 <listitem> 2473 <para> 2474 The client locates the cookie and generates its own 2475 randomly-generated challenge string. The client then concatenates 2476 the server's decoded challenge, a ":" character, its own challenge, 2477 another ":" character, and the cookie. It computes the SHA-1 hash 2478 of this composite string as a hex digest. It concatenates the 2479 client's challenge string, a space character, and the SHA-1 hex 2480 digest, hex-encodes the result and sends it back to the server. 2481 </para> 2482 </listitem> 2483 <listitem> 2484 <para> 2485 The server generates the same concatenated string used by the 2486 client and computes its SHA-1 hash. It compares the hash with 2487 the hash received from the client; if the two hashes match, the 2488 client is authenticated. 2489 </para> 2490 </listitem> 2491 </itemizedlist> 2492 </para> 2493 <para> 2494 Each server has a "cookie context," which is a name that identifies a 2495 set of cookies that apply to that server. A sample context might be 2496 "org_freedesktop_session_bus". Context names must be valid ASCII, 2497 nonzero length, and may not contain the characters slash ("/"), 2498 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"), 2499 tab ("\t"), or period ("."). There is a default context, 2500 "org_freedesktop_general" that's used by servers that do not specify 2501 otherwise. 2502 </para> 2503 <para> 2504 Cookies are stored in a user's home directory, in the directory 2505 <filename>~/.dbus-keyrings/</filename>. This directory must 2506 not be readable or writable by other users. If it is, 2507 clients and servers must ignore it. The directory 2508 contains cookie files named after the cookie context. 2509 </para> 2510 <para> 2511 A cookie file contains one cookie per line. Each line 2512 has three space-separated fields: 2513 <itemizedlist> 2514 <listitem> 2515 <para> 2516 The cookie ID number, which must be a non-negative integer and 2517 may not be used twice in the same file. 2518 </para> 2519 </listitem> 2520 <listitem> 2521 <para> 2522 The cookie's creation time, in UNIX seconds-since-the-epoch 2523 format. 2524 </para> 2525 </listitem> 2526 <listitem> 2527 <para> 2528 The cookie itself, a hex-encoded random block of bytes. The cookie 2529 may be of any length, though obviously security increases 2530 as the length increases. 2531 </para> 2532 </listitem> 2533 </itemizedlist> 2534 </para> 2535 <para> 2536 Only server processes modify the cookie file. 2537 They must do so with this procedure: 2538 <itemizedlist> 2539 <listitem> 2540 <para> 2541 Create a lockfile name by appending ".lock" to the name of the 2542 cookie file. The server should attempt to create this file 2543 using <literal>O_CREAT | O_EXCL</literal>. If file creation 2544 fails, the lock fails. Servers should retry for a reasonable 2545 period of time, then they may choose to delete an existing lock 2546 to keep users from having to manually delete a stale 2547 lock. <footnote><para>Lockfiles are used instead of real file 2548 locking <literal>fcntl()</literal> because real locking 2549 implementations are still flaky on network 2550 filesystems.</para></footnote> 2551 </para> 2552 </listitem> 2553 <listitem> 2554 <para> 2555 Once the lockfile has been created, the server loads the cookie 2556 file. It should then delete any cookies that are old (the 2557 timeout can be fairly short), or more than a reasonable 2558 time in the future (so that cookies never accidentally 2559 become permanent, if the clock was set far into the future 2560 at some point). If no recent keys remain, the 2561 server may generate a new key. 2562 </para> 2563 </listitem> 2564 <listitem> 2565 <para> 2566 The pruned and possibly added-to cookie file 2567 must be resaved atomically (using a temporary 2568 file which is rename()'d). 2569 </para> 2570 </listitem> 2571 <listitem> 2572 <para> 2573 The lock must be dropped by deleting the lockfile. 2574 </para> 2575 </listitem> 2576 </itemizedlist> 2577 </para> 2578 <para> 2579 Clients need not lock the file in order to load it, 2580 because servers are required to save the file atomically. 2581 </para> 2582 </sect3> 2583 </sect2> 2584 </sect1> 2585 <sect1 id="addresses"> 2586 <title>Server Addresses</title> 2587 <para> 2588 Server addresses consist of a transport name followed by a colon, and 2589 then an optional, comma-separated list of keys and values in the form key=value. 2590 Each value is escaped. 2591 </para> 2592 <para> 2593 For example: 2594 <programlisting>unix:path=/tmp/dbus-test</programlisting> 2595 Which is the address to a unix socket with the path /tmp/dbus-test. 2596 </para> 2597 <para> 2598 Value escaping is similar to URI escaping but simpler. 2599 <itemizedlist> 2600 <listitem> 2601 <para> 2602 The set of optionally-escaped bytes is: 2603 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each 2604 <emphasis>byte</emphasis> (note, not character) which is not in the 2605 set of optionally-escaped bytes must be replaced with an ASCII 2606 percent (<literal>%</literal>) and the value of the byte in hex. 2607 The hex value must always be two digits, even if the first digit is 2608 zero. The optionally-escaped bytes may be escaped if desired. 2609 </para> 2610 </listitem> 2611 <listitem> 2612 <para> 2613 To unescape, append each byte in the value; if a byte is an ASCII 2614 percent (<literal>%</literal>) character then append the following 2615 hex value instead. It is an error if a <literal>%</literal> byte 2616 does not have two hex digits following. It is an error if a 2617 non-optionally-escaped byte is seen unescaped. 2618 </para> 2619 </listitem> 2620 </itemizedlist> 2621 The set of optionally-escaped bytes is intended to preserve address 2622 readability and convenience. 2623 </para> 2624 2625 <para> 2626 A server may specify a key-value pair with the key <literal>guid</literal> 2627 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/> 2628 describes the format of the <literal>guid</literal> field. If present, 2629 this UUID may be used to distinguish one server address from another. A 2630 server should use a different UUID for each address it listens on. For 2631 example, if a message bus daemon offers both UNIX domain socket and TCP 2632 connections, but treats clients the same regardless of how they connect, 2633 those two connections are equivalent post-connection but should have 2634 distinct UUIDs to distinguish the kinds of connection. 2635 </para> 2636 2637 <para> 2638 The intent of the address UUID feature is to allow a client to avoid 2639 opening multiple identical connections to the same server, by allowing the 2640 client to check whether an address corresponds to an already-existing 2641 connection. Comparing two addresses is insufficient, because addresses 2642 can be recycled by distinct servers, and equivalent addresses may look 2643 different if simply compared as strings (for example, the host in a TCP 2644 address can be given as an IP address or as a hostname). 2645 </para> 2646 2647 <para> 2648 Note that the address key is <literal>guid</literal> even though the 2649 rest of the API and documentation says "UUID," for historical reasons. 2650 </para> 2651 2652 <para> 2653 [FIXME clarify if attempting to connect to each is a requirement 2654 or just a suggestion] 2655 When connecting to a server, multiple server addresses can be 2656 separated by a semi-colon. The library will then try to connect 2657 to the first address and if that fails, it'll try to connect to 2658 the next one specified, and so forth. For example 2659 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting> 2660 </para> 2661 2662 </sect1> 2663 2664 <sect1 id="transports"> 2665 <title>Transports</title> 2666 <para> 2667 [FIXME we need to specify in detail each transport and its possible arguments] 2668 2669 Current transports include: unix domain sockets (including 2670 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport 2671 using in-process pipes. Future possible transports include one that 2672 tunnels over X11 protocol. 2673 </para> 2674 2675 <sect2 id="transports-unix-domain-sockets"> 2676 <title>Unix Domain Sockets</title> 2677 <para> 2678 Unix domain sockets can be either paths in the file system or on Linux 2679 kernels, they can be abstract which are similar to paths but 2680 do not show up in the file system. 2681 </para> 2682 2683 <para> 2684 When a socket is opened by the D-Bus library it truncates the path 2685 name right before the first trailing Nul byte. This is true for both 2686 normal paths and abstract paths. Note that this is a departure from 2687 previous versions of D-Bus that would create sockets with a fixed 2688 length path name. Names which were shorter than the fixed length 2689 would be padded by Nul bytes. 2690 </para> 2691 <para> 2692 Unix domain sockets are not available on Windows. 2693 </para> 2694 <sect3 id="transports-unix-domain-sockets-addresses"> 2695 <title>Server Address Format</title> 2696 <para> 2697 Unix domain socket addresses are identified by the "unix:" prefix 2698 and support the following key/value pairs: 2699 </para> 2700 <informaltable> 2701 <tgroup cols="3"> 2702 <thead> 2703 <row> 2704 <entry>Name</entry> 2705 <entry>Values</entry> 2706 <entry>Description</entry> 2707 </row> 2708 </thead> 2709 <tbody> 2710 <row> 2711 <entry>path</entry> 2712 <entry>(path)</entry> 2713 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry> 2714 </row> 2715 <row> 2716 <entry>tmpdir</entry> 2717 <entry>(path)</entry> 2718 <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> 2719 </row> 2720 <row> 2721 <entry>abstract</entry> 2722 <entry>(string)</entry> 2723 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry> 2724 </row> 2725 </tbody> 2726 </tgroup> 2727 </informaltable> 2728 </sect3> 2729 </sect2> 2730 <sect2 id="transports-launchd"> 2731 <title>launchd</title> 2732 <para> 2733 launchd is an open-source server management system that replaces init, inetd 2734 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session 2735 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX. 2736 </para> 2737 2738 <para> 2739 launchd allocates a socket and provides it with the unix path through the 2740 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process 2741 spawned by launchd (or dbus-daemon, if it was started by launchd) can access 2742 it through its environment. 2743 Other processes can query for the launchd socket by executing: 2744 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET 2745 This is normally done by the D-Bus client library so doesn't have to be done 2746 manually. 2747 </para> 2748 <para> 2749 launchd is not available on Microsoft Windows. 2750 </para> 2751 <sect3 id="transports-launchd-addresses"> 2752 <title>Server Address Format</title> 2753 <para> 2754 launchd addresses are identified by the "launchd:" prefix 2755 and support the following key/value pairs: 2756 </para> 2757 <informaltable> 2758 <tgroup cols="3"> 2759 <thead> 2760 <row> 2761 <entry>Name</entry> 2762 <entry>Values</entry> 2763 <entry>Description</entry> 2764 </row> 2765 </thead> 2766 <tbody> 2767 <row> 2768 <entry>env</entry> 2769 <entry>(environment variable)</entry> 2770 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry> 2771 </row> 2772 </tbody> 2773 </tgroup> 2774 </informaltable> 2775 </sect3> 2776 </sect2> 2777 <sect2 id="transports-systemd"> 2778 <title>systemd</title> 2779 <para> 2780 systemd is an open-source server management system that 2781 replaces init and inetd on newer Linux systems. It supports 2782 socket activation. The D-Bus systemd transport is used to acquire 2783 socket activation file descriptors from systemd and use them 2784 as D-Bus transport when the current process is spawned by 2785 socket activation from it. 2786 </para> 2787 <para> 2788 The systemd transport accepts only one or more Unix domain or 2789 TCP streams sockets passed in via socket activation. 2790 </para> 2791 <para> 2792 The systemd transport is not available on non-Linux operating systems. 2793 </para> 2794 <para> 2795 The systemd transport defines no parameter keys. 2796 </para> 2797 </sect2> 2798 <sect2 id="transports-tcp-sockets"> 2799 <title>TCP Sockets</title> 2800 <para> 2801 The tcp transport provides TCP/IP based connections between clients 2802 located on the same or different hosts. 2803 </para> 2804 <para> 2805 Using tcp transport without any additional secure authentification mechanismus 2806 over a network is unsecure. 2807 </para> 2808 <para> 2809 Windows notes: Because of the tcp stack on Windows does not provide sending 2810 credentials over a tcp connection, the EXTERNAL authentification 2811 mechanismus does not work. 2812 </para> 2813 <sect3 id="transports-tcp-sockets-addresses"> 2814 <title>Server Address Format</title> 2815 <para> 2816 TCP/IP socket addresses are identified by the "tcp:" prefix 2817 and support the following key/value pairs: 2818 </para> 2819 <informaltable> 2820 <tgroup cols="3"> 2821 <thead> 2822 <row> 2823 <entry>Name</entry> 2824 <entry>Values</entry> 2825 <entry>Description</entry> 2826 </row> 2827 </thead> 2828 <tbody> 2829 <row> 2830 <entry>host</entry> 2831 <entry>(string)</entry> 2832 <entry>dns name or ip address</entry> 2833 </row> 2834 <row> 2835 <entry>port</entry> 2836 <entry>(number)</entry> 2837 <entry>The tcp port the server will open. A zero value let the server 2838 choose a free port provided from the underlaying operating system. 2839 libdbus is able to retrieve the real used port from the server. 2840 </entry> 2841 </row> 2842 <row> 2843 <entry>family</entry> 2844 <entry>(string)</entry> 2845 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry> 2846 </row> 2847 </tbody> 2848 </tgroup> 2849 </informaltable> 2850 </sect3> 2851 </sect2> 2852 <sect2 id="transports-nonce-tcp-sockets"> 2853 <title>Nonce-secured TCP Sockets</title> 2854 <para> 2855 The nonce-tcp transport provides a secured TCP transport, using a 2856 simple authentication mechanism to ensure that only clients with read 2857 access to a certain location in the filesystem can connect to the server. 2858 The server writes a secret, the nonce, to a file and an incoming client 2859 connection is only accepted if the client sends the nonce right after 2860 the connect. The nonce mechanism requires no setup and is orthogonal to 2861 the higher-level authentication mechanisms described in the 2862 Authentication section. 2863 </para> 2864 2865 <para> 2866 On start, the server generates a random 16 byte nonce and writes it 2867 to a file in the user's temporary directory. The nonce file location 2868 is published as part of the server's D-Bus address using the 2869 "noncefile" key-value pair. 2870 2871 After an accept, the server reads 16 bytes from the socket. If the 2872 read bytes do not match the nonce stored in the nonce file, the 2873 server MUST immediately drop the connection. 2874 If the nonce match the received byte sequence, the client is accepted 2875 and the transport behaves like an unsecured tcp transport. 2876 </para> 2877 <para> 2878 After a successful connect to the server socket, the client MUST read 2879 the nonce from the file published by the server via the noncefile= 2880 key-value pair and send it over the socket. After that, the 2881 transport behaves like an unsecured tcp transport. 2882 </para> 2883 <sect3 id="transports-nonce-tcp-sockets-addresses"> 2884 <title>Server Address Format</title> 2885 <para> 2886 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix 2887 and support the following key/value pairs: 2888 </para> 2889 <informaltable> 2890 <tgroup cols="3"> 2891 <thead> 2892 <row> 2893 <entry>Name</entry> 2894 <entry>Values</entry> 2895 <entry>Description</entry> 2896 </row> 2897 </thead> 2898 <tbody> 2899 <row> 2900 <entry>host</entry> 2901 <entry>(string)</entry> 2902 <entry>dns name or ip address</entry> 2903 </row> 2904 <row> 2905 <entry>port</entry> 2906 <entry>(number)</entry> 2907 <entry>The tcp port the server will open. A zero value let the server 2908 choose a free port provided from the underlaying operating system. 2909 libdbus is able to retrieve the real used port from the server. 2910 </entry> 2911 </row> 2912 <row> 2913 <entry>family</entry> 2914 <entry>(string)</entry> 2915 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry> 2916 </row> 2917 <row> 2918 <entry>noncefile</entry> 2919 <entry>(path)</entry> 2920 <entry>file location containing the secret</entry> 2921 </row> 2922 </tbody> 2923 </tgroup> 2924 </informaltable> 2925 </sect3> 2926 </sect2> 2927 <sect2 id="transports-exec"> 2928 <title>Executed Subprocesses on Unix</title> 2929 <para> 2930 This transport forks off a process and connects its standard 2931 input and standard output with an anonymous Unix domain 2932 socket. This socket is then used for communication by the 2933 transport. This transport may be used to use out-of-process 2934 forwarder programs as basis for the D-Bus protocol. 2935 </para> 2936 <para> 2937 The forked process will inherit the standard error output and 2938 process group from the parent process. 2939 </para> 2940 <para> 2941 Executed subprocesses are not available on Windows. 2942 </para> 2943 <sect3 id="transports-exec-addresses"> 2944 <title>Server Address Format</title> 2945 <para> 2946 Executed subprocess addresses are identified by the "unixexec:" prefix 2947 and support the following key/value pairs: 2948 </para> 2949 <informaltable> 2950 <tgroup cols="3"> 2951 <thead> 2952 <row> 2953 <entry>Name</entry> 2954 <entry>Values</entry> 2955 <entry>Description</entry> 2956 </row> 2957 </thead> 2958 <tbody> 2959 <row> 2960 <entry>path</entry> 2961 <entry>(path)</entry> 2962 <entry>Path of the binary to execute, either an absolute 2963 path or a binary name that is searched for in the default 2964 search path of the OS. This corresponds to the first 2965 argument of execlp(). This key is mandatory.</entry> 2966 </row> 2967 <row> 2968 <entry>argv0</entry> 2969 <entry>(string)</entry> 2970 <entry>The program name to use when executing the 2971 binary. If omitted the same value as specified for path= 2972 will be used. This corresponds to the second argument of 2973 execlp().</entry> 2974 </row> 2975 <row> 2976 <entry>argv1, argv2, ...</entry> 2977 <entry>(string)</entry> 2978 <entry>Arguments to pass to the binary. This corresponds 2979 to the third and later arguments of execlp(). If a 2980 specific argvX is not specified no further argvY for Y > X 2981 are taken into account.</entry> 2982 </row> 2983 </tbody> 2984 </tgroup> 2985 </informaltable> 2986 </sect3> 2987 </sect2> 2988 </sect1> 2989 <sect1 id="meta-transports"> 2990 <title>Meta Transports</title> 2991 <para> 2992 Meta transports are a kind of transport with special enhancements or 2993 behavior. Currently available meta transports include: autolaunch 2994 </para> 2995 2996 <sect2 id="meta-transports-autolaunch"> 2997 <title>Autolaunch</title> 2998 <para>The autolaunch transport provides a way for dbus clients to autodetect 2999 a running dbus session bus and to autolaunch a session bus if not present. 3000 </para> 3001 <sect3 id="meta-transports-autolaunch-addresses"> 3002 <title>Server Address Format</title> 3003 <para> 3004 Autolaunch addresses uses the "autolaunch:" prefix and support the 3005 following key/value pairs: 3006 </para> 3007 <informaltable> 3008 <tgroup cols="3"> 3009 <thead> 3010 <row> 3011 <entry>Name</entry> 3012 <entry>Values</entry> 3013 <entry>Description</entry> 3014 </row> 3015 </thead> 3016 <tbody> 3017 <row> 3018 <entry>scope</entry> 3019 <entry>(string)</entry> 3020 <entry>scope of autolaunch (Windows only) 3021 <itemizedlist> 3022 <listitem> 3023 <para> 3024 "*install-path" - limit session bus to dbus installation path. 3025 The dbus installation path is determined from the location of 3026 the shared dbus library. If the library is located in a 'bin' 3027 subdirectory the installation root is the directory above, 3028 otherwise the directory where the library lives is taken as 3029 installation root. 3030 <programlisting> 3031 <install-root>/bin/[lib]dbus-1.dll 3032 <install-root>/[lib]dbus-1.dll 3033 </programlisting> 3034 </para> 3035 </listitem> 3036 <listitem> 3037 <para> 3038 "*user" - limit session bus to the recent user. 3039 </para> 3040 </listitem> 3041 <listitem> 3042 <para> 3043 other values - specify dedicated session bus like "release", 3044 "debug" or other 3045 </para> 3046 </listitem> 3047 </itemizedlist> 3048 </entry> 3049 </row> 3050 </tbody> 3051 </tgroup> 3052 </informaltable> 3053 </sect3> 3054 3055 <sect3 id="meta-transports-autolaunch-windows-implementation"> 3056 <title>Windows implementation</title> 3057 <para> 3058 On start, the server opens a platform specific transport, creates a mutex 3059 and a shared memory section containing the related session bus address. 3060 This mutex will be inspected by the dbus client library to detect a 3061 running dbus session bus. The access to the mutex and the shared memory 3062 section are protected by global locks. 3063 </para> 3064 <para> 3065 In the recent implementation the autolaunch transport uses a tcp transport 3066 on localhost with a port choosen from the operating system. This detail may 3067 change in the future. 3068 </para> 3069 <para> 3070 Disclaimer: The recent implementation is in an early state and may not 3071 work in all cirumstances and/or may have security issues. Because of this 3072 the implementation is not documentated yet. 3073 </para> 3074 </sect3> 3075 </sect2> 3076 </sect1> 3077 3078 <sect1 id="uuids"> 3079 <title>UUIDs</title> 3080 <para> 3081 A working D-Bus implementation uses universally-unique IDs in two places. 3082 First, each server address has a UUID identifying the address, 3083 as described in <xref linkend="addresses"/>. Second, each operating 3084 system kernel instance running a D-Bus client or server has a UUID 3085 identifying that kernel, retrieved by invoking the method 3086 org.freedesktop.DBus.Peer.GetMachineId() (see <xref 3087 linkend="standard-interfaces-peer"/>). 3088 </para> 3089 <para> 3090 The term "UUID" in this document is intended literally, i.e. an 3091 identifier that is universally unique. It is not intended to refer to 3092 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC. 3093 </para> 3094 <para> 3095 The UUID must contain 128 bits of data and be hex-encoded. The 3096 hex-encoded string may not contain hyphens or other non-hex-digit 3097 characters, and it must be exactly 32 characters long. To generate a 3098 UUID, the current reference implementation concatenates 96 bits of random 3099 data followed by the 32-bit time in seconds since the UNIX epoch (in big 3100 endian byte order). 3101 </para> 3102 <para> 3103 It would also be acceptable and probably better to simply generate 128 3104 bits of random data, as long as the random number generator is of high 3105 quality. The timestamp could conceivably help if the random bits are not 3106 very random. With a quality random number generator, collisions are 3107 extremely unlikely even with only 96 bits, so it's somewhat academic. 3108 </para> 3109 <para> 3110 Implementations should, however, stick to random data for the first 96 bits 3111 of the UUID. 3112 </para> 3113 </sect1> 3114 3115 <sect1 id="standard-interfaces"> 3116 <title>Standard Interfaces</title> 3117 <para> 3118 See <xref linkend="message-protocol-types-notation"/> for details on 3119 the notation used in this section. There are some standard interfaces 3120 that may be useful across various D-Bus applications. 3121 </para> 3122 <sect2 id="standard-interfaces-peer"> 3123 <title><literal>org.freedesktop.DBus.Peer</literal></title> 3124 <para> 3125 The <literal>org.freedesktop.DBus.Peer</literal> interface 3126 has two methods: 3127 <programlisting> 3128 org.freedesktop.DBus.Peer.Ping () 3129 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid) 3130 </programlisting> 3131 </para> 3132 <para> 3133 On receipt of the <literal>METHOD_CALL</literal> message 3134 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do 3135 nothing other than reply with a <literal>METHOD_RETURN</literal> as 3136 usual. It does not matter which object path a ping is sent to. The 3137 reference implementation handles this method automatically. 3138 </para> 3139 <para> 3140 On receipt of the <literal>METHOD_CALL</literal> message 3141 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should 3142 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded 3143 UUID representing the identity of the machine the process is running on. 3144 This UUID must be the same for all processes on a single system at least 3145 until that system next reboots. It should be the same across reboots 3146 if possible, but this is not always possible to implement and is not 3147 guaranteed. 3148 It does not matter which object path a GetMachineId is sent to. The 3149 reference implementation handles this method automatically. 3150 </para> 3151 <para> 3152 The UUID is intended to be per-instance-of-the-operating-system, so may represent 3153 a virtual machine running on a hypervisor, rather than a physical machine. 3154 Basically if two processes see the same UUID, they should also see the same 3155 shared memory, UNIX domain sockets, process IDs, and other features that require 3156 a running OS kernel in common between the processes. 3157 </para> 3158 <para> 3159 The UUID is often used where other programs might use a hostname. Hostnames 3160 can change without rebooting, however, or just be "localhost" - so the UUID 3161 is more robust. 3162 </para> 3163 <para> 3164 <xref linkend="uuids"/> explains the format of the UUID. 3165 </para> 3166 </sect2> 3167 3168 <sect2 id="standard-interfaces-introspectable"> 3169 <title><literal>org.freedesktop.DBus.Introspectable</literal></title> 3170 <para> 3171 This interface has one method: 3172 <programlisting> 3173 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data) 3174 </programlisting> 3175 </para> 3176 <para> 3177 Objects instances may implement 3178 <literal>Introspect</literal> which returns an XML description of 3179 the object, including its interfaces (with signals and methods), objects 3180 below it in the object path tree, and its properties. 3181 </para> 3182 <para> 3183 <xref linkend="introspection-format"/> describes the format of this XML string. 3184 </para> 3185 </sect2> 3186 <sect2 id="standard-interfaces-properties"> 3187 <title><literal>org.freedesktop.DBus.Properties</literal></title> 3188 <para> 3189 Many native APIs will have a concept of object <firstterm>properties</firstterm> 3190 or <firstterm>attributes</firstterm>. These can be exposed via the 3191 <literal>org.freedesktop.DBus.Properties</literal> interface. 3192 </para> 3193 <para> 3194 <programlisting> 3195 org.freedesktop.DBus.Properties.Get (in STRING interface_name, 3196 in STRING property_name, 3197 out VARIANT value); 3198 org.freedesktop.DBus.Properties.Set (in STRING interface_name, 3199 in STRING property_name, 3200 in VARIANT value); 3201 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name, 3202 out DICT<STRING,VARIANT> props); 3203 </programlisting> 3204 </para> 3205 <para> 3206 It is conventional to give D-Bus properties names consisting of 3207 capitalized words without punctuation ("CamelCase"), like 3208 <link linkend="message-protocol-names-member">member names</link>. 3209 For instance, the GObject property 3210 <literal>connection-status</literal> or the Qt property 3211 <literal>connectionStatus</literal> could be represented on D-Bus 3212 as <literal>ConnectionStatus</literal>. 3213 </para> 3214 <para> 3215 Strictly speaking, D-Bus property names are not required to follow 3216 the same naming restrictions as member names, but D-Bus property 3217 names that would not be valid member names (in particular, 3218 GObject-style dash-separated property names) can cause interoperability 3219 problems and should be avoided. 3220 </para> 3221 <para> 3222 The available properties and whether they are writable can be determined 3223 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>, 3224 see <xref linkend="standard-interfaces-introspectable"/>. 3225 </para> 3226 <para> 3227 An empty string may be provided for the interface name; in this case, 3228 if there are multiple properties on an object with the same name, 3229 the results are undefined (picking one by according to an arbitrary 3230 deterministic rule, or returning an error, are the reasonable 3231 possibilities). 3232 </para> 3233 <para> 3234 If one or more properties change on an object, the 3235 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal> 3236 signal may be emitted (this signal was added in 0.14): 3237 </para> 3238 <para> 3239 <programlisting> 3240 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name, 3241 DICT<STRING,VARIANT> changed_properties, 3242 ARRAY<STRING> invalidated_properties); 3243 </programlisting> 3244 </para> 3245 <para> 3246 where <literal>changed_properties</literal> is a dictionary 3247 containing the changed properties with the new values and 3248 <literal>invalidated_properties</literal> is an array of 3249 properties that changed but the value is not conveyed. 3250 </para> 3251 <para> 3252 Whether the <literal>PropertiesChanged</literal> signal is 3253 supported can be determined by calling 3254 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note 3255 that the signal may be supported for an object but it may 3256 differ how whether and how it is used on a per-property basis 3257 (for e.g. performance or security reasons). Each property (or 3258 the parent interface) must be annotated with the 3259 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal> 3260 annotation to convey this (usually the default value 3261 <literal>true</literal> is sufficient meaning that the 3262 annotation does not need to be used). See <xref 3263 linkend="introspection-format"/> for details on this 3264 annotation. 3265 </para> 3266 </sect2> 3267 3268 <sect2 id="standard-interfaces-objectmanager"> 3269 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title> 3270 <para> 3271 An API can optionally make use of this interface for one or 3272 more sub-trees of objects. The root of each sub-tree implements 3273 this interface so other applications can get all objects, 3274 interfaces and properties in a single method call. It is 3275 appropriate to use this interface if users of the tree of 3276 objects are expected to be interested in all interfaces of all 3277 objects in the tree; a more granular API should be used if 3278 users of the objects are expected to be interested in a small 3279 subset of the objects, a small subset of their interfaces, or 3280 both. 3281 </para> 3282 <para> 3283 The method that applications can use to get all objects and 3284 properties is <literal>GetManagedObjects</literal>: 3285 </para> 3286 <para> 3287 <programlisting> 3288 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties); 3289 </programlisting> 3290 </para> 3291 <para> 3292 The return value of this method is a dict whose keys are 3293 object paths. All returned object paths are children of the 3294 object path implementing this interface, i.e. their object 3295 paths start with the ObjectManager's object path plus '/'. 3296 </para> 3297 <para> 3298 Each value is a dict whose keys are interfaces names. Each 3299 value in this inner dict is the same dict that would be 3300 returned by the <link 3301 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link> 3302 method for that combination of object path and interface. If 3303 an interface has no properties, the empty dict is returned. 3304 </para> 3305 <para> 3306 Changes are emitted using the following two signals: 3307 </para> 3308 <para> 3309 <programlisting> 3310 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path, 3311 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties); 3312 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path, 3313 ARRAY<STRING> interfaces); 3314 </programlisting> 3315 </para> 3316 <para> 3317 The <literal>InterfacesAdded</literal> signal is emitted when 3318 either a new object is added or when an existing object gains 3319 one or more interfaces. The 3320 <literal>InterfacesRemoved</literal> signal is emitted 3321 whenever an object is removed or it loses one or more 3322 interfaces. The second parameter of the 3323 <literal>InterfacesAdded</literal> signal contains a dict with 3324 the interfaces and properties (if any) that have been added to 3325 the given object path. Similarly, the second parameter of the 3326 <literal>InterfacesRemoved</literal> signal contains an array 3327 of the interfaces that were removed. Note that changes on 3328 properties on existing interfaces are not reported using this 3329 interface - an application should also monitor the existing <link 3330 linkend="standard-interfaces-properties">PropertiesChanged</link> 3331 signal on each object. 3332 </para> 3333 <para> 3334 Applications SHOULD NOT export objects that are children of an 3335 object (directly or otherwise) implementing this interface but 3336 which are not returned in the reply from the 3337 <literal>GetManagedObjects()</literal> method of this 3338 interface on the given object. 3339 </para> 3340 <para> 3341 The intent of the <literal>ObjectManager</literal> interface 3342 is to make it easy to write a robust client 3343 implementation. The trivial client implementation only needs 3344 to make two method calls: 3345 </para> 3346 <para> 3347 <programlisting> 3348 org.freedesktop.DBus.AddMatch (bus_proxy, 3349 "type='signal',name='org.example.App',path_namespace='/org/example/App'"); 3350 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy); 3351 </programlisting> 3352 </para> 3353 <para> 3354 on the message bus and the remote application's 3355 <literal>ObjectManager</literal>, respectively. Whenever a new 3356 remote object is created (or an existing object gains a new 3357 interface), the <literal>InterfacesAdded</literal> signal is 3358 emitted, and since this signal contains all properties for the 3359 interfaces, no calls to the 3360 <literal>org.freedesktop.Properties</literal> interface on the 3361 remote object are needed. Additionally, since the initial 3362 <literal>AddMatch()</literal> rule already includes signal 3363 messages from the newly created child object, no new 3364 <literal>AddMatch()</literal> call is needed. 3365 </para> 3366 3367 <para> 3368 <emphasis> 3369 The <literal>org.freedesktop.DBus.ObjectManager</literal> 3370 interface was added in version 0.17 of the D-Bus 3371 specification. 3372 </emphasis> 3373 </para> 3374 </sect2> 3375 </sect1> 3376 3377 <sect1 id="introspection-format"> 3378 <title>Introspection Data Format</title> 3379 <para> 3380 As described in <xref linkend="standard-interfaces-introspectable"/>, 3381 objects may be introspected at runtime, returning an XML string 3382 that describes the object. The same XML format may be used in 3383 other contexts as well, for example as an "IDL" for generating 3384 static language bindings. 3385 </para> 3386 <para> 3387 Here is an example of introspection data: 3388 <programlisting> 3389 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN" 3390 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd"> 3391 <node name="/org/freedesktop/sample_object"> 3392 <interface name="org.freedesktop.SampleInterface"> 3393 <method name="Frobate"> 3394 <arg name="foo" type="i" direction="in"/> 3395 <arg name="bar" type="s" direction="out"/> 3396 <arg name="baz" type="a{us}" direction="out"/> 3397 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/> 3398 </method> 3399 <method name="Bazify"> 3400 <arg name="bar" type="(iiu)" direction="in"/> 3401 <arg name="bar" type="v" direction="out"/> 3402 </method> 3403 <method name="Mogrify"> 3404 <arg name="bar" type="(iiav)" direction="in"/> 3405 </method> 3406 <signal name="Changed"> 3407 <arg name="new_value" type="b"/> 3408 </signal> 3409 <property name="Bar" type="y" access="readwrite"/> 3410 </interface> 3411 <node name="child_of_sample_object"/> 3412 <node name="another_child_of_sample_object"/> 3413 </node> 3414 </programlisting> 3415 </para> 3416 <para> 3417 A more formal DTD and spec needs writing, but here are some quick notes. 3418 <itemizedlist> 3419 <listitem> 3420 <para> 3421 Only the root <node> element can omit the node name, as it's 3422 known to be the object that was introspected. If the root 3423 <node> does have a name attribute, it must be an absolute 3424 object path. If child <node> have object paths, they must be 3425 relative. 3426 </para> 3427 </listitem> 3428 <listitem> 3429 <para> 3430 If a child <node> has any sub-elements, then they 3431 must represent a complete introspection of the child. 3432 If a child <node> is empty, then it may or may 3433 not have sub-elements; the child must be introspected 3434 in order to find out. The intent is that if an object 3435 knows that its children are "fast" to introspect 3436 it can go ahead and return their information, but 3437 otherwise it can omit it. 3438 </para> 3439 </listitem> 3440 <listitem> 3441 <para> 3442 The direction element on <arg> may be omitted, 3443 in which case it defaults to "in" for method calls 3444 and "out" for signals. Signals only allow "out" 3445 so while direction may be specified, it's pointless. 3446 </para> 3447 </listitem> 3448 <listitem> 3449 <para> 3450 The possible directions are "in" and "out", 3451 unlike CORBA there is no "inout" 3452 </para> 3453 </listitem> 3454 <listitem> 3455 <para> 3456 The possible property access flags are 3457 "readwrite", "read", and "write" 3458 </para> 3459 </listitem> 3460 <listitem> 3461 <para> 3462 Multiple interfaces can of course be listed for 3463 one <node>. 3464 </para> 3465 </listitem> 3466 <listitem> 3467 <para> 3468 The "name" attribute on arguments is optional. 3469 </para> 3470 </listitem> 3471 </itemizedlist> 3472 </para> 3473 <para> 3474 Method, interface, property, and signal elements may have 3475 "annotations", which are generic key/value pairs of metadata. 3476 They are similar conceptually to Java's annotations and C# attributes. 3477 Well-known annotations: 3478 </para> 3479 <informaltable> 3480 <tgroup cols="3"> 3481 <thead> 3482 <row> 3483 <entry>Name</entry> 3484 <entry>Values (separated by ,)</entry> 3485 <entry>Description</entry> 3486 </row> 3487 </thead> 3488 <tbody> 3489 <row> 3490 <entry>org.freedesktop.DBus.Deprecated</entry> 3491 <entry>true,false</entry> 3492 <entry>Whether or not the entity is deprecated; defaults to false</entry> 3493 </row> 3494 <row> 3495 <entry>org.freedesktop.DBus.GLib.CSymbol</entry> 3496 <entry>(string)</entry> 3497 <entry>The C symbol; may be used for methods and interfaces</entry> 3498 </row> 3499 <row> 3500 <entry>org.freedesktop.DBus.Method.NoReply</entry> 3501 <entry>true,false</entry> 3502 <entry>If set, don't expect a reply to the method call; defaults to false.</entry> 3503 </row> 3504 <row> 3505 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry> 3506 <entry>true,invalidates,false</entry> 3507 <entry> 3508 <para> 3509 If set to <literal>false</literal>, the 3510 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal> 3511 signal, see <xref 3512 linkend="standard-interfaces-properties"/> is not 3513 guaranteed to be emitted if the property changes. 3514 </para> 3515 <para> 3516 If set to <literal>invalidates</literal> the signal 3517 is emitted but the value is not included in the 3518 signal. 3519 </para> 3520 <para> 3521 If set to <literal>true</literal> the signal is 3522 emitted with the value included. 3523 </para> 3524 <para> 3525 The value for the annotation defaults to 3526 <literal>true</literal> if the enclosing interface 3527 element does not specify the annotation. Otherwise it 3528 defaults to the value specified in the enclosing 3529 interface element. 3530 </para> 3531 </entry> 3532 </row> 3533 </tbody> 3534 </tgroup> 3535 </informaltable> 3536 </sect1> 3537 <sect1 id="message-bus"> 3538 <title>Message Bus Specification</title> 3539 <sect2 id="message-bus-overview"> 3540 <title>Message Bus Overview</title> 3541 <para> 3542 The message bus accepts connections from one or more applications. 3543 Once connected, applications can exchange messages with other 3544 applications that are also connected to the bus. 3545 </para> 3546 <para> 3547 In order to route messages among connections, the message bus keeps a 3548 mapping from names to connections. Each connection has one 3549 unique-for-the-lifetime-of-the-bus name automatically assigned. 3550 Applications may request additional names for a connection. Additional 3551 names are usually "well-known names" such as 3552 "org.freedesktop.TextEditor". When a name is bound to a connection, 3553 that connection is said to <firstterm>own</firstterm> the name. 3554 </para> 3555 <para> 3556 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>. 3557 This name routes messages to the bus, allowing applications to make 3558 administrative requests. For example, applications can ask the bus 3559 to assign a name to a connection. 3560 </para> 3561 <para> 3562 Each name may have <firstterm>queued owners</firstterm>. When an 3563 application requests a name for a connection and the name is already in 3564 use, the bus will optionally add the connection to a queue waiting for 3565 the name. If the current owner of the name disconnects or releases 3566 the name, the next connection in the queue will become the new owner. 3567 </para> 3568 3569 <para> 3570 This feature causes the right thing to happen if you start two text 3571 editors for example; the first one may request "org.freedesktop.TextEditor", 3572 and the second will be queued as a possible owner of that name. When 3573 the first exits, the second will take over. 3574 </para> 3575 3576 <para> 3577 Applications may send <firstterm>unicast messages</firstterm> to 3578 a specific recipient or to the message bus itself, or 3579 <firstterm>broadcast messages</firstterm> to all interested recipients. 3580 See <xref linkend="message-bus-routing"/> for details. 3581 </para> 3582 </sect2> 3583 3584 <sect2 id="message-bus-names"> 3585 <title>Message Bus Names</title> 3586 <para> 3587 Each connection has at least one name, assigned at connection time and 3588 returned in response to the 3589 <literal>org.freedesktop.DBus.Hello</literal> method call. This 3590 automatically-assigned name is called the connection's <firstterm>unique 3591 name</firstterm>. Unique names are never reused for two different 3592 connections to the same bus. 3593 </para> 3594 <para> 3595 Ownership of a unique name is a prerequisite for interaction with 3596 the message bus. It logically follows that the unique name is always 3597 the first name that an application comes to own, and the last 3598 one that it loses ownership of. 3599 </para> 3600 <para> 3601 Unique connection names must begin with the character ':' (ASCII colon 3602 character); bus names that are not unique names must not begin 3603 with this character. (The bus must reject any attempt by an application 3604 to manually request a name beginning with ':'.) This restriction 3605 categorically prevents "spoofing"; messages sent to a unique name 3606 will always go to the expected connection. 3607 </para> 3608 <para> 3609 When a connection is closed, all the names that it owns are deleted (or 3610 transferred to the next connection in the queue if any). 3611 </para> 3612 <para> 3613 A connection can request additional names to be associated with it using 3614 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref 3615 linkend="message-protocol-names-bus"/> describes the format of a valid 3616 name. These names can be released again using the 3617 <literal>org.freedesktop.DBus.ReleaseName</literal> message. 3618 </para> 3619 3620 <sect3 id="bus-messages-request-name"> 3621 <title><literal>org.freedesktop.DBus.RequestName</literal></title> 3622 <para> 3623 As a method: 3624 <programlisting> 3625 UINT32 RequestName (in STRING name, in UINT32 flags) 3626 </programlisting> 3627 Message arguments: 3628 <informaltable> 3629 <tgroup cols="3"> 3630 <thead> 3631 <row> 3632 <entry>Argument</entry> 3633 <entry>Type</entry> 3634 <entry>Description</entry> 3635 </row> 3636 </thead> 3637 <tbody> 3638 <row> 3639 <entry>0</entry> 3640 <entry>STRING</entry> 3641 <entry>Name to request</entry> 3642 </row> 3643 <row> 3644 <entry>1</entry> 3645 <entry>UINT32</entry> 3646 <entry>Flags</entry> 3647 </row> 3648 </tbody> 3649 </tgroup> 3650 </informaltable> 3651 Reply arguments: 3652 <informaltable> 3653 <tgroup cols="3"> 3654 <thead> 3655 <row> 3656 <entry>Argument</entry> 3657 <entry>Type</entry> 3658 <entry>Description</entry> 3659 </row> 3660 </thead> 3661 <tbody> 3662 <row> 3663 <entry>0</entry> 3664 <entry>UINT32</entry> 3665 <entry>Return value</entry> 3666 </row> 3667 </tbody> 3668 </tgroup> 3669 </informaltable> 3670 </para> 3671 <para> 3672 This method call should be sent to 3673 <literal>org.freedesktop.DBus</literal> and asks the message bus to 3674 assign the given name to the method caller. Each name maintains a 3675 queue of possible owners, where the head of the queue is the primary 3676 or current owner of the name. Each potential owner in the queue 3677 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and 3678 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName 3679 call. When RequestName is invoked the following occurs: 3680 <itemizedlist> 3681 <listitem> 3682 <para> 3683 If the method caller is currently the primary owner of the name, 3684 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE 3685 values are updated with the values from the new RequestName call, 3686 and nothing further happens. 3687 </para> 3688 </listitem> 3689 3690 <listitem> 3691 <para> 3692 If the current primary owner (head of the queue) has 3693 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName 3694 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then 3695 the caller of RequestName replaces the current primary owner at 3696 the head of the queue and the current primary owner moves to the 3697 second position in the queue. If the caller of RequestName was 3698 in the queue previously its flags are updated with the values from 3699 the new RequestName in addition to moving it to the head of the queue. 3700 </para> 3701 </listitem> 3702 3703 <listitem> 3704 <para> 3705 If replacement is not possible, and the method caller is 3706 currently in the queue but not the primary owner, its flags are 3707 updated with the values from the new RequestName call. 3708 </para> 3709 </listitem> 3710 3711 <listitem> 3712 <para> 3713 If replacement is not possible, and the method caller is 3714 currently not in the queue, the method caller is appended to the 3715 queue. 3716 </para> 3717 </listitem> 3718 3719 <listitem> 3720 <para> 3721 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE 3722 set and is not the primary owner, it is removed from the 3723 queue. This can apply to the previous primary owner (if it 3724 was replaced) or the method caller (if it updated the 3725 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the 3726 queue, or if it was just added to the queue with that flag set). 3727 </para> 3728 </listitem> 3729 </itemizedlist> 3730 </para> 3731 <para> 3732 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the 3733 queue," even if another application already in the queue had specified 3734 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner 3735 that does not allow replacement goes away, and the next primary owner 3736 does allow replacement. In this case, queued items that specified 3737 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis> 3738 automatically replace the new primary owner. In other words, 3739 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the 3740 time RequestName is called. This is deliberate to avoid an infinite loop 3741 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT 3742 and DBUS_NAME_FLAG_REPLACE_EXISTING. 3743 </para> 3744 <para> 3745 The flags argument contains any of the following values logically ORed 3746 together: 3747 3748 <informaltable> 3749 <tgroup cols="3"> 3750 <thead> 3751 <row> 3752 <entry>Conventional Name</entry> 3753 <entry>Value</entry> 3754 <entry>Description</entry> 3755 </row> 3756 </thead> 3757 <tbody> 3758 <row> 3759 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry> 3760 <entry>0x1</entry> 3761 <entry> 3762 3763 If an application A specifies this flag and succeeds in 3764 becoming the owner of the name, and another application B 3765 later calls RequestName with the 3766 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A 3767 will lose ownership and receive a 3768 <literal>org.freedesktop.DBus.NameLost</literal> signal, and 3769 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT 3770 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING 3771 is not specified by application B, then application B will not replace 3772 application A as the owner. 3773 3774 </entry> 3775 </row> 3776 <row> 3777 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry> 3778 <entry>0x2</entry> 3779 <entry> 3780 3781 Try to replace the current owner if there is one. If this 3782 flag is not set the application will only become the owner of 3783 the name if there is no current owner. If this flag is set, 3784 the application will replace the current owner if 3785 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT. 3786 3787 </entry> 3788 </row> 3789 <row> 3790 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry> 3791 <entry>0x4</entry> 3792 <entry> 3793 3794 Without this flag, if an application requests a name that is 3795 already owned, the application will be placed in a queue to 3796 own the name when the current owner gives it up. If this 3797 flag is given, the application will not be placed in the 3798 queue, the request for the name will simply fail. This flag 3799 also affects behavior when an application is replaced as 3800 name owner; by default the application moves back into the 3801 waiting queue, unless this flag was provided when the application 3802 became the name owner. 3803 3804 </entry> 3805 </row> 3806 </tbody> 3807 </tgroup> 3808 </informaltable> 3809 3810 The return code can be one of the following values: 3811 3812 <informaltable> 3813 <tgroup cols="3"> 3814 <thead> 3815 <row> 3816 <entry>Conventional Name</entry> 3817 <entry>Value</entry> 3818 <entry>Description</entry> 3819 </row> 3820 </thead> 3821 <tbody> 3822 <row> 3823 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry> 3824 <entry>1</entry> <entry>The caller is now the primary owner of 3825 the name, replacing any previous owner. Either the name had no 3826 owner before, or the caller specified 3827 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified 3828 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry> 3829 </row> 3830 <row> 3831 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry> 3832 <entry>2</entry> 3833 3834 <entry>The name already had an owner, 3835 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either 3836 the current owner did not specify 3837 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting 3838 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING. 3839 </entry> 3840 </row> 3841 <row> 3842 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry> 3843 <entry>The name already has an owner, 3844 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either 3845 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the 3846 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not 3847 specified by the requesting application.</entry> 3848 </row> 3849 <row> 3850 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry> 3851 <entry>4</entry> 3852 <entry>The application trying to request ownership of a name is already the owner of it.</entry> 3853 </row> 3854 </tbody> 3855 </tgroup> 3856 </informaltable> 3857 </para> 3858 </sect3> 3859 3860 <sect3 id="bus-messages-release-name"> 3861 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title> 3862 <para> 3863 As a method: 3864 <programlisting> 3865 UINT32 ReleaseName (in STRING name) 3866 </programlisting> 3867 Message arguments: 3868 <informaltable> 3869 <tgroup cols="3"> 3870 <thead> 3871 <row> 3872 <entry>Argument</entry> 3873 <entry>Type</entry> 3874 <entry>Description</entry> 3875 </row> 3876 </thead> 3877 <tbody> 3878 <row> 3879 <entry>0</entry> 3880 <entry>STRING</entry> 3881 <entry>Name to release</entry> 3882 </row> 3883 </tbody> 3884 </tgroup> 3885 </informaltable> 3886 Reply arguments: 3887 <informaltable> 3888 <tgroup cols="3"> 3889 <thead> 3890 <row> 3891 <entry>Argument</entry> 3892 <entry>Type</entry> 3893 <entry>Description</entry> 3894 </row> 3895 </thead> 3896 <tbody> 3897 <row> 3898 <entry>0</entry> 3899 <entry>UINT32</entry> 3900 <entry>Return value</entry> 3901 </row> 3902 </tbody> 3903 </tgroup> 3904 </informaltable> 3905 </para> 3906 <para> 3907 This method call should be sent to 3908 <literal>org.freedesktop.DBus</literal> and asks the message bus to 3909 release the method caller's claim to the given name. If the caller is 3910 the primary owner, a new primary owner will be selected from the 3911 queue if any other owners are waiting. If the caller is waiting in 3912 the queue for the name, the caller will removed from the queue and 3913 will not be made an owner of the name if it later becomes available. 3914 If there are no other owners in the queue for the name, it will be 3915 removed from the bus entirely. 3916 3917 The return code can be one of the following values: 3918 3919 <informaltable> 3920 <tgroup cols="3"> 3921 <thead> 3922 <row> 3923 <entry>Conventional Name</entry> 3924 <entry>Value</entry> 3925 <entry>Description</entry> 3926 </row> 3927 </thead> 3928 <tbody> 3929 <row> 3930 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry> 3931 <entry>1</entry> <entry>The caller has released his claim on 3932 the given name. Either the caller was the primary owner of 3933 the name, and the name is now unused or taken by somebody 3934 waiting in the queue for the name, or the caller was waiting 3935 in the queue for the name and has now been removed from the 3936 queue.</entry> 3937 </row> 3938 <row> 3939 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry> 3940 <entry>2</entry> 3941 <entry>The given name does not exist on this bus.</entry> 3942 </row> 3943 <row> 3944 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry> 3945 <entry>3</entry> 3946 <entry>The caller was not the primary owner of this name, 3947 and was also not waiting in the queue to own this name.</entry> 3948 </row> 3949 </tbody> 3950 </tgroup> 3951 </informaltable> 3952 </para> 3953 </sect3> 3954 3955 <sect3 id="bus-messages-list-queued-owners"> 3956 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title> 3957 <para> 3958 As a method: 3959 <programlisting> 3960 ARRAY of STRING ListQueuedOwners (in STRING name) 3961 </programlisting> 3962 Message arguments: 3963 <informaltable> 3964 <tgroup cols="3"> 3965 <thead> 3966 <row> 3967 <entry>Argument</entry> 3968 <entry>Type</entry> 3969 <entry>Description</entry> 3970 </row> 3971 </thead> 3972 <tbody> 3973 <row> 3974 <entry>0</entry> 3975 <entry>STRING</entry> 3976 <entry>The well-known bus name to query, such as 3977 <literal>com.example.cappuccino</literal></entry> 3978 </row> 3979 </tbody> 3980 </tgroup> 3981 </informaltable> 3982 Reply arguments: 3983 <informaltable> 3984 <tgroup cols="3"> 3985 <thead> 3986 <row> 3987 <entry>Argument</entry> 3988 <entry>Type</entry> 3989 <entry>Description</entry> 3990 </row> 3991 </thead> 3992 <tbody> 3993 <row> 3994 <entry>0</entry> 3995 <entry>ARRAY of STRING</entry> 3996 <entry>The unique bus names of connections currently queued 3997 for the name</entry> 3998 </row> 3999 </tbody> 4000 </tgroup> 4001 </informaltable> 4002 </para> 4003 <para> 4004 This method call should be sent to 4005 <literal>org.freedesktop.DBus</literal> and lists the connections 4006 currently queued for a bus name (see 4007 <xref linkend="term-queued-owner"/>). 4008 </para> 4009 </sect3> 4010 </sect2> 4011 4012 <sect2 id="message-bus-routing"> 4013 <title>Message Bus Message Routing</title> 4014 4015 <para> 4016 Messages may have a <literal>DESTINATION</literal> field (see <xref 4017 linkend="message-protocol-header-fields"/>), resulting in a 4018 <firstterm>unicast message</firstterm>. If the 4019 <literal>DESTINATION</literal> field is present, it specifies a message 4020 recipient by name. Method calls and replies normally specify this field. 4021 The message bus must send messages (of any type) with the 4022 <literal>DESTINATION</literal> field set to the specified recipient, 4023 regardless of whether the recipient has set up a match rule matching 4024 the message. 4025 </para> 4026 4027 <para> 4028 When the message bus receives a signal, if the 4029 <literal>DESTINATION</literal> field is absent, it is considered to 4030 be a <firstterm>broadcast signal</firstterm>, and is sent to all 4031 applications with <firstterm>message matching rules</firstterm> that 4032 match the message. Most signal messages are broadcasts. 4033 </para> 4034 4035 <para> 4036 Unicast signal messages (those with a <literal>DESTINATION</literal> 4037 field) are not commonly used, but they are treated like any unicast 4038 message: they are delivered to the specified receipient, 4039 regardless of its match rules. One use for unicast signals is to 4040 avoid a race condition in which a signal is emitted before the intended 4041 recipient can call <xref linkend="bus-messages-add-match"/> to 4042 receive that signal: if the signal is sent directly to that recipient 4043 using a unicast message, it does not need to add a match rule at all, 4044 and there is no race condition. Another use for unicast signals, 4045 on message buses whose security policy prevents eavesdropping, is to 4046 send sensitive information which should only be visible to one 4047 recipient. 4048 </para> 4049 4050 <para> 4051 When the message bus receives a method call, if the 4052 <literal>DESTINATION</literal> field is absent, the call is taken to be 4053 a standard one-to-one message and interpreted by the message bus 4054 itself. For example, sending an 4055 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no 4056 <literal>DESTINATION</literal> will cause the message bus itself to 4057 reply to the ping immediately; the message bus will not make this 4058 message visible to other applications. 4059 </para> 4060 4061 <para> 4062 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if 4063 the ping message were sent with a <literal>DESTINATION</literal> name of 4064 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be 4065 forwarded, and the Yoyodyne Corporation screensaver application would be 4066 expected to reply to the ping. 4067 </para> 4068 4069 <para> 4070 Message bus implementations may impose a security policy which 4071 prevents certain messages from being sent or received. 4072 When a message cannot be sent or received due to a security 4073 policy, the message bus should send an error reply, unless the 4074 original message had the <literal>NO_REPLY</literal> flag. 4075 </para> 4076 4077 <sect3 id="message-bus-routing-eavesdropping"> 4078 <title>Eavesdropping</title> 4079 <para> 4080 Receiving a unicast message whose <literal>DESTINATION</literal> 4081 indicates a different recipient is called 4082 <firstterm>eavesdropping</firstterm>. On a message bus which acts as 4083 a security boundary (like the standard system bus), the security 4084 policy should usually prevent eavesdropping, since unicast messages 4085 are normally kept private and may contain security-sensitive 4086 information. 4087 </para> 4088 4089 <para> 4090 Eavesdropping is mainly useful for debugging tools, such as 4091 the <literal>dbus-monitor</literal> tool in the reference 4092 implementation of D-Bus. Tools which eavesdrop on the message bus 4093 should be careful to avoid sending a reply or error in response to 4094 messages intended for a different client. 4095 </para> 4096 4097 <para> 4098 Clients may attempt to eavesdrop by adding match rules 4099 (see <xref linkend="message-bus-routing-match-rules"/>) containing 4100 the <literal>eavesdrop='true'</literal> match. If the message bus' 4101 security policy does not allow eavesdropping, the match rule can 4102 still be added, but will not have any practical effect. For 4103 compatibility with older message bus implementations, if adding such 4104 a match rule results in an error reply, the client may fall back to 4105 adding the same rule with the <literal>eavesdrop</literal> match 4106 omitted. 4107 </para> 4108 </sect3> 4109 4110 <sect3 id="message-bus-routing-match-rules"> 4111 <title>Match Rules</title> 4112 <para> 4113 An important part of the message bus routing protocol is match 4114 rules. Match rules describe the messages that should be sent to a 4115 client, based on the contents of the message. Broadcast signals 4116 are only sent to clients which have a suitable match rule: this 4117 avoids waking up client processes to deal with signals that are 4118 not relevant to that client. 4119 </para> 4120 <para> 4121 Messages that list a client as their <literal>DESTINATION</literal> 4122 do not need to match the client's match rules, and are sent to that 4123 client regardless. As a result, match rules are mainly used to 4124 receive a subset of broadcast signals. 4125 </para> 4126 <para> 4127 Match rules can also be used for eavesdropping 4128 (see <xref linkend="message-bus-routing-eavesdropping"/>), 4129 if the security policy of the message bus allows it. 4130 </para> 4131 <para> 4132 Match rules are added using the AddMatch bus method 4133 (see <xref linkend="bus-messages-add-match"/>). Rules are 4134 specified as a string of comma separated key/value pairs. 4135 Excluding a key from the rule indicates a wildcard match. 4136 For instance excluding the the member from a match rule but 4137 adding a sender would let all messages from that sender through. 4138 An example of a complete rule would be 4139 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'" 4140 </para> 4141 <para> 4142 The following table describes the keys that can be used to create 4143 a match rule: 4144 The following table summarizes the D-Bus types. 4145 <informaltable> 4146 <tgroup cols="3"> 4147 <thead> 4148 <row> 4149 <entry>Key</entry> 4150 <entry>Possible Values</entry> 4151 <entry>Description</entry> 4152 </row> 4153 </thead> 4154 <tbody> 4155 <row> 4156 <entry><literal>type</literal></entry> 4157 <entry>'signal', 'method_call', 'method_return', 'error'</entry> 4158 <entry>Match on the message type. An example of a type match is type='signal'</entry> 4159 </row> 4160 <row> 4161 <entry><literal>sender</literal></entry> 4162 <entry>A bus or unique name (see <xref linkend="term-bus-name"/> 4163 and <xref linkend="term-unique-name"/> respectively) 4164 </entry> 4165 <entry>Match messages sent by a particular sender. An example of a sender match 4166 is sender='org.freedesktop.Hal'</entry> 4167 </row> 4168 <row> 4169 <entry><literal>interface</literal></entry> 4170 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry> 4171 <entry>Match messages sent over or to a particular interface. An example of an 4172 interface match is interface='org.freedesktop.Hal.Manager'. 4173 If a message omits the interface header, it must not match any rule 4174 that specifies this key.</entry> 4175 </row> 4176 <row> 4177 <entry><literal>member</literal></entry> 4178 <entry>Any valid method or signal name</entry> 4179 <entry>Matches messages which have the give method or signal name. An example of 4180 a member match is member='NameOwnerChanged'</entry> 4181 </row> 4182 <row> 4183 <entry><literal>path</literal></entry> 4184 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry> 4185 <entry>Matches messages which are sent from or to the given object. An example of a 4186 path match is path='/org/freedesktop/Hal/Manager'</entry> 4187 </row> 4188 <row> 4189 <entry><literal>path_namespace</literal></entry> 4190 <entry>An object path</entry> 4191 <entry> 4192 <para> 4193 Matches messages which are sent from or to an 4194 object for which the object path is either the 4195 given value, or that value followed by one or 4196 more path components. 4197 </para> 4198 4199 <para> 4200 For example, 4201 <literal>path_namespace='/com/example/foo'</literal> 4202 would match signals sent by 4203 <literal>/com/example/foo</literal> 4204 or by 4205 <literal>/com/example/foo/bar</literal>, 4206 but not by 4207 <literal>/com/example/foobar</literal>. 4208 </para> 4209 4210 <para> 4211 Using both <literal>path</literal> and 4212 <literal>path_namespace</literal> in the same match 4213 rule is not allowed. 4214 </para> 4215 4216 <para> 4217 <emphasis> 4218 This match key was added in version 0.16 of the 4219 D-Bus specification and implemented by the bus 4220 daemon in dbus 1.5.0 and later. 4221 </emphasis> 4222 </para> 4223 </entry> 4224 </row> 4225 <row> 4226 <entry><literal>destination</literal></entry> 4227 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry> 4228 <entry>Matches messages which are being sent to the given unique name. An 4229 example of a destination match is destination=':1.0'</entry> 4230 </row> 4231 <row> 4232 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry> 4233 <entry>Any string</entry> 4234 <entry>Arg matches are special and are used for further restricting the 4235 match based on the arguments in the body of a message. Only arguments of type 4236 STRING can be matched in this way. An example of an argument match 4237 would be arg3='Foo'. Only argument indexes from 0 to 63 should be 4238 accepted.</entry> 4239 </row> 4240 <row> 4241 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry> 4242 <entry>Any string</entry> 4243 <entry> 4244 <para>Argument path matches provide a specialised form of wildcard matching for 4245 path-like namespaces. They can match arguments whose type is either STRING or 4246 OBJECT_PATH. As with normal argument matches, 4247 if the argument is exactly equal to the string given in the match 4248 rule then the rule is satisfied. Additionally, there is also a 4249 match when either the string given in the match rule or the 4250 appropriate message argument ends with '/' and is a prefix of the 4251 other. An example argument path match is arg0path='/aa/bb/'. This 4252 would match messages with first arguments of '/', '/aa/', 4253 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match 4254 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para> 4255 4256 <para>This is intended for monitoring directories in file system-like 4257 hierarchies, as used in the <citetitle>dconf</citetitle> configuration 4258 system. An application interested in all nodes in a particular hierarchy would 4259 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could 4260 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to 4261 represent a modification to the bar property, or a signal with zeroth 4262 argument <literal>"/ca/example/"</literal> to represent atomic modification of 4263 many properties within that directory, and the interested application would be 4264 notified in both cases.</para> 4265 <para> 4266 <emphasis> 4267 This match key was added in version 0.12 of the 4268 D-Bus specification, implemented for STRING 4269 arguments by the bus daemon in dbus 1.2.0 and later, 4270 and implemented for OBJECT_PATH arguments in dbus 1.5.0 4271 and later. 4272 </emphasis> 4273 </para> 4274 </entry> 4275 </row> 4276 <row> 4277 <entry><literal>arg0namespace</literal></entry> 4278 <entry>Like a bus name, except that the string is not 4279 required to contain a '.' (period)</entry> 4280 <entry> 4281 <para>Match messages whose first argument is of type STRING, and is a bus name 4282 or interface name within the specified namespace. This is primarily intended 4283 for watching name owner changes for a group of related bus names, rather than 4284 for a single name or all name changes.</para> 4285 4286 <para>Because every valid interface name is also a valid 4287 bus name, this can also be used for messages whose 4288 first argument is an interface name.</para> 4289 4290 <para>For example, the match rule 4291 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal> 4292 matches name owner changes for bus names such as 4293 <literal>com.example.backend.foo</literal>, 4294 <literal>com.example.backend.foo.bar</literal>, and 4295 <literal>com.example.backend</literal> itself.</para> 4296 4297 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para> 4298 <para> 4299 <emphasis> 4300 This match key was added in version 0.16 of the 4301 D-Bus specification and implemented by the bus 4302 daemon in dbus 1.5.0 and later. 4303 </emphasis> 4304 </para> 4305 </entry> 4306 </row> 4307 <row> 4308 <entry><literal>eavesdrop</literal></entry> 4309 <entry><literal>'true'</literal>, <literal>'false'</literal></entry> 4310 <entry>Since D-Bus 1.5.6, match rules do not 4311 match messages which have a <literal>DESTINATION</literal> 4312 field unless the match rule specifically 4313 requests this 4314 (see <xref linkend="message-bus-routing-eavesdropping"/>) 4315 by specifying <literal>eavesdrop='true'</literal> 4316 in the match rule. <literal>eavesdrop='false'</literal> 4317 restores the default behaviour. Messages are 4318 delivered to their <literal>DESTINATION</literal> 4319 regardless of match rules, so this match does not 4320 affect normal delivery of unicast messages. 4321 If the message bus has a security policy which forbids 4322 eavesdropping, this match may still be used without error, 4323 but will not have any practical effect. 4324 In older versions of D-Bus, this match was not allowed 4325 in match rules, and all match rules behaved as if 4326 <literal>eavesdrop='true'</literal> had been used. 4327 </entry> 4328 </row> 4329 </tbody> 4330 </tgroup> 4331 </informaltable> 4332 </para> 4333 </sect3> 4334 </sect2> 4335 <sect2 id="message-bus-starting-services"> 4336 <title>Message Bus Starting Services</title> 4337 <para> 4338 The message bus can start applications on behalf of other applications. 4339 In CORBA terms, this would be called <firstterm>activation</firstterm>. 4340 An application that can be started in this way is called a 4341 <firstterm>service</firstterm>. 4342 </para> 4343 <para> 4344 With D-Bus, starting a service is normally done by name. That is, 4345 applications ask the message bus to start some program that will own a 4346 well-known name, such as <literal>org.freedesktop.TextEditor</literal>. 4347 This implies a contract documented along with the name 4348 <literal>org.freedesktop.TextEditor</literal> for which objects 4349 the owner of that name will provide, and what interfaces those 4350 objects will have. 4351 </para> 4352 <para> 4353 To find an executable corresponding to a particular name, the bus daemon 4354 looks for <firstterm>service description files</firstterm>. Service 4355 description files define a mapping from names to executables. Different 4356 kinds of message bus will look for these files in different places, see 4357 <xref linkend="message-bus-types"/>. 4358 </para> 4359 <para> 4360 Service description files have the ".service" file 4361 extension. The message bus will only load service description files 4362 ending with .service; all other files will be ignored. The file format 4363 is similar to that of <ulink 4364 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop 4365 entries</ulink>. All service description files must be in UTF-8 4366 encoding. To ensure that there will be no name collisions, service files 4367 must be namespaced using the same mechanism as messages and service 4368 names. 4369 </para> 4370 4371 <para> 4372 [FIXME the file format should be much better specified than "similar to 4373 .desktop entries" esp. since desktop entries are already 4374 badly-specified. ;-)] 4375 These sections from the specification apply to service files as well: 4376 4377 <itemizedlist> 4378 <listitem><para>General syntax</para></listitem> 4379 <listitem><para>Comment format</para></listitem> 4380 </itemizedlist> 4381 4382 <figure> 4383 <title>Example service description file</title> 4384 <programlisting> 4385 # Sample service description file 4386 [D-BUS Service] 4387 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf; 4388 Exec=/usr/libexec/gconfd-2 4389 </programlisting> 4390 </figure> 4391 </para> 4392 <para> 4393 When an application asks to start a service by name, the bus daemon tries to 4394 find a service that will own that name. It then tries to spawn the 4395 executable associated with it. If this fails, it will report an 4396 error. [FIXME what happens if two .service files offer the same service; 4397 what kind of error is reported, should we have a way for the client to 4398 choose one?] 4399 </para> 4400 <para> 4401 The executable launched will have the environment variable 4402 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the 4403 message bus so it can connect and request the appropriate names. 4404 </para> 4405 <para> 4406 The executable being launched may want to know whether the message bus 4407 starting it is one of the well-known message buses (see <xref 4408 linkend="message-bus-types"/>). To facilitate this, the bus must also set 4409 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one 4410 of the well-known buses. The currently-defined values for this variable 4411 are <literal>system</literal> for the systemwide message bus, 4412 and <literal>session</literal> for the per-login-session message 4413 bus. The new executable must still connect to the address given 4414 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the 4415 resulting connection is to the well-known bus. 4416 </para> 4417 <para> 4418 [FIXME there should be a timeout somewhere, either specified 4419 in the .service file, by the client, or just a global value 4420 and if the client being activated fails to connect within that 4421 timeout, an error should be sent back.] 4422 </para> 4423 4424 <sect3 id="message-bus-starting-services-scope"> 4425 <title>Message Bus Service Scope</title> 4426 <para> 4427 The "scope" of a service is its "per-", such as per-session, 4428 per-machine, per-home-directory, or per-display. The reference 4429 implementation doesn't yet support starting services in a different 4430 scope from the message bus itself. So e.g. if you start a service 4431 on the session bus its scope is per-session. 4432 </para> 4433 <para> 4434 We could add an optional scope to a bus name. For example, for 4435 per-(display,session pair), we could have a unique ID for each display 4436 generated automatically at login and set on screen 0 by executing a 4437 special "set display ID" binary. The ID would be stored in a 4438 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of 4439 random bytes. This ID would then be used to scope names. 4440 Starting/locating a service could be done by ID-name pair rather than 4441 only by name. 4442 </para> 4443 <para> 4444 Contrast this with a per-display scope. To achieve that, we would 4445 want a single bus spanning all sessions using a given display. 4446 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal> 4447 property on screen 0 of the display, pointing to this bus. 4448 </para> 4449 </sect3> 4450 </sect2> 4451 4452 <sect2 id="message-bus-types"> 4453 <title>Well-known Message Bus Instances</title> 4454 <para> 4455 Two standard message bus instances are defined here, along with how 4456 to locate them and where their service files live. 4457 </para> 4458 <sect3 id="message-bus-types-login"> 4459 <title>Login session message bus</title> 4460 <para> 4461 Each time a user logs in, a <firstterm>login session message 4462 bus</firstterm> may be started. All applications in the user's login 4463 session may interact with one another using this message bus. 4464 </para> 4465 <para> 4466 The address of the login session message bus is given 4467 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment 4468 variable. If that variable is not set, applications may 4469 also try to read the address from the X Window System root 4470 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>. 4471 The root window property must have type <literal>STRING</literal>. 4472 The environment variable should have precedence over the 4473 root window property. 4474 </para> 4475 <para>The address of the login session message bus is given in the 4476 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If 4477 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string 4478 "autolaunch:", the system should use platform-specific methods of 4479 locating a running D-Bus session server, or starting one if a running 4480 instance cannot be found. Note that this mechanism is not recommended 4481 for attempting to determine if a daemon is running. It is inherently 4482 racy to attempt to make this determination, since the bus daemon may 4483 be started just before or just after the determination is made. 4484 Therefore, it is recommended that applications do not try to make this 4485 determination for their functionality purposes, and instead they 4486 should attempt to start the server.</para> 4487 4488 <sect4 id="message-bus-types-login-x-windows"> 4489 <title>X Windowing System</title> 4490 <para> 4491 For the X Windowing System, the application must locate the 4492 window owner of the selection represented by the atom formed by 4493 concatenating: 4494 <itemizedlist> 4495 <listitem> 4496 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para> 4497 </listitem> 4498 4499 <listitem> 4500 <para>the current user's username</para> 4501 </listitem> 4502 4503 <listitem> 4504 <para>the literal character '_' (underscore)</para> 4505 </listitem> 4506 4507 <listitem> 4508 <para>the machine's ID</para> 4509 </listitem> 4510 </itemizedlist> 4511 </para> 4512 4513 <para> 4514 The following properties are defined for the window that owns 4515 this X selection: 4516 <informaltable frame="all"> 4517 <tgroup cols="2"> 4518 <tbody> 4519 <row> 4520 <entry> 4521 <para>Atom</para> 4522 </entry> 4523 4524 <entry> 4525 <para>meaning</para> 4526 </entry> 4527 </row> 4528 4529 <row> 4530 <entry> 4531 <para>_DBUS_SESSION_BUS_ADDRESS</para> 4532 </entry> 4533 4534 <entry> 4535 <para>the actual address of the server socket</para> 4536 </entry> 4537 </row> 4538 4539 <row> 4540 <entry> 4541 <para>_DBUS_SESSION_BUS_PID</para> 4542 </entry> 4543 4544 <entry> 4545 <para>the PID of the server process</para> 4546 </entry> 4547 </row> 4548 </tbody> 4549 </tgroup> 4550 </informaltable> 4551 </para> 4552 4553 <para> 4554 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be 4555 present in this window. 4556 </para> 4557 4558 <para> 4559 If the X selection cannot be located or if reading the 4560 properties from the window fails, the implementation MUST conclude 4561 that there is no D-Bus server running and proceed to start a new 4562 server. (See below on concurrency issues) 4563 </para> 4564 4565 <para> 4566 Failure to connect to the D-Bus server address thus obtained 4567 MUST be treated as a fatal connection error and should be reported 4568 to the application. 4569 </para> 4570 4571 <para> 4572 As an alternative, an implementation MAY find the information 4573 in the following file located in the current user's home directory, 4574 in subdirectory .dbus/session-bus/: 4575 <itemizedlist> 4576 <listitem> 4577 <para>the machine's ID</para> 4578 </listitem> 4579 4580 <listitem> 4581 <para>the literal character '-' (dash)</para> 4582 </listitem> 4583 4584 <listitem> 4585 <para>the X display without the screen number, with the 4586 following prefixes removed, if present: ":", "localhost:" 4587 ."localhost.localdomain:". That is, a display of 4588 "localhost:10.0" produces just the number "10"</para> 4589 </listitem> 4590 </itemizedlist> 4591 </para> 4592 4593 <para> 4594 The contents of this file NAME=value assignment pairs and 4595 lines starting with # are comments (no comments are allowed 4596 otherwise). The following variable names are defined: 4597 <informaltable 4598 frame="all"> 4599 <tgroup cols="2"> 4600 <tbody> 4601 <row> 4602 <entry> 4603 <para>Variable</para> 4604 </entry> 4605 4606 <entry> 4607 <para>meaning</para> 4608 </entry> 4609 </row> 4610 4611 <row> 4612 <entry> 4613 <para>DBUS_SESSION_BUS_ADDRESS</para> 4614 </entry> 4615 4616 <entry> 4617 <para>the actual address of the server socket</para> 4618 </entry> 4619 </row> 4620 4621 <row> 4622 <entry> 4623 <para>DBUS_SESSION_BUS_PID</para> 4624 </entry> 4625 4626 <entry> 4627 <para>the PID of the server process</para> 4628 </entry> 4629 </row> 4630 4631 <row> 4632 <entry> 4633 <para>DBUS_SESSION_BUS_WINDOWID</para> 4634 </entry> 4635 4636 <entry> 4637 <para>the window ID</para> 4638 </entry> 4639 </row> 4640 </tbody> 4641 </tgroup> 4642 </informaltable> 4643 </para> 4644 4645 <para> 4646 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present 4647 in this file. 4648 </para> 4649 4650 <para> 4651 Failure to open this file MUST be interpreted as absence of a 4652 running server. Therefore, the implementation MUST proceed to 4653 attempting to launch a new bus server if the file cannot be 4654 opened. 4655 </para> 4656 4657 <para> 4658 However, success in opening this file MUST NOT lead to the 4659 conclusion that the server is running. Thus, a failure to connect to 4660 the bus address obtained by the alternative method MUST NOT be 4661 considered a fatal error. If the connection cannot be established, 4662 the implementation MUST proceed to check the X selection settings or 4663 to start the server on its own. 4664 </para> 4665 4666 <para> 4667 If the implementation concludes that the D-Bus server is not 4668 running it MUST attempt to start a new server and it MUST also 4669 ensure that the daemon started as an effect of the "autolaunch" 4670 mechanism provides the lookup mechanisms described above, so 4671 subsequent calls can locate the newly started server. The 4672 implementation MUST also ensure that if two or more concurrent 4673 initiations happen, only one server remains running and all other 4674 initiations are able to obtain the address of this server and 4675 connect to it. In other words, the implementation MUST ensure that 4676 the X selection is not present when it attempts to set it, without 4677 allowing another process to set the selection between the 4678 verification and the setting (e.g., by using XGrabServer / 4679 XungrabServer). 4680 </para> 4681 </sect4> 4682 <sect4> 4683 <title></title> 4684 <para> 4685 On Unix systems, the session bus should search for .service files 4686 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined 4687 by the 4688 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>. 4689 Implementations may also search additional locations, which 4690 should be searched with lower priority than anything in 4691 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults; 4692 for example, the reference implementation also 4693 looks in <literal>${datadir}/dbus-1/services</literal> as 4694 set at compile time. 4695 </para> 4696 <para> 4697 As described in the XDG Base Directory Specification, software 4698 packages should install their session .service files to their 4699 configured <literal>${datadir}/dbus-1/services</literal>, 4700 where <literal>${datadir}</literal> is as defined by the GNU 4701 coding standards. System administrators or users can arrange 4702 for these service files to be read by setting XDG_DATA_DIRS or by 4703 symlinking them into the default locations. 4704 </para> 4705 </sect4> 4706 </sect3> 4707 <sect3 id="message-bus-types-system"> 4708 <title>System message bus</title> 4709 <para> 4710 A computer may have a <firstterm>system message bus</firstterm>, 4711 accessible to all applications on the system. This message bus may be 4712 used to broadcast system events, such as adding new hardware devices, 4713 changes in the printer queue, and so forth. 4714 </para> 4715 <para> 4716 The address of the system message bus is given 4717 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment 4718 variable. If that variable is not set, applications should try 4719 to connect to the well-known address 4720 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>. 4721 <footnote> 4722 <para> 4723 The D-Bus reference implementation actually honors the 4724 <literal>$(localstatedir)</literal> configure option 4725 for this address, on both client and server side. 4726 </para> 4727 </footnote> 4728 </para> 4729 <para> 4730 On Unix systems, the system bus should default to searching 4731 for .service files in 4732 <literal>/usr/local/share/dbus-1/system-services</literal>, 4733 <literal>/usr/share/dbus-1/system-services</literal> and 4734 <literal>/lib/dbus-1/system-services</literal>, with that order 4735 of precedence. It may also search other implementation-specific 4736 locations, but should not vary these locations based on environment 4737 variables. 4738 <footnote> 4739 <para> 4740 The system bus is security-sensitive and is typically executed 4741 by an init system with a clean environment. Its launch helper 4742 process is particularly security-sensitive, and specifically 4743 clears its own environment. 4744 </para> 4745 </footnote> 4746 </para> 4747 <para> 4748 Software packages should install their system .service 4749 files to their configured 4750 <literal>${datadir}/dbus-1/system-services</literal>, 4751 where <literal>${datadir}</literal> is as defined by the GNU 4752 coding standards. System administrators can arrange 4753 for these service files to be read by editing the system bus' 4754 configuration file or by symlinking them into the default 4755 locations. 4756 </para> 4757 </sect3> 4758 </sect2> 4759 4760 <sect2 id="message-bus-messages"> 4761 <title>Message Bus Messages</title> 4762 <para> 4763 The special message bus name <literal>org.freedesktop.DBus</literal> 4764 responds to a number of additional messages. 4765 </para> 4766 4767 <sect3 id="bus-messages-hello"> 4768 <title><literal>org.freedesktop.DBus.Hello</literal></title> 4769 <para> 4770 As a method: 4771 <programlisting> 4772 STRING Hello () 4773 </programlisting> 4774 Reply arguments: 4775 <informaltable> 4776 <tgroup cols="3"> 4777 <thead> 4778 <row> 4779 <entry>Argument</entry> 4780 <entry>Type</entry> 4781 <entry>Description</entry> 4782 </row> 4783 </thead> 4784 <tbody> 4785 <row> 4786 <entry>0</entry> 4787 <entry>STRING</entry> 4788 <entry>Unique name assigned to the connection</entry> 4789 </row> 4790 </tbody> 4791 </tgroup> 4792 </informaltable> 4793 </para> 4794 <para> 4795 Before an application is able to send messages to other applications 4796 it must send the <literal>org.freedesktop.DBus.Hello</literal> message 4797 to the message bus to obtain a unique name. If an application without 4798 a unique name tries to send a message to another application, or a 4799 message to the message bus itself that isn't the 4800 <literal>org.freedesktop.DBus.Hello</literal> message, it will be 4801 disconnected from the bus. 4802 </para> 4803 <para> 4804 There is no corresponding "disconnect" request; if a client wishes to 4805 disconnect from the bus, it simply closes the socket (or other 4806 communication channel). 4807 </para> 4808 </sect3> 4809 <sect3 id="bus-messages-list-names"> 4810 <title><literal>org.freedesktop.DBus.ListNames</literal></title> 4811 <para> 4812 As a method: 4813 <programlisting> 4814 ARRAY of STRING ListNames () 4815 </programlisting> 4816 Reply arguments: 4817 <informaltable> 4818 <tgroup cols="3"> 4819 <thead> 4820 <row> 4821 <entry>Argument</entry> 4822 <entry>Type</entry> 4823 <entry>Description</entry> 4824 </row> 4825 </thead> 4826 <tbody> 4827 <row> 4828 <entry>0</entry> 4829 <entry>ARRAY of STRING</entry> 4830 <entry>Array of strings where each string is a bus name</entry> 4831 </row> 4832 </tbody> 4833 </tgroup> 4834 </informaltable> 4835 </para> 4836 <para> 4837 Returns a list of all currently-owned names on the bus. 4838 </para> 4839 </sect3> 4840 <sect3 id="bus-messages-list-activatable-names"> 4841 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title> 4842 <para> 4843 As a method: 4844 <programlisting> 4845 ARRAY of STRING ListActivatableNames () 4846 </programlisting> 4847 Reply arguments: 4848 <informaltable> 4849 <tgroup cols="3"> 4850 <thead> 4851 <row> 4852 <entry>Argument</entry> 4853 <entry>Type</entry> 4854 <entry>Description</entry> 4855 </row> 4856 </thead> 4857 <tbody> 4858 <row> 4859 <entry>0</entry> 4860 <entry>ARRAY of STRING</entry> 4861 <entry>Array of strings where each string is a bus name</entry> 4862 </row> 4863 </tbody> 4864 </tgroup> 4865 </informaltable> 4866 </para> 4867 <para> 4868 Returns a list of all names that can be activated on the bus. 4869 </para> 4870 </sect3> 4871 <sect3 id="bus-messages-name-exists"> 4872 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title> 4873 <para> 4874 As a method: 4875 <programlisting> 4876 BOOLEAN NameHasOwner (in STRING name) 4877 </programlisting> 4878 Message arguments: 4879 <informaltable> 4880 <tgroup cols="3"> 4881 <thead> 4882 <row> 4883 <entry>Argument</entry> 4884 <entry>Type</entry> 4885 <entry>Description</entry> 4886 </row> 4887 </thead> 4888 <tbody> 4889 <row> 4890 <entry>0</entry> 4891 <entry>STRING</entry> 4892 <entry>Name to check</entry> 4893 </row> 4894 </tbody> 4895 </tgroup> 4896 </informaltable> 4897 Reply arguments: 4898 <informaltable> 4899 <tgroup cols="3"> 4900 <thead> 4901 <row> 4902 <entry>Argument</entry> 4903 <entry>Type</entry> 4904 <entry>Description</entry> 4905 </row> 4906 </thead> 4907 <tbody> 4908 <row> 4909 <entry>0</entry> 4910 <entry>BOOLEAN</entry> 4911 <entry>Return value, true if the name exists</entry> 4912 </row> 4913 </tbody> 4914 </tgroup> 4915 </informaltable> 4916 </para> 4917 <para> 4918 Checks if the specified name exists (currently has an owner). 4919 </para> 4920 </sect3> 4921 4922 <sect3 id="bus-messages-name-owner-changed"> 4923 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title> 4924 <para> 4925 This is a signal: 4926 <programlisting> 4927 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner) 4928 </programlisting> 4929 Message arguments: 4930 <informaltable> 4931 <tgroup cols="3"> 4932 <thead> 4933 <row> 4934 <entry>Argument</entry> 4935 <entry>Type</entry> 4936 <entry>Description</entry> 4937 </row> 4938 </thead> 4939 <tbody> 4940 <row> 4941 <entry>0</entry> 4942 <entry>STRING</entry> 4943 <entry>Name with a new owner</entry> 4944 </row> 4945 <row> 4946 <entry>1</entry> 4947 <entry>STRING</entry> 4948 <entry>Old owner or empty string if none</entry> 4949 </row> 4950 <row> 4951 <entry>2</entry> 4952 <entry>STRING</entry> 4953 <entry>New owner or empty string if none</entry> 4954 </row> 4955 </tbody> 4956 </tgroup> 4957 </informaltable> 4958 </para> 4959 <para> 4960 This signal indicates that the owner of a name has changed. 4961 It's also the signal to use to detect the appearance of 4962 new names on the bus. 4963 </para> 4964 </sect3> 4965 <sect3 id="bus-messages-name-lost"> 4966 <title><literal>org.freedesktop.DBus.NameLost</literal></title> 4967 <para> 4968 This is a signal: 4969 <programlisting> 4970 NameLost (STRING name) 4971 </programlisting> 4972 Message arguments: 4973 <informaltable> 4974 <tgroup cols="3"> 4975 <thead> 4976 <row> 4977 <entry>Argument</entry> 4978 <entry>Type</entry> 4979 <entry>Description</entry> 4980 </row> 4981 </thead> 4982 <tbody> 4983 <row> 4984 <entry>0</entry> 4985 <entry>STRING</entry> 4986 <entry>Name which was lost</entry> 4987 </row> 4988 </tbody> 4989 </tgroup> 4990 </informaltable> 4991 </para> 4992 <para> 4993 This signal is sent to a specific application when it loses 4994 ownership of a name. 4995 </para> 4996 </sect3> 4997 4998 <sect3 id="bus-messages-name-acquired"> 4999 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title> 5000 <para> 5001 This is a signal: 5002 <programlisting> 5003 NameAcquired (STRING name) 5004 </programlisting> 5005 Message arguments: 5006 <informaltable> 5007 <tgroup cols="3"> 5008 <thead> 5009 <row> 5010 <entry>Argument</entry> 5011 <entry>Type</entry> 5012 <entry>Description</entry> 5013 </row> 5014 </thead> 5015 <tbody> 5016 <row> 5017 <entry>0</entry> 5018 <entry>STRING</entry> 5019 <entry>Name which was acquired</entry> 5020 </row> 5021 </tbody> 5022 </tgroup> 5023 </informaltable> 5024 </para> 5025 <para> 5026 This signal is sent to a specific application when it gains 5027 ownership of a name. 5028 </para> 5029 </sect3> 5030 5031 <sect3 id="bus-messages-start-service-by-name"> 5032 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title> 5033 <para> 5034 As a method: 5035 <programlisting> 5036 UINT32 StartServiceByName (in STRING name, in UINT32 flags) 5037 </programlisting> 5038 Message arguments: 5039 <informaltable> 5040 <tgroup cols="3"> 5041 <thead> 5042 <row> 5043 <entry>Argument</entry> 5044 <entry>Type</entry> 5045 <entry>Description</entry> 5046 </row> 5047 </thead> 5048 <tbody> 5049 <row> 5050 <entry>0</entry> 5051 <entry>STRING</entry> 5052 <entry>Name of the service to start</entry> 5053 </row> 5054 <row> 5055 <entry>1</entry> 5056 <entry>UINT32</entry> 5057 <entry>Flags (currently not used)</entry> 5058 </row> 5059 </tbody> 5060 </tgroup> 5061 </informaltable> 5062 Reply arguments: 5063 <informaltable> 5064 <tgroup cols="3"> 5065 <thead> 5066 <row> 5067 <entry>Argument</entry> 5068 <entry>Type</entry> 5069 <entry>Description</entry> 5070 </row> 5071 </thead> 5072 <tbody> 5073 <row> 5074 <entry>0</entry> 5075 <entry>UINT32</entry> 5076 <entry>Return value</entry> 5077 </row> 5078 </tbody> 5079 </tgroup> 5080 </informaltable> 5081 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>. 5082 5083 </para> 5084 <para> 5085 The return value can be one of the following values: 5086 <informaltable> 5087 <tgroup cols="3"> 5088 <thead> 5089 <row> 5090 <entry>Identifier</entry> 5091 <entry>Value</entry> 5092 <entry>Description</entry> 5093 </row> 5094 </thead> 5095 <tbody> 5096 <row> 5097 <entry>DBUS_START_REPLY_SUCCESS</entry> 5098 <entry>1</entry> 5099 <entry>The service was successfully started.</entry> 5100 </row> 5101 <row> 5102 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry> 5103 <entry>2</entry> 5104 <entry>A connection already owns the given name.</entry> 5105 </row> 5106 </tbody> 5107 </tgroup> 5108 </informaltable> 5109 </para> 5110 5111 </sect3> 5112 5113 <sect3 id="bus-messages-update-activation-environment"> 5114 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title> 5115 <para> 5116 As a method: 5117 <programlisting> 5118 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment) 5119 </programlisting> 5120 Message arguments: 5121 <informaltable> 5122 <tgroup cols="3"> 5123 <thead> 5124 <row> 5125 <entry>Argument</entry> 5126 <entry>Type</entry> 5127 <entry>Description</entry> 5128 </row> 5129 </thead> 5130 <tbody> 5131 <row> 5132 <entry>0</entry> 5133 <entry>ARRAY of DICT<STRING,STRING></entry> 5134 <entry>Environment to add or update</entry> 5135 </row> 5136 </tbody> 5137 </tgroup> 5138 </informaltable> 5139 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services. 5140 </para> 5141 <para> 5142 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers. 5143 </para> 5144 <para> 5145 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. 5146 </para> 5147 5148 </sect3> 5149 5150 <sect3 id="bus-messages-get-name-owner"> 5151 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title> 5152 <para> 5153 As a method: 5154 <programlisting> 5155 STRING GetNameOwner (in STRING name) 5156 </programlisting> 5157 Message arguments: 5158 <informaltable> 5159 <tgroup cols="3"> 5160 <thead> 5161 <row> 5162 <entry>Argument</entry> 5163 <entry>Type</entry> 5164 <entry>Description</entry> 5165 </row> 5166 </thead> 5167 <tbody> 5168 <row> 5169 <entry>0</entry> 5170 <entry>STRING</entry> 5171 <entry>Name to get the owner of</entry> 5172 </row> 5173 </tbody> 5174 </tgroup> 5175 </informaltable> 5176 Reply arguments: 5177 <informaltable> 5178 <tgroup cols="3"> 5179 <thead> 5180 <row> 5181 <entry>Argument</entry> 5182 <entry>Type</entry> 5183 <entry>Description</entry> 5184 </row> 5185 </thead> 5186 <tbody> 5187 <row> 5188 <entry>0</entry> 5189 <entry>STRING</entry> 5190 <entry>Return value, a unique connection name</entry> 5191 </row> 5192 </tbody> 5193 </tgroup> 5194 </informaltable> 5195 Returns the unique connection name of the primary owner of the name 5196 given. If the requested name doesn't have an owner, returns a 5197 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error. 5198 </para> 5199 </sect3> 5200 5201 <sect3 id="bus-messages-get-connection-unix-user"> 5202 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title> 5203 <para> 5204 As a method: 5205 <programlisting> 5206 UINT32 GetConnectionUnixUser (in STRING bus_name) 5207 </programlisting> 5208 Message arguments: 5209 <informaltable> 5210 <tgroup cols="3"> 5211 <thead> 5212 <row> 5213 <entry>Argument</entry> 5214 <entry>Type</entry> 5215 <entry>Description</entry> 5216 </row> 5217 </thead> 5218 <tbody> 5219 <row> 5220 <entry>0</entry> 5221 <entry>STRING</entry> 5222 <entry>Unique or well-known bus name of the connection to 5223 query, such as <literal>:12.34</literal> or 5224 <literal>com.example.tea</literal></entry> 5225 </row> 5226 </tbody> 5227 </tgroup> 5228 </informaltable> 5229 Reply arguments: 5230 <informaltable> 5231 <tgroup cols="3"> 5232 <thead> 5233 <row> 5234 <entry>Argument</entry> 5235 <entry>Type</entry> 5236 <entry>Description</entry> 5237 </row> 5238 </thead> 5239 <tbody> 5240 <row> 5241 <entry>0</entry> 5242 <entry>UINT32</entry> 5243 <entry>Unix user ID</entry> 5244 </row> 5245 </tbody> 5246 </tgroup> 5247 </informaltable> 5248 Returns the Unix user ID of the process connected to the server. If 5249 unable to determine it (for instance, because the process is not on the 5250 same machine as the bus daemon), an error is returned. 5251 </para> 5252 </sect3> 5253 5254 <sect3 id="bus-messages-get-connection-unix-process-id"> 5255 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title> 5256 <para> 5257 As a method: 5258 <programlisting> 5259 UINT32 GetConnectionUnixProcessID (in STRING bus_name) 5260 </programlisting> 5261 Message arguments: 5262 <informaltable> 5263 <tgroup cols="3"> 5264 <thead> 5265 <row> 5266 <entry>Argument</entry> 5267 <entry>Type</entry> 5268 <entry>Description</entry> 5269 </row> 5270 </thead> 5271 <tbody> 5272 <row> 5273 <entry>0</entry> 5274 <entry>STRING</entry> 5275 <entry>Unique or well-known bus name of the connection to 5276 query, such as <literal>:12.34</literal> or 5277 <literal>com.example.tea</literal></entry> 5278 </row> 5279 </tbody> 5280 </tgroup> 5281 </informaltable> 5282 Reply arguments: 5283 <informaltable> 5284 <tgroup cols="3"> 5285 <thead> 5286 <row> 5287 <entry>Argument</entry> 5288 <entry>Type</entry> 5289 <entry>Description</entry> 5290 </row> 5291 </thead> 5292 <tbody> 5293 <row> 5294 <entry>0</entry> 5295 <entry>UINT32</entry> 5296 <entry>Unix process id</entry> 5297 </row> 5298 </tbody> 5299 </tgroup> 5300 </informaltable> 5301 Returns the Unix process ID of the process connected to the server. If 5302 unable to determine it (for instance, because the process is not on the 5303 same machine as the bus daemon), an error is returned. 5304 </para> 5305 </sect3> 5306 5307 <sect3 id="bus-messages-add-match"> 5308 <title><literal>org.freedesktop.DBus.AddMatch</literal></title> 5309 <para> 5310 As a method: 5311 <programlisting> 5312 AddMatch (in STRING rule) 5313 </programlisting> 5314 Message arguments: 5315 <informaltable> 5316 <tgroup cols="3"> 5317 <thead> 5318 <row> 5319 <entry>Argument</entry> 5320 <entry>Type</entry> 5321 <entry>Description</entry> 5322 </row> 5323 </thead> 5324 <tbody> 5325 <row> 5326 <entry>0</entry> 5327 <entry>STRING</entry> 5328 <entry>Match rule to add to the connection</entry> 5329 </row> 5330 </tbody> 5331 </tgroup> 5332 </informaltable> 5333 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>). 5334 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal> 5335 error is returned. 5336 </para> 5337 </sect3> 5338 <sect3 id="bus-messages-remove-match"> 5339 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title> 5340 <para> 5341 As a method: 5342 <programlisting> 5343 RemoveMatch (in STRING rule) 5344 </programlisting> 5345 Message arguments: 5346 <informaltable> 5347 <tgroup cols="3"> 5348 <thead> 5349 <row> 5350 <entry>Argument</entry> 5351 <entry>Type</entry> 5352 <entry>Description</entry> 5353 </row> 5354 </thead> 5355 <tbody> 5356 <row> 5357 <entry>0</entry> 5358 <entry>STRING</entry> 5359 <entry>Match rule to remove from the connection</entry> 5360 </row> 5361 </tbody> 5362 </tgroup> 5363 </informaltable> 5364 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>). 5365 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal> 5366 error is returned. 5367 </para> 5368 </sect3> 5369 5370 <sect3 id="bus-messages-get-id"> 5371 <title><literal>org.freedesktop.DBus.GetId</literal></title> 5372 <para> 5373 As a method: 5374 <programlisting> 5375 GetId (out STRING id) 5376 </programlisting> 5377 Reply arguments: 5378 <informaltable> 5379 <tgroup cols="3"> 5380 <thead> 5381 <row> 5382 <entry>Argument</entry> 5383 <entry>Type</entry> 5384 <entry>Description</entry> 5385 </row> 5386 </thead> 5387 <tbody> 5388 <row> 5389 <entry>0</entry> 5390 <entry>STRING</entry> 5391 <entry>Unique ID identifying the bus daemon</entry> 5392 </row> 5393 </tbody> 5394 </tgroup> 5395 </informaltable> 5396 Gets the unique ID of the bus. The unique ID here is shared among all addresses the 5397 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in 5398 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique 5399 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related. 5400 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned 5401 by org.freedesktop.DBus.Peer.GetMachineId(). 5402 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session. 5403 </para> 5404 </sect3> 5405 5406 </sect2> 5407 5408 </sect1> 5409 <!-- 5410 <appendix id="implementation-notes"> 5411 <title>Implementation notes</title> 5412 <sect1 id="implementation-notes-subsection"> 5413 <title></title> 5414 <para> 5415 </para> 5416 </sect1> 5417 </appendix> 5418 --> 5419 5420 <glossary><title>Glossary</title> 5421 <para> 5422 This glossary defines some of the terms used in this specification. 5423 </para> 5424 5425 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm> 5426 <glossdef> 5427 <para> 5428 The message bus maintains an association between names and 5429 connections. (Normally, there's one connection per application.) A 5430 bus name is simply an identifier used to locate connections. For 5431 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal> 5432 name might be used to send a message to a screensaver from Yoyodyne 5433 Corporation. An application is said to <firstterm>own</firstterm> a 5434 name if the message bus has associated the application's connection 5435 with the name. Names may also have <firstterm>queued 5436 owners</firstterm> (see <xref linkend="term-queued-owner"/>). 5437 The bus assigns a unique name to each connection, 5438 see <xref linkend="term-unique-name"/>. Other names 5439 can be thought of as "well-known names" and are 5440 used to find applications that offer specific functionality. 5441 </para> 5442 5443 <para> 5444 See <xref linkend="message-protocol-names-bus"/> for details of 5445 the syntax and naming conventions for bus names. 5446 </para> 5447 </glossdef> 5448 </glossentry> 5449 5450 <glossentry id="term-message"><glossterm>Message</glossterm> 5451 <glossdef> 5452 <para> 5453 A message is the atomic unit of communication via the D-Bus 5454 protocol. It consists of a <firstterm>header</firstterm> and a 5455 <firstterm>body</firstterm>; the body is made up of 5456 <firstterm>arguments</firstterm>. 5457 </para> 5458 </glossdef> 5459 </glossentry> 5460 5461 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm> 5462 <glossdef> 5463 <para> 5464 The message bus is a special application that forwards 5465 or routes messages between a group of applications 5466 connected to the message bus. It also manages 5467 <firstterm>names</firstterm> used for routing 5468 messages. 5469 </para> 5470 </glossdef> 5471 </glossentry> 5472 5473 <glossentry id="term-name"><glossterm>Name</glossterm> 5474 <glossdef> 5475 <para> 5476 See <xref linkend="term-bus-name"/>. "Name" may 5477 also be used to refer to some of the other names 5478 in D-Bus, such as interface names. 5479 </para> 5480 </glossdef> 5481 </glossentry> 5482 5483 <glossentry id="namespace"><glossterm>Namespace</glossterm> 5484 <glossdef> 5485 <para> 5486 Used to prevent collisions when defining new interfaces, bus names 5487 etc. The convention used is the same one Java uses for defining 5488 classes: a reversed domain name. 5489 See <xref linkend="message-protocol-names-bus"/>, 5490 <xref linkend="message-protocol-names-interface"/>, 5491 <xref linkend="message-protocol-names-error"/>, 5492 <xref linkend="message-protocol-marshaling-object-path"/>. 5493 </para> 5494 </glossdef> 5495 </glossentry> 5496 5497 <glossentry id="term-object"><glossterm>Object</glossterm> 5498 <glossdef> 5499 <para> 5500 Each application contains <firstterm>objects</firstterm>, which have 5501 <firstterm>interfaces</firstterm> and 5502 <firstterm>methods</firstterm>. Objects are referred to by a name, 5503 called a <firstterm>path</firstterm>. 5504 </para> 5505 </glossdef> 5506 </glossentry> 5507 5508 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm> 5509 <glossdef> 5510 <para> 5511 An application talking directly to another application, without going 5512 through a message bus. One-to-one connections may be "peer to peer" or 5513 "client to server." The D-Bus protocol has no concept of client 5514 vs. server after a connection has authenticated; the flow of messages 5515 is symmetrical (full duplex). 5516 </para> 5517 </glossdef> 5518 </glossentry> 5519 5520 <glossentry id="term-path"><glossterm>Path</glossterm> 5521 <glossdef> 5522 <para> 5523 Object references (object names) in D-Bus are organized into a 5524 filesystem-style hierarchy, so each object is named by a path. As in 5525 LDAP, there's no difference between "files" and "directories"; a path 5526 can refer to an object, while still having child objects below it. 5527 </para> 5528 </glossdef> 5529 </glossentry> 5530 5531 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm> 5532 <glossdef> 5533 <para> 5534 Each bus name has a primary owner; messages sent to the name go to the 5535 primary owner. However, certain names also maintain a queue of 5536 secondary owners "waiting in the wings." If the primary owner releases 5537 the name, then the first secondary owner in the queue automatically 5538 becomes the new owner of the name. 5539 </para> 5540 </glossdef> 5541 </glossentry> 5542 5543 <glossentry id="term-service"><glossterm>Service</glossterm> 5544 <glossdef> 5545 <para> 5546 A service is an executable that can be launched by the bus daemon. 5547 Services normally guarantee some particular features, for example they 5548 may guarantee that they will request a specific name such as 5549 "org.freedesktop.Screensaver", have a singleton object 5550 "/org/freedesktop/Application", and that object will implement the 5551 interface "org.freedesktop.ScreensaverControl". 5552 </para> 5553 </glossdef> 5554 </glossentry> 5555 5556 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm> 5557 <glossdef> 5558 <para> 5559 ".service files" tell the bus about service applications that can be 5560 launched (see <xref linkend="term-service"/>). Most importantly they 5561 provide a mapping from bus names to services that will request those 5562 names when they start up. 5563 </para> 5564 </glossdef> 5565 </glossentry> 5566 5567 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm> 5568 <glossdef> 5569 <para> 5570 The special name automatically assigned to each connection by the 5571 message bus. This name will never change owner, and will be unique 5572 (never reused during the lifetime of the message bus). 5573 It will begin with a ':' character. 5574 </para> 5575 </glossdef> 5576 </glossentry> 5577 5578 </glossary> 5579 </article> 5580
