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src/spec/dbus-specification.xml
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David Rheinsberg
spec: clarify copyright
30 июн 2026, 15:45
30 июн 2026, 15:45a29e0f5
<!-- SPDX-License-Identifier: GPL-2.0-or-later -->
<!-- SPDX-FileCopyrightText: 2003 Red Hat, Inc. -->
<?xml version="1.0" standalone="no" ?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd"
[
]>
<article id="index" xmlns="http://docbook.org/ns/docbook">
  <articleinfo>
    <title>D-Bus Specification</title>
    <releaseinfo>Version 0.43</releaseinfo>
    <date>2024-10-29</date>
    <authorgroup>
      <author>
        <firstname>Havoc</firstname>
        <surname>Pennington</surname>
        <affiliation>
          <orgname>Red Hat, Inc.</orgname>
          <address>
            <email>hp@pobox.com</email>
          </address>
        </affiliation>
      </author>
      <author>
        <firstname>Anders</firstname>
        <surname>Carlsson</surname>
        <affiliation>
          <orgname>CodeFactory AB</orgname>
          <address>
            <email>andersca@codefactory.se</email>
          </address>
        </affiliation>
      </author>
      <author>
        <firstname>Alexander</firstname>
        <surname>Larsson</surname>
        <affiliation>
          <orgname>Red Hat, Inc.</orgname>
          <address>
            <email>alexl@redhat.com</email>
          </address>
        </affiliation>
      </author>
      <author>
        <firstname>Sven</firstname>
        <surname>Herzberg</surname>
        <affiliation>
          <orgname>Imendio AB</orgname>
          <address>
            <email>sven@imendio.com</email>
          </address>
        </affiliation>
      </author>
      <author>
        <firstname>Simon</firstname>
        <surname>McVittie</surname>
        <affiliation>
          <orgname>Collabora Ltd.</orgname>
          <address>
            <email>smcv@collabora.com</email>
          </address>
        </affiliation>
      </author>
      <author>
        <firstname>David</firstname>
        <surname>Zeuthen</surname>
        <affiliation>
          <address>
            <email>zeuthen@gmail.com</email>
          </address>
        </affiliation>
      </author>
    </authorgroup>
   <revhistory>
     <revision>
       <revnumber>latest</revnumber>
       <date></date>
       <authorinitials></authorinitials>
       <revremark>
         See <ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink>
       </revremark>
     </revision>
     <revision>
       <revnumber>0.43</revnumber>
       <date>2024-10-29</date>
       <authorinitials></authorinitials>
       <revdescription>
         <itemizedlist>
           <listitem><simpara>Recommend loading system services from /etc, /run</simpara></listitem>
           <listitem><simpara>Document the Stats interface</simpara></listitem>
           <listitem><simpara>Document the Verbose interface</simpara></listitem>
         </itemizedlist>
       </revdescription>
     </revision>
     <revision>
       <revnumber>0.42</revnumber>
       <date>2023-08-21</date>
       <authorinitials></authorinitials>
       <revdescription>
         <itemizedlist>
           <listitem><simpara>GetConnectionCredentials can return ProcessFD</simpara></listitem>
         </itemizedlist>
       </revdescription>
     </revision>
     <revision>
       <revnumber>0.41</revnumber>
       <date>2023-02-08</date>
       <authorinitials></authorinitials>
       <revdescription>
         <itemizedlist>
           <listitem><simpara>Clarify intended handling of /run vs. /var/run</simpara></listitem>
         </itemizedlist>
       </revdescription>
     </revision>
     <revision>
       <revnumber>0.40</revnumber>
       <date>2022-10-05</date>
       <authorinitials></authorinitials>
       <revdescription>
         <itemizedlist>
           <listitem><simpara>Clarify that unix:tmpdir is not required to use an abstract socket on Linux</simpara></listitem>
           <listitem><simpara>Mention implications of abstract sockets for Linux namespacing</simpara></listitem>
         </itemizedlist>
       </revdescription>
     </revision>
     <revision>
       <revnumber>0.39</revnumber>
       <date>2022-09-22</date>
       <authorinitials></authorinitials>
       <revdescription>
         <itemizedlist>
           <listitem><simpara>Document a recommendation for IDNs in reversed domain names</simpara></listitem>
           <listitem><simpara>Clarify documentation regarding AF_UNIX sockets</simpara></listitem>
         </itemizedlist>
       </revdescription>
     </revision>
     <revision>
       <revnumber>0.38</revnumber>
       <date>2022-02-23</date>
       <authorinitials></authorinitials>
       <revdescription>
         <itemizedlist>
           <listitem><simpara>Add ActivatableServicesChanged signal and feature flag</simpara></listitem>
           <listitem><simpara><literal>*</literal> is optionally-escaped in addresses</simpara></listitem>
         </itemizedlist>
       </revdescription>
     </revision>
     <revision>
       <revnumber>0.37</revnumber>
       <date>2021-12-17</date>
       <authorinitials></authorinitials>
       <revdescription>
         <itemizedlist>
           <listitem><simpara>Update recommendations for interoperable DBUS_COOKIE_SHA1 timeouts</simpara></listitem>
           <listitem><simpara>Clarify padding requirements for arrays and variants</simpara></listitem>
           <listitem><simpara>Describe where the interoperable machine ID comes from</simpara></listitem>
           <listitem><simpara>Clarify use of dictionary (array of dict-entry) types</simpara></listitem>
         </itemizedlist>
       </revdescription>
     </revision>
     <revision>
       <revnumber>0.36</revnumber>
       <date>2020-04-21</date>
       <authorinitials></authorinitials>
       <revdescription>
         <itemizedlist>
           <listitem><simpara>Fix a typo in an annotated hexdump of part of a message</simpara></listitem>
         </itemizedlist>
       </revdescription>
     </revision>
     <revision>
       <revnumber>0.35</revnumber>
       <date>2019-05-13</date>
       <authorinitials></authorinitials>
       <revdescription>
         <itemizedlist>
           <listitem><simpara>Add UnixGroupIDs to GetConnectionCredentials</simpara></listitem>
           <listitem><simpara>Avoid redundancy in defining interface name syntax</simpara></listitem>
         </itemizedlist>
       </revdescription>
     </revision>
     <revision>
       <revnumber>0.34</revnumber>
       <date>2018-12-04</date>
       <authorinitials>pwithnall</authorinitials>
       <revdescription>
         <itemizedlist>
           <listitem><simpara>Correct ObjectManager example AddMatch rule</simpara></listitem>
         </itemizedlist>
       </revdescription>
     </revision>
     <revision>
       <revnumber>0.33</revnumber>
       <date>2018-04-27</date>
       <authorinitials>smcv</authorinitials>
       <revdescription>
         <itemizedlist>
           <listitem><simpara>Deprecate TCP on Unix</simpara></listitem>
           <listitem><simpara>Deprecate non-local TCP everywhere</simpara></listitem>
         </itemizedlist>
       </revdescription>
     </revision>
     <revision>
       <revnumber>0.32</revnumber>
       <date>2018-01-30</date>
       <authorinitials>smcv</authorinitials>
       <revdescription>
         <itemizedlist>
           <listitem><simpara>Deprecate hyphen/minus in bus names, with
             underscore as the recommended replacement</simpara></listitem>
           <listitem><simpara>Document the convention for escaping
             leading digits in interface and bus names
             (org._7_zip)</simpara></listitem>
           <listitem><simpara>Recommend using SASL EXTERNAL where
             possible, or DBUS_COOKIE_SHA1 otherwise</simpara></listitem>
           <listitem><simpara>Message buses should not accept SASL
             ANONYMOUS</simpara></listitem>
           <listitem><simpara>Document the meaning of non-empty SASL
             authorization identity strings</simpara></listitem>
           <listitem><simpara>Document the optional argument to SASL
             ERROR</simpara></listitem>
           <listitem><simpara>Document who sends each SASL command, and
             the possible replies</simpara></listitem>
           <listitem><simpara>Document the authentication states used to
             negotiate Unix fd-passing</simpara></listitem>
           <listitem><simpara>Servers that relay messages should remove
             header fields they do not understand</simpara></listitem>
           <listitem><simpara>Clarify who controls each header
             field</simpara></listitem>
           <listitem><simpara>Document the HeaderFiltering message bus
             feature flag</simpara></listitem>
           <listitem><simpara>Non-message-bus servers may use the SENDER
             and DESTINATION fields</simpara></listitem>
         </itemizedlist>
       </revdescription>
     </revision>
     <revision>
       <revnumber>0.31</revnumber>
       <date>2017-06-29</date>
       <authorinitials>smcv, TG</authorinitials>
       <revdescription>
         <itemizedlist>
           <listitem><simpara>Don't require implementation-specific search
               paths to be lowest priority</simpara></listitem>
           <listitem><simpara>Correct regex syntax for optionally-escaped
               bytes in addresses so it includes hyphen-minus, forward slash
               and underscore as intended</simpara></listitem>
           <listitem><simpara>Describe all message bus methods in the same
               section</simpara></listitem>
           <listitem><simpara>Clarify the correct object path for method calls
               to the message bus</simpara></listitem>
           <listitem><simpara>Document that the message bus implements
               Introspectable, Peer and Properties</simpara></listitem>
           <listitem><simpara>Add new Features and Interfaces properties for
               message bus feature-discovery</simpara></listitem>
           <listitem><simpara>Add unix:dir=..., which resembles
               unix:tmpdir=... but never uses abstract
               sockets</simpara></listitem>
           <listitem><simpara>Don't require eavesdrop='true' to be accepted
               from connections not sufficiently privileged to use it
               successfully</simpara></listitem>
           <listitem><simpara>Formally deprecate eavesdropping in favour of
               BecomeMonitor</simpara></listitem>
         </itemizedlist>
       </revdescription>
     </revision>
     <revision>
       <revnumber>0.30</revnumber>
       <date>2016-11-28</date>
       <authorinitials>smcv, PW</authorinitials>
       <revremark>
         Define the jargon terms service activation and auto-starting more
         clearly. Document the SystemdService key in service files.
         Document how AppArmor interacts with service activation, and the
         new AssumedAppArmorLabel key in service files (dbus-daemon 1.11.8).
         Clarify intended behaviour of Properties.GetAll.
         Use versioned interface and bus names in most examples.
       </revremark>
     </revision>
     <revision>
       <revnumber>0.29</revnumber>
       <date>2016-10-10</date>
       <authorinitials>PW</authorinitials>
       <revremark>
         Introspection arguments may contain annotations; recommend against
         using the object path '/'
       </revremark>
     </revision>
     <revision>
       <revnumber>0.28</revnumber>
       <date>2016-08-15</date>
       <authorinitials>PW</authorinitials>
       <revremark>Clarify serialization</revremark>
     </revision>
     <revision>
       <revnumber>0.27</revnumber>
       <date>2015-12-02</date>
       <authorinitials>LU</authorinitials>
       <revremark>Services should not send unwanted replies</revremark>
     </revision>
     <revision>
       <revnumber>0.26</revnumber>
       <date>2015-02-19</date>
       <authorinitials>smcv, rh</authorinitials>
       <revremark>
         GetConnectionCredentials can return LinuxSecurityLabel or
         WindowsSID; add privileged BecomeMonitor method
       </revremark>
     </revision>
     <revision>
       <revnumber>0.25</revnumber>
       <date>2014-11-10</date>
       <authorinitials>smcv, lennart</authorinitials>
       <revremark>
         ALLOW_INTERACTIVE_AUTHORIZATION flag, EmitsChangedSignal=const
       </revremark>
     </revision>
     <revision>
       <revnumber>0.24</revnumber>
       <date>2014-10-01</date>
       <authorinitials>SMcV</authorinitials>
       <revremark>
         non-method-calls never expect a reply even without NO_REPLY_EXPECTED;
         document how to quote match rules
       </revremark>
     </revision>
     <revision>
       <revnumber>0.23</revnumber>
       <date>2014-01-06</date>
       <authorinitials>SMcV, CY</authorinitials>
       <revremark>
         method call messages with no INTERFACE may be considered an error;
         document tcp:bind=... and nonce-tcp:bind=...; define listenable
         and connectable addresses
       </revremark>
     </revision>
     <revision>
       <revnumber>0.22</revnumber>
       <date>2013-10-09</date>
       <authorinitials></authorinitials>
       <revremark>add GetConnectionCredentials, document
        GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
        document and correct .service file syntax and naming
      </revremark>
     </revision>
     <revision>
       <revnumber>0.21</revnumber>
       <date>2013-04-25</date>
       <authorinitials>smcv</authorinitials>
       <revremark>allow Unicode noncharacters in UTF-8 (Unicode
         Corrigendum #9)</revremark>
     </revision>
     <revision>
       <revnumber>0.20</revnumber>
       <date>22 February 2013</date>
       <authorinitials>smcv, walters</authorinitials>
       <revremark>reorganise for clarity, remove false claims about
         basic types, mention /o/fd/DBus</revremark>
     </revision>
     <revision>
       <revnumber>0.19</revnumber>
       <date>20 February 2012</date>
       <authorinitials>smcv/lp</authorinitials>
       <revremark>formally define unique connection names and well-known
        bus names; document best practices for interface, bus, member and
        error names, and object paths; document the search path for session
        and system services on Unix; document the systemd transport</revremark>
     </revision>
     <revision>
       <revnumber>0.18</revnumber>
       <date>29 July 2011</date>
       <authorinitials>smcv</authorinitials>
       <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
         match keyword; promote type system to a top-level section</revremark>
     </revision>
     <revision>
       <revnumber>0.17</revnumber>
       <date>1 June 2011</date>
       <authorinitials>smcv/davidz</authorinitials>
       <revremark>define ObjectManager; reserve extra pseudo-type-codes used
         by GVariant</revremark>
     </revision>
     <revision>
       <revnumber>0.16</revnumber>
       <date>11 April 2011</date>
       <authorinitials></authorinitials>
       <revremark>add path_namespace, arg0namespace; argNpath matches object
        paths</revremark>
     </revision>
     <revision>
       <revnumber>0.15</revnumber>
       <date>3 November 2010</date>
       <authorinitials></authorinitials>
       <revremark></revremark>
     </revision>
     <revision>
       <revnumber>0.14</revnumber>
       <date>12 May 2010</date>
       <authorinitials></authorinitials>
       <revremark></revremark>
     </revision>
     <revision>
       <revnumber>0.13</revnumber>
       <date>23 Dezember 2009</date>
       <authorinitials></authorinitials>
       <revremark></revremark>
     </revision>
     <revision>
       <revnumber>0.12</revnumber>
       <date>7 November, 2006</date>
       <authorinitials></authorinitials>
       <revremark></revremark>
     </revision>
     <revision>
       <revnumber>0.11</revnumber>
       <date>6 February 2005</date>
       <authorinitials></authorinitials>
       <revremark></revremark>
     </revision>
     <revision>
       <revnumber>0.10</revnumber>
       <date>28 January 2005</date>
       <authorinitials></authorinitials>
       <revremark></revremark>
     </revision>
     <revision>
       <revnumber>0.9</revnumber>
       <date>7 Januar 2005</date>
       <authorinitials></authorinitials>
       <revremark></revremark>
     </revision>
     <revision>
       <revnumber>0.8</revnumber>
       <date>06 September 2003</date>
       <authorinitials></authorinitials>
       <revremark>First released document.</revremark>
     </revision>
   </revhistory>
  </articleinfo>

  <sect1 id="introduction">
    <title>Introduction</title>
    <para>
      D-Bus is a system for low-overhead, easy to use
      interprocess communication (IPC). In more detail:
      <itemizedlist>
        <listitem>
          <para>
            D-Bus is <emphasis>low-overhead</emphasis> because it uses a
            binary protocol, and does not have to convert to and from a text
            format such as XML. Because D-Bus is intended for potentially
            high-resolution same-machine IPC, not primarily for Internet IPC,
            this is an interesting optimization. D-Bus is also designed to
            avoid round trips and allow asynchronous operation, much like
            the X protocol.
          </para>
        </listitem>
        <listitem>
          <para>
            D-Bus is <emphasis>easy to use</emphasis> because it works in terms
            of <firstterm>messages</firstterm> rather than byte streams, and
            automatically handles a lot of the hard IPC issues. Also, the D-Bus
            library is designed to be wrapped in a way that lets developers use
            their framework's existing object/type system, rather than learning
            a new one specifically for IPC.
          </para>
        </listitem>
      </itemizedlist>
    </para>

    <para>
      The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
      protocol, specified in <xref linkend="message-protocol"/>. That is, it is
      a system for one application to talk to a single other
      application. However, the primary intended application of the protocol is the
      D-Bus <firstterm>message bus</firstterm>, specified in <xref
      linkend="message-bus"/>. The message bus is a special application that
      accepts connections from multiple other applications, and forwards
      messages among them.
    </para>

    <para>
      Uses of D-Bus include notification of system changes (notification of when
      a camera is plugged in to a computer, or a new version of some software
      has been installed), or desktop interoperability, for example a file
      monitoring service or a configuration service.
    </para>

    <para>
      D-Bus is designed for two specific use cases:
      <itemizedlist>
        <listitem>
          <para>
            A "system bus" for notifications from the system to user sessions,
            and to allow the system to request input from user sessions.
          </para>
        </listitem>
        <listitem>
          <para>
            A "session bus" used to implement desktop environments such as
            GNOME and KDE.
          </para>
        </listitem>
      </itemizedlist>
      D-Bus is not intended to be a generic IPC system for any possible
      application, and intentionally omits many features found in other
      IPC systems for this reason.
    </para>

    <para>
      At the same time, the bus daemons offer a number of features not found in
      other IPC systems, such as single-owner "bus names" (similar to X
      selections), on-demand startup of services, and security policies.
      In many ways, these features are the primary motivation for developing
      D-Bus; other systems would have sufficed if IPC were the only goal.
    </para>

    <para>
      D-Bus may turn out to be useful in unanticipated applications, but future
      versions of this spec and the reference implementation probably will not
      incorporate features that interfere with the core use cases.
    </para>

    <para>
      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
      "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
      document are to be interpreted as described in RFC 2119. However, the
      document could use a serious audit to be sure it makes sense to do
      so. Also, they are not capitalized.
    </para>

    <sect2 id="stability">
      <title>Protocol and Specification Stability</title>
      <para>
        The D-Bus protocol is frozen (only compatible extensions are allowed) as
        of November 8, 2006.  However, this specification could still use a fair
        bit of work to make interoperable reimplementation possible without
        reference to the D-Bus reference implementation. Thus, this
        specification is not marked 1.0. To mark it 1.0, we'd like to see
        someone invest significant effort in clarifying the specification
        language, and growing the specification to cover more aspects of the
        reference implementation's behavior.
      </para>
      <para>
        Until this work is complete, any attempt to reimplement D-Bus will
        probably require looking at the reference implementation and/or asking
        questions on the D-Bus mailing list about intended behavior.
        Questions on the list are very welcome.
      </para>
      <para>
        Nonetheless, this document should be a useful starting point and is
        to our knowledge accurate, though incomplete.
      </para>
    </sect2>

  </sect1>

  <sect1 id="type-system">
    <title>Type System</title>

    <para>
      D-Bus has a type system, in which values of various types can be
      serialized into a sequence of bytes referred to as the
      <firstterm>wire format</firstterm> in a standard way.
      Converting a value from some other representation into the wire
      format is called <firstterm>marshaling</firstterm> and converting
      it back from the wire format is <firstterm>unmarshaling</firstterm>.
    </para>

    <para>
      The D-Bus protocol does not include type tags in the marshaled data; a
      block of marshaled values must have a known <firstterm>type
        signature</firstterm>. The type signature is made up of zero or more
      <firstterm id="term-single-complete-type">single complete
        types</firstterm>, each made up of one or more
      <firstterm>type codes</firstterm>.
    </para>

    <para>
      A type code is an ASCII character representing the
      type of a value. Because ASCII characters are used, the type signature
      will always form a valid ASCII string. A simple string compare
      determines whether two type signatures are equivalent.
    </para>

    <para>
      A single complete type is a sequence of type codes that fully describes
      one type: either a basic type, or a single fully-described container type.
      A single complete type is a basic type code, a variant type code,
      an array with its element type, or a struct with its fields (all of which
      are defined below). So the following signatures are not single complete
      types:
      <programlisting>
        "aa"
      </programlisting>
      <programlisting>
        "(ii"
      </programlisting>
      <programlisting>
        "ii)"
      </programlisting>
      And the following signatures contain multiple complete types:
      <programlisting>
        "ii"
      </programlisting>
      <programlisting>
        "aiai"
      </programlisting>
      <programlisting>
        "(ii)(ii)"
      </programlisting>
      Note however that a single complete type may <emphasis>contain</emphasis>
      multiple other single complete types, by containing a struct or dict
      entry.
    </para>

    <sect2 id="basic-types">
      <title>Basic types</title>

      <para>
        The simplest type codes are the <firstterm id="term-basic-type">basic
          types</firstterm>, which are the types whose structure is entirely
        defined by their 1-character type code. Basic types consist of
        fixed types and string-like types.
      </para>

      <para>
        The <firstterm id="term-fixed-type">fixed types</firstterm>
        are basic types whose values have a fixed length, namely BYTE,
        BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
        16, 32 or 64 bits.
      </para>

      <para>
        As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
        the ASCII character 'i'. So the signature for a block of values
        containing a single <literal>INT32</literal> would be:
        <programlisting>
          "i"
        </programlisting>
        A block of values containing two <literal>INT32</literal> would have this signature:
        <programlisting>
          "ii"
        </programlisting>
      </para>

      <para>
        The characteristics of the fixed types are listed in this table.

        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Conventional name</entry>
                <entry>ASCII type-code</entry>
                <entry>Encoding</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry><literal>BYTE</literal></entry>
                <entry><literal>y</literal> (121)</entry>
                <entry>Unsigned 8-bit integer</entry>
              </row>
              <row>
                <entry><literal>BOOLEAN</literal></entry>
                <entry><literal>b</literal> (98)</entry>
                <entry>Boolean value: 0 is false, 1 is true, any other value
                  allowed by the marshalling format is invalid</entry>
              </row>
              <row>
                <entry><literal>INT16</literal></entry>
                <entry><literal>n</literal> (110)</entry>
                <entry>Signed (two's complement) 16-bit integer</entry>
              </row>
              <row>
                <entry><literal>UINT16</literal></entry>
                <entry><literal>q</literal> (113)</entry>
                <entry>Unsigned 16-bit integer</entry>
              </row>
              <row>
                <entry><literal>INT32</literal></entry>
                <entry><literal>i</literal> (105)</entry>
                <entry>Signed (two's complement) 32-bit integer</entry>
              </row>
              <row>
                <entry><literal>UINT32</literal></entry>
                <entry><literal>u</literal> (117)</entry>
                <entry>Unsigned 32-bit integer</entry>
              </row>
              <row>
                <entry><literal>INT64</literal></entry>
                <entry><literal>x</literal> (120)</entry>
                <entry>Signed (two's complement) 64-bit integer
                  (mnemonic: x and t are the first characters in "sixty" not
                  already used for something more common)</entry>
              </row>
              <row>
                <entry><literal>UINT64</literal></entry>
                <entry><literal>t</literal> (116)</entry>
                <entry>Unsigned 64-bit integer</entry>
              </row>
              <row>
                <entry><literal>DOUBLE</literal></entry>
                <entry><literal>d</literal> (100)</entry>
                <entry>IEEE 754 double-precision floating point</entry>
              </row>
              <row>
                <entry><literal>UNIX_FD</literal></entry>
                <entry><literal>h</literal> (104)</entry>
                <entry>Unsigned 32-bit integer representing an index into an
                  out-of-band array of file descriptors, transferred via some
                  platform-specific mechanism (mnemonic: h for handle)</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>

      <para>
        The <firstterm id="term-string-like-type">string-like types</firstterm>
        are basic types with a variable length. The value of any string-like
        type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
        none of which may be U+0000. The UTF-8 text must be validated
        strictly: in particular, it must not contain overlong sequences
        or codepoints above U+10FFFF.
      </para>

      <para>
        Since D-Bus Specification version 0.21, in accordance with Unicode
        Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
        U+nFFFF are allowed in UTF-8 strings (but note that older versions of
        D-Bus rejected these noncharacters).
      </para>

      <para>
        The marshalling formats for the string-like types all end with a
        single zero (NUL) byte, but that byte is not considered to be part of
        the text.
      </para>

      <para>
        The characteristics of the string-like types are listed in this table.

        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Conventional name</entry>
                <entry>ASCII type-code</entry>
                <entry>Validity constraints</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry><literal>STRING</literal></entry>
                <entry><literal>s</literal> (115)</entry>
                <entry>No extra constraints</entry>
              </row>
              <row>
                <entry><literal>OBJECT_PATH</literal></entry>
                <entry><literal>o</literal> (111)</entry>
                <entry>Must be
                  <link linkend="message-protocol-marshaling-object-path">a
                    syntactically valid object path</link></entry>
              </row>
              <row>
                <entry><literal>SIGNATURE</literal></entry>
                <entry><literal>g</literal> (103)</entry>
                <entry>Zero or more
                  <firstterm linkend="term-single-complete-type">single
                    complete types</firstterm></entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>

      <sect3 id="message-protocol-marshaling-object-path">
        <title>Valid Object Paths</title>

        <para>
          An object path is a name used to refer to an object instance.
          Conceptually, each participant in a D-Bus message exchange may have
          any number of object instances (think of C++ or Java objects) and each
          such instance will have a path. Like a filesystem, the object
          instances in an application form a hierarchical tree.
        </para>

        <para>
          Object paths are often namespaced by starting with a reversed
          domain name and containing an interface version number, in the
          same way as
          <link linkend="message-protocol-names-interface">interface
            names</link> and
          <link linkend="message-protocol-names-bus">well-known
            bus names</link>.
          This makes it possible to implement more than one service, or
          more than one version of a service, in the same process,
          even if the services share a connection but cannot otherwise
          co-operate (for instance, if they are implemented by different
          plugins).
        </para>

        <para>
          Using an object path of <literal>/</literal> is allowed, but
          recommended against, as it makes versioning of interfaces hard. Any
          signals emitted from a D-Bus object have the service’s unique bus name
          associated with them, rather than its well-known name. This means that
          receipients of the signals must rely entirely on the signal name and
          object path to work out which interface the signal originated from.
        </para>

        <para>
          For instance, if the owner of <literal>example.com</literal> is
          developing a D-Bus API for a music player, they might use the
          hierarchy of object paths that start with
          <literal>/com/example/MusicPlayer1</literal> for its objects.
        </para>

        <para>
          The following rules define a valid object path. Implementations must
          not send or accept messages with invalid object paths.
          <itemizedlist>
            <listitem>
              <para>
                The path may be of any length.
              </para>
            </listitem>
            <listitem>
              <para>
                The path must begin with an ASCII '/' (integer 47) character,
                and must consist of elements separated by slash characters.
              </para>
            </listitem>
            <listitem>
              <para>
                Each element must only contain the ASCII characters
                "[A-Z][a-z][0-9]_"
              </para>
            </listitem>
            <listitem>
              <para>
                No element may be the empty string.
              </para>
            </listitem>
            <listitem>
              <para>
                Multiple '/' characters cannot occur in sequence.
              </para>
            </listitem>
            <listitem>
              <para>
                A trailing '/' character is not allowed unless the
                path is the root path (a single '/' character).
              </para>
            </listitem>
          </itemizedlist>
        </para>

      </sect3>

      <sect3 id="message-protocol-marshaling-signature">
        <title>Valid Signatures</title>
        <para>
          An implementation must not send or accept invalid signatures.
          Valid signatures will conform to the following rules:
          <itemizedlist>
            <listitem>
              <para>
                The signature is a list of single complete types.
                Arrays must have element types, and structs must
                have both open and close parentheses.
              </para>
            </listitem>
            <listitem>
              <para>
                Only type codes, open and close parentheses, and open and
                close curly brackets are allowed in the signature. The
                <literal>STRUCT</literal> type code
                is not allowed in signatures, because parentheses
                are used instead. Similarly, the
                <literal>DICT_ENTRY</literal> type code is not allowed in
                signatures, because curly brackets are used instead.
              </para>
            </listitem>
            <listitem>
              <para>
                The maximum depth of container type nesting is 32 array type
                codes and 32 open parentheses. This implies that the maximum
                total depth of recursion is 64, for an "array of array of array
                of ... struct of struct of struct of ..."  where there are 32
                array and 32 struct.
              </para>
            </listitem>
            <listitem>
              <para>
                The maximum length of a signature is 255.
              </para>
            </listitem>
          </itemizedlist>
        </para>

        <para>
          When signatures appear in messages, the marshalling format
          guarantees that they will be followed by a nul byte (which can
          be interpreted as either C-style string termination or the INVALID
          type-code), but this is not conceptually part of the signature.
        </para>
      </sect3>

    </sect2>

    <sect2 id="container-types">
      <title>Container types</title>

      <para>
        In addition to basic types, there are four <firstterm>container</firstterm>
        types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
        and <literal>DICT_ENTRY</literal>.
      </para>

      <para>
        <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
        code does not appear in signatures. Instead, ASCII characters
        '(' and ')' are used to mark the beginning and end of the struct.
        So for example, a struct containing two integers would have this
        signature:
        <programlisting>
          "(ii)"
        </programlisting>
        Structs can be nested, so for example a struct containing
        an integer and another struct:
        <programlisting>
          "(i(ii))"
        </programlisting>
        The value block storing that struct would contain three integers; the
        type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
        "(iii)" or "iii".
      </para>

      <para>
        The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
        but is useful in code that implements the protocol. This type code
        is specified to allow such code to interoperate in non-protocol contexts.
      </para>

      <para>
        Empty structures are not allowed; there must be at least one
        type code between the parentheses.
      </para>

      <para>
        <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
        followed by a <firstterm>single complete type</firstterm>. The single
        complete type following the array is the type of each array element. So
        the simple example is:
        <programlisting>
          "ai"
        </programlisting>
        which is an array of 32-bit integers. But an array can be of any type,
        such as this array-of-struct-with-two-int32-fields:
        <programlisting>
          "a(ii)"
        </programlisting>
        Or this array of array of integer:
        <programlisting>
          "aai"
        </programlisting>
      </para>

      <para>
        <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
        type <literal>VARIANT</literal> will have the signature of a single complete type as part
        of the <emphasis>value</emphasis>.  This signature will be followed by a
        marshaled value of that type.
      </para>

      <para>
        Unlike a message signature, the variant signature can
        contain only a single complete type.  So "i", "ai"
        or "(ii)" is OK, but "ii" is not.  Use of variants may not
        cause a total message depth to be larger than 64, including
        other container types such as structures.
      </para>

      <para>
        A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
        than parentheses it uses curly braces, and it has more restrictions.
        The restrictions are: it occurs only as an array element type; it has
        exactly two single complete types inside the curly braces; the first
        single complete type (the "key") must be a basic type rather than a
        container type. Implementations must not accept dict entries outside of
        arrays, must not accept dict entries with zero, one, or more than two
        fields, and must not accept dict entries with non-basic-typed keys. A
        dict entry is always a key-value pair.
      </para>

      <para>
        The first field in the <literal>DICT_ENTRY</literal> is always the key.
        A message is considered corrupt if the same key occurs twice in the same
        array of <literal>DICT_ENTRY</literal>. However, for performance reasons
        implementations are not required to reject dicts with duplicate keys.
      </para>

      <para>
        In most languages, an array of dict entry would be represented as a
        map, hash table, or dict object.
      </para>
    </sect2>

    <sect2>
      <title>Summary of types</title>

      <para>
        The following table summarizes the D-Bus types.
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Category</entry>
                <entry>Conventional Name</entry>
                <entry>Code</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>reserved</entry>
                <entry><literal>INVALID</literal></entry>
                <entry>0 (ASCII NUL)</entry>
                <entry>Not a valid type code, used to terminate signatures</entry>
              </row><row>
                <entry>fixed, basic</entry>
                <entry><literal>BYTE</literal></entry>
                <entry>121 (ASCII 'y')</entry>
                <entry>8-bit unsigned integer</entry>
              </row><row>
                <entry>fixed, basic</entry>
                <entry><literal>BOOLEAN</literal></entry>
                <entry>98 (ASCII 'b')</entry>
                <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
              </row><row>
                <entry>fixed, basic</entry>
                <entry><literal>INT16</literal></entry>
                <entry>110 (ASCII 'n')</entry>
                <entry>16-bit signed integer</entry>
              </row><row>
                <entry>fixed, basic</entry>
                <entry><literal>UINT16</literal></entry>
                <entry>113 (ASCII 'q')</entry>
                <entry>16-bit unsigned integer</entry>
              </row><row>
                <entry>fixed, basic</entry>
                <entry><literal>INT32</literal></entry>
                <entry>105 (ASCII 'i')</entry>
                <entry>32-bit signed integer</entry>
              </row><row>
                <entry>fixed, basic</entry>
                <entry><literal>UINT32</literal></entry>
                <entry>117 (ASCII 'u')</entry>
                <entry>32-bit unsigned integer</entry>
              </row><row>
                <entry>fixed, basic</entry>
                <entry><literal>INT64</literal></entry>
                <entry>120 (ASCII 'x')</entry>
                <entry>64-bit signed integer</entry>
              </row><row>
                <entry>fixed, basic</entry>
                <entry><literal>UINT64</literal></entry>
                <entry>116 (ASCII 't')</entry>
                <entry>64-bit unsigned integer</entry>
              </row><row>
                <entry>fixed, basic</entry>
                <entry><literal>DOUBLE</literal></entry>
                <entry>100 (ASCII 'd')</entry>
                <entry>IEEE 754 double</entry>
              </row><row>
                <entry>string-like, basic</entry>
                <entry><literal>STRING</literal></entry>
                <entry>115 (ASCII 's')</entry>
                <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
              </row><row>
                <entry>string-like, basic</entry>
                <entry><literal>OBJECT_PATH</literal></entry>
                <entry>111 (ASCII 'o')</entry>
                <entry>Name of an object instance</entry>
              </row><row>
                <entry>string-like, basic</entry>
                <entry><literal>SIGNATURE</literal></entry>
                <entry>103 (ASCII 'g')</entry>
                <entry>A type signature</entry>
              </row><row>
                <entry>container</entry>
                <entry><literal>ARRAY</literal></entry>
                <entry>97 (ASCII 'a')</entry>
                <entry>Array</entry>
              </row><row>
                <entry>container</entry>
                <entry><literal>STRUCT</literal></entry>
                <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
                <entry>Struct; type code 114 'r' is reserved for use in
                  bindings and implementations to represent the general
                  concept of a struct, and must not appear in signatures
                  used on D-Bus.</entry>
              </row><row>
                <entry>container</entry>
                <entry><literal>VARIANT</literal></entry>
                <entry>118 (ASCII 'v') </entry>
                <entry>Variant type (the type of the value is part of the value itself)</entry>
              </row><row>
                <entry>container</entry>
                <entry><literal>DICT_ENTRY</literal></entry>
                <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
                <entry>Entry in a dict or map (array of key-value pairs).
                  Type code 101 'e' is reserved for use in bindings and
                  implementations to represent the general concept of a
                  dict or dict-entry, and must not appear in signatures
                  used on D-Bus.</entry>
              </row><row>
                <entry>fixed, basic</entry>
                <entry><literal>UNIX_FD</literal></entry>
                <entry>104 (ASCII 'h')</entry>
                <entry>Unix file descriptor</entry>
              </row>
              <row>
                <entry>reserved</entry>
                <entry>(reserved)</entry>
                <entry>109 (ASCII 'm')</entry>
                <entry>Reserved for <ulink
                    url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
                  'maybe' type compatible with the one in GVariant</ulink>,
                  and must not appear in signatures used on D-Bus until
                  specified here</entry>
              </row>
              <row>
                <entry>reserved</entry>
                <entry>(reserved)</entry>
                <entry>42 (ASCII '*')</entry>
                <entry>Reserved for use in bindings/implementations to
                  represent any <firstterm>single complete type</firstterm>,
                  and must not appear in signatures used on D-Bus.</entry>
              </row>
              <row>
                <entry>reserved</entry>
                <entry>(reserved)</entry>
                <entry>63 (ASCII '?')</entry>
                <entry>Reserved for use in bindings/implementations to
                  represent any <firstterm>basic type</firstterm>, and must
                  not appear in signatures used on D-Bus.</entry>
              </row>
              <row>
                <entry>reserved</entry>
                <entry>(reserved)</entry>
                <entry>64 (ASCII '@'), 38 (ASCII '&amp;'),
                  94 (ASCII '^')</entry>
                <entry>Reserved for internal use by bindings/implementations,
                  and must not appear in signatures used on D-Bus.
                  GVariant uses these type-codes to encode calling
                  conventions.</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>

    </sect2>
  </sect1>

  <sect1 id="message-protocol-marshaling">
    <title>Marshaling (Wire Format)</title>

    <para>
      D-Bus defines a marshalling format for its type system, which is
      used in D-Bus messages. This is not the only possible marshalling
      format for the type system: for instance, GVariant (part of GLib)
      re-uses the D-Bus type system but implements an alternative marshalling
      format.
    </para>

    <sect2>
      <title>Byte order and alignment</title>

      <para>
        Given a type signature, a block of bytes can be converted into typed
        values. This section describes the format of the block of bytes.  Byte
        order and alignment issues are handled uniformly for all D-Bus types.
      </para>

      <para>
        A block of bytes has an associated byte order. The byte order
        has to be discovered in some way; for D-Bus messages, the
        byte order is part of the message header as described in
        <xref linkend="message-protocol-messages"/>. For now, assume
        that the byte order is known to be either little endian or big
          endian.
      </para>

      <para>
        Each value in a block of bytes is aligned "naturally," for example
        4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
        8-byte boundary. Boundaries are calculated globally, with respect to
        the first byte in the message. To properly align a value,
        <firstterm>alignment padding</firstterm> may be necessary before the
        value. The alignment padding must always
        be the minimum required padding to properly align the following value;
        and it must always be made up of nul bytes. The alignment padding must
        not be left uninitialized (it can't contain garbage), and more padding
        than required must not be used.
      </para>

      <para>
        As an exception to natural alignment, <literal>STRUCT</literal> and
        <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
        boundary, regardless of the alignments of their contents.
      </para>
    </sect2>

    <sect2>
      <title>Marshalling basic types</title>

      <para>
        To marshal and unmarshal fixed types, you simply read one value
        from the data block corresponding to each type code in the signature.
        All signed integer values are encoded in two's complement, DOUBLE
        values are IEEE 754 double-precision floating-point, and BOOLEAN
        values are encoded in 32 bits (of which only the least significant
        bit is used).
      </para>

      <para>
        The string-like types (STRING, OBJECT_PATH and SIGNATURE) are all
        marshalled as a
        fixed-length unsigned integer <varname>n</varname> giving the
        length of the variable part, followed by <varname>n</varname>
        nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
        which is not considered to be part of the text. The alignment
        of the string-like type is the same as the alignment of
        <varname>n</varname>: any padding required for <varname>n</varname>
        appears immediately before <varname>n</varname> itself. There is never
        any alignment padding between <varname>n</varname> and the string text,
        or between the string text and the trailing nul. The alignment padding
        for the next value in the message (if there is one) starts after the
        trailing nul.
      </para>

      <para>
        For the STRING and OBJECT_PATH types, <varname>n</varname> is
        encoded in 4 bytes (a <literal>UINT32</literal>), leading to 4-byte
        alignment. For the SIGNATURE type, <varname>n</varname> is encoded as a
        single byte (a <literal>UINT8</literal>). As a result, alignment
        padding is never required before a SIGNATURE.
      </para>

      <para>
        For example, if the current position is a multiple of 8 bytes from the
        beginning of a little-endian message, strings ‘foo’, ‘+’ and ‘bar’
        would be serialized in sequence as follows:

        <screen>
                                          <lineannotation>no padding required, we are already at a multiple of 4</lineannotation>
0x03 0x00 0x00 0x00                       <lineannotation>length of ‘foo’ = 3</lineannotation>
                    0x66 0x6f 0x6f        <lineannotation>‘foo’</lineannotation>
                                   0x00   <lineannotation>trailing nul</lineannotation>

                                          <lineannotation>no padding required, we are already at a multiple of 4</lineannotation>
0x01 0x00 0x00 0x00                       <lineannotation>length of ‘+’ = 1</lineannotation>
                    0x2b                  <lineannotation>‘+’</lineannotation>
                         0x00             <lineannotation>trailing nul</lineannotation>

                               0x00 0x00  <lineannotation>2 bytes of padding to reach next multiple of 4</lineannotation>
0x03 0x00 0x00 0x00                       <lineannotation>length of ‘bar’ = 3</lineannotation>
                    0x62 0x61 0x72        <lineannotation>‘bar’</lineannotation>
                                    0x00  <lineannotation>trailing nul</lineannotation>
        </screen>
      </para>
    </sect2>

    <sect2>
      <title>Marshalling containers</title>

      <para>
        Arrays are marshalled as a <literal>UINT32</literal>
        <varname>n</varname> giving the length of the array data in bytes,
        followed by alignment padding to the alignment boundary of the array
        element type, followed by the <varname>n</varname> bytes of the
        array elements marshalled in sequence. <varname>n</varname> does not
        include the padding after the length, or any padding after the
        last element. i.e. <varname>n</varname> should be divisible by the
        number of elements in the array. Note that the alignment padding for the
        first element is required even if there is no first element (an
        empty array, where <varname>n</varname> is zero).
      </para>

      <para>
        For instance, if the current position in the message is a multiple
        of 8 bytes and the byte-order is big-endian, an array containing only
        the 64-bit integer 5 would be marshalled as:

        <screen>
00 00 00 08               <lineannotation><varname>n</varname> = 8 bytes of data</lineannotation>
00 00 00 00               <lineannotation>padding to 8-byte boundary</lineannotation>
00 00 00 00  00 00 00 05  <lineannotation>first element = 5</lineannotation>
        </screen>
      </para>

      <para>
        Arrays have a maximum length defined to be 2 to the 26th power or
        67108864 (64 MiB). Implementations must not send or accept arrays
        exceeding this length.
      </para>

      <para>
        Structs and dict entries are marshalled in the same way as their
        contents, but their alignment is always to an 8-byte boundary,
        even if their contents would normally be less strictly aligned.
      </para>

      <para>
        Variants are marshalled as the <literal>SIGNATURE</literal> of
        the contents (which must be a single complete type), followed by a
        marshalled value with the type given by that signature. The
        variant has the same 1-byte alignment as the signature, which means
        that alignment padding before a variant is never needed.
        Use of variants must not cause a total message depth to be larger
        than 64, including other container types such as structures.
        (See <link linkend="message-protocol-marshaling-signature">Valid
        Signatures</link>.)
      </para>

      <para>
        It should be noted that while a variant itself does not require any
        alignment padding, the contained value does need to be padded
        according to the alignment rules of its type.
      </para>

      <para>
        For instance, if the current position in the message is at a multiple of
        8 bytes and the byte-order is big-endian, a variant containing a 64-bit
        integer 5 would be marshalled as:

        <screen>
0x01 0x74 0x00                          <lineannotation>signature bytes (length = 1, signature = 't' and trailing nul)</lineannotation>
               0x00 0x00 0x00 0x00 0x00 <lineannotation>padding to 8-byte boundary</lineannotation>
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x05 <lineannotation>8 bytes of contained value</lineannotation>
        </screen>
      </para>
    </sect2>

    <sect2>
      <title>Summary of D-Bus marshalling</title>

      <para>
        Given all this, the types are marshaled on the wire as follows:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Conventional Name</entry>
                <entry>Encoding</entry>
                <entry>Alignment</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry><literal>INVALID</literal></entry>
                <entry>Not applicable; cannot be marshaled.</entry>
                <entry>N/A</entry>
              </row><row>
                <entry><literal>BYTE</literal></entry>
                <entry>A single 8-bit byte.</entry>
                <entry>1</entry>
              </row><row>
                <entry><literal>BOOLEAN</literal></entry>
                <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
                <entry>4</entry>
              </row><row>
                <entry><literal>INT16</literal></entry>
                <entry>16-bit signed integer in the message's byte order.</entry>
                <entry>2</entry>
              </row><row>
                <entry><literal>UINT16</literal></entry>
                <entry>16-bit unsigned integer in the message's byte order.</entry>
                <entry>2</entry>
              </row><row>
                <entry><literal>INT32</literal></entry>
                <entry>32-bit signed integer in the message's byte order.</entry>
                <entry>4</entry>
              </row><row>
                <entry><literal>UINT32</literal></entry>
                <entry>32-bit unsigned integer in the message's byte order.</entry>
                <entry>4</entry>
              </row><row>
                <entry><literal>INT64</literal></entry>
                <entry>64-bit signed integer in the message's byte order.</entry>
                <entry>8</entry>
              </row><row>
                <entry><literal>UINT64</literal></entry>
                <entry>64-bit unsigned integer in the message's byte order.</entry>
                <entry>8</entry>
              </row><row>
                <entry><literal>DOUBLE</literal></entry>
                <entry>64-bit IEEE 754 double in the message's byte order.</entry>
                <entry>8</entry>
              </row><row>
                <entry><literal>STRING</literal></entry>
                <entry>A <literal>UINT32</literal> indicating the string's
                  length in bytes excluding its terminating nul, followed by
                  non-nul string data of the given length, followed by a terminating nul
                  byte.
                </entry>
                <entry>
                  4 (for the length)
                </entry>
              </row><row>
                <entry><literal>OBJECT_PATH</literal></entry>
                <entry>Exactly the same as <literal>STRING</literal> except the
                  content must be a valid object path (see above).
                </entry>
                <entry>
                  4 (for the length)
                </entry>
              </row><row>
                <entry><literal>SIGNATURE</literal></entry>
                <entry>The same as <literal>STRING</literal> except the length is a single
                  byte (thus signatures have a maximum length of 255)
                  and the content must be a valid signature (see above).
                </entry>
                <entry>
                  1
                </entry>
              </row><row>
                <entry><literal>ARRAY</literal></entry>
                <entry>
                  A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
                  alignment padding to the alignment boundary of the array element type,
                  followed by each array element.
                </entry>
                <entry>
                  4 (for the length)
                </entry>
              </row><row>
                <entry><literal>STRUCT</literal></entry>
                <entry>
                  A struct must start on an 8-byte boundary regardless of the
                  type of the struct fields. The struct value consists of each
                  field marshaled in sequence starting from that 8-byte
                  alignment boundary.
                </entry>
                <entry>
                  8
                </entry>
              </row><row>
                <entry><literal>VARIANT</literal></entry>
                <entry>
                  The marshaled <literal>SIGNATURE</literal> of a single
                  complete type, followed by a marshaled value with the type
                  given in the signature.
                </entry>
                <entry>
                  1 (alignment of the signature)
                </entry>
              </row><row>
                <entry><literal>DICT_ENTRY</literal></entry>
                <entry>
                  Identical to STRUCT.
                </entry>
                <entry>
                  8
                </entry>
              </row><row>
                <entry><literal>UNIX_FD</literal></entry>
                <entry>32-bit unsigned integer in the message's byte
                order. The actual file descriptors need to be
                transferred out-of-band via some platform specific
                mechanism. On the wire, values of this type store the index to the
                file descriptor in the array of file descriptors that
                accompany the message.</entry>
                <entry>4</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>

    </sect2>

  </sect1>

  <sect1 id="message-protocol">
    <title>Message Protocol</title>

    <para>
      A <firstterm>message</firstterm> consists of a
      <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
      think of a message as a package, the header is the address, and the body
      contains the package contents. The message delivery system uses the header
      information to figure out where to send the message and how to interpret
      it; the recipient interprets the body of the message.
    </para>

    <para>
      The body of the message is made up of zero or more
      <firstterm>arguments</firstterm>, which are typed values, such as an
      integer or a byte array.
    </para>

    <para>
      Both header and body use the D-Bus <link linkend="type-system">type
        system</link> and format for serializing data.
    </para>

    <sect2 id="message-protocol-messages">
      <title>Message Format</title>

      <para>
        A message consists of a header and a body. The header is a block of
        values with a fixed signature and meaning.  The body is a separate block
        of values, with a signature specified in the header.
      </para>

      <para>
        The length of the header must be a multiple of 8, allowing the body to
        begin on an 8-byte boundary when storing the entire message in a single
        buffer. If the header does not naturally end on an 8-byte boundary
        up to 7 bytes of nul-initialized alignment padding must be added.
      </para>

      <para>
        The message body need not end on an 8-byte boundary.
      </para>

      <para>
        The maximum length of a message, including header, header alignment padding,
        and body is 2 to the 27th power or 134217728 (128 MiB).
        Implementations must not send or accept messages exceeding this size.
      </para>

      <para>
        The signature of the header is:
        <programlisting>
          "yyyyuua(yv)"
        </programlisting>
        Written out more readably, this is:
        <programlisting>
          BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
        </programlisting>
      </para>

      <para>
        These values have the following meanings:
        <informaltable>
          <tgroup cols="2">
            <thead>
              <row>
                <entry>Value</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>1st <literal>BYTE</literal></entry>
                <entry>Endianness flag; ASCII 'l' for little-endian
                  or ASCII 'B' for big-endian. Both header and body are
                in this endianness.</entry>
              </row>
              <row>
                <entry>2nd <literal>BYTE</literal></entry>
                <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
                  Currently-defined types are described below.
                </entry>
              </row>
              <row>
                <entry>3rd <literal>BYTE</literal></entry>
                <entry>Bitwise OR of flags. Unknown flags
                  must be ignored. Currently-defined flags are described below.
                </entry>
              </row>
              <row>
                <entry>4th <literal>BYTE</literal></entry>
                <entry>Major protocol version of the sending application.  If
                the major protocol version of the receiving application does not
                match, the applications will not be able to communicate and the
                D-Bus connection must be disconnected. The major protocol
                version for this version of the specification is 1.
                </entry>
              </row>
              <row>
                <entry>1st <literal>UINT32</literal></entry>
                <entry>Length in bytes of the message body, starting
                  from the end of the header. The header ends after
                  its alignment padding to an 8-boundary.
                </entry>
              </row>
              <row>
                <entry>2nd <literal>UINT32</literal></entry>
                <entry>The serial of this message, used as a cookie
                  by the sender to identify the reply corresponding
                  to this request. This must not be zero.
                </entry>
              </row>
              <row>
                <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
                <entry>An array of zero or more <firstterm>header
                  fields</firstterm> where the byte is the field code, and the
                  variant is the field value. The message type determines
                  which fields are required.
                </entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
      <para>
        <firstterm>Message types</firstterm> that can appear in the second byte
        of the header are:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Conventional name</entry>
                <entry>Decimal value</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry><literal>INVALID</literal></entry>
                <entry>0</entry>
                <entry>This is an invalid type.</entry>
              </row>
              <row>
                <entry><literal>METHOD_CALL</literal></entry>
                <entry>1</entry>
                <entry>Method call. This message type may prompt a
                  reply.</entry>
              </row>
              <row>
                <entry><literal>METHOD_RETURN</literal></entry>
                <entry>2</entry>
                <entry>Method reply with returned data.</entry>
              </row>
              <row>
                <entry><literal>ERROR</literal></entry>
                <entry>3</entry>
                <entry>Error reply. If the first argument exists and is a
                string, it is an error message.</entry>
              </row>
              <row>
                <entry><literal>SIGNAL</literal></entry>
                <entry>4</entry>
                <entry>Signal emission.</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
      <para>
        Flags that can appear in the third byte of the header:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Conventional name</entry>
                <entry>Hex value</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry><literal>NO_REPLY_EXPECTED</literal></entry>
                <entry>0x1</entry>
                <entry>
                  <para>
                    This message does not expect method return replies or
                    error replies, even if it is of a type that can
                    have a reply; the reply should be omitted.
                  </para>
                  <para>
                    Note that METHOD_CALL is the only message type currently
                    defined in this specification that can expect a reply,
                    so the presence or absence of this flag in the other
                    three message types that are currently
                    documented is meaningless: replies to those message
                    types should not be sent, whether this flag is present
                    or not.
                  </para>
                </entry>
              </row>
              <row>
                <entry><literal>NO_AUTO_START</literal></entry>
                <entry>0x2</entry>
                <entry>The bus must not launch an owner
                  for the destination name in response to this message.
                </entry>
              </row>
              <row>
                <entry><literal>ALLOW_INTERACTIVE_AUTHORIZATION</literal></entry>
                <entry>0x4</entry>
                <entry>
                  <para>
                    This flag may be set on a method call message to
                    inform the receiving side that the caller is prepared
                    to wait for interactive authorization, which might
                    take a considerable time to complete. For instance,
                    if this flag is set, it would be appropriate to
                    query the user for passwords or confirmation via
                    Polkit or a similar framework.
                  </para>
                  <para>
                    This flag is only useful when
                    unprivileged code calls a more privileged method call,
                    and an authorization framework is deployed that allows
                    possibly interactive authorization. If no such framework
                    is deployed it has no effect. This flag should not
                    be set by default by client implementations. If it is
                    set, the caller should also set a suitably long timeout
                    on the method call to make sure the user interaction
                    may complete. This flag is only valid for method call
                    messages, and shall be ignored otherwise.
                  </para>
                  <para>
                    Interaction that takes place as a part of the
                    effect of the method being called is outside the scope
                    of this flag, even if it could also be characterized
                    as authentication or authorization. For instance, in
                    a method call that directs a network management service
                    to attempt to connect to a virtual private network,
                    this flag should control how the network management
                    service makes the decision "is this user allowed to
                    change system network configuration?", but it should
                    not affect how or whether the network management
                    service interacts with the user to obtain the credentials
                    that are required for access to the VPN.
                  </para>
                  <para>
                    If a this flag is not set on a method call, and a
                    service determines that the requested operation is
                    not allowed without interactive authorization, but
                    could be allowed after successful interactive
                    authorization, it may return the
                    <literal>org.freedesktop.DBus.Error.InteractiveAuthorizationRequired</literal>
                    error.
                  </para>
                  <para>
                    The absence of this flag does not guarantee that
                    interactive authorization will not be applied, since
                    existing services that pre-date this flag might
                    already use interactive authorization. However,
                    existing D-Bus APIs that will use interactive
                    authorization should document that the call may take
                    longer than usual, and new D-Bus APIs should avoid
                    interactive authorization in the absence of this flag.
                  </para>
                </entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>

      <sect3 id="message-protocol-header-fields">
        <title>Header Fields</title>

        <para>
          The array at the end of the header contains <firstterm>header
          fields</firstterm>, where each field is a 1-byte field code followed
          by a field value. A header must contain the required header fields for
          its message type, and zero or more of any optional header
          fields. Future versions of this protocol specification may add new
          fields. Implementations must not invent their own header fields;
          only changes to this specification may introduce new header fields.
        </para>

        <para>
          If an implementation sees a header field code that it does not
          expect, it must accept and ignore that field, as it will be part
          of a new
          (but compatible) version of this specification. This also applies
          to known header fields appearing in unexpected messages, for
          example: if a signal has a reply serial it must be ignored
          even though it has no meaning as of this version of the spec.
        </para>

        <para>
          However, implementations must not send or accept known header fields
          with the wrong type stored in the field value. So for example a
          message with an <literal>INTERFACE</literal> field of type
          <literal>UINT32</literal> would be considered corrupt.
        </para>

        <para>
          Server implementations that might relay messages from one
          mutually-distrustful client to another, such as the message
          bus, should remove header fields that the server does not
          recognise. However, a client must assume that the server has
          not done so, unless it has evidence to the contrary,
          such as having checked for the <literal>HeaderFiltering</literal>
          <link linkend="message-bus-properties-features">message bus
            feature</link>.
        </para>

        <para>
          New header fields controlled by the message bus
          (similar to <literal>SENDER</literal>) might be added to this
          specification in future. Such message fields should normally
          only be added to messages that are going to be delivered to a
          client that specifically requested them (for example by calling
          some method), and the message bus should remove those header
          fields from all other messages that it relays. This design
          principle serves two main purposes. One is to avoid unnecessary
          memory and throughput overhead when delivering messages to
          clients that are not interested in the new header fields.
          The other is to give clients a reason to call the method that
          requests those messages (otherwise, the clients would not work).
          This is desirable because looking at the reply to that method
          call is a natural way to check that the message bus guarantees
          to filter out faked header fields that might have been sent by
          malicious peers.
        </para>

        <para>
          Here are the currently-defined header fields:
          <informaltable>
            <tgroup cols="5">
              <thead>
                <row>
                  <entry>Conventional Name</entry>
                  <entry>Decimal Code</entry>
                  <entry>Type</entry>
                  <entry>Required In</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry><literal>INVALID</literal></entry>
                  <entry>0</entry>
                  <entry>N/A</entry>
                  <entry>not allowed</entry>
                  <entry>Not a valid field name (error if it appears in a message)</entry>
                </row>
                <row>
                  <entry><literal>PATH</literal></entry>
                  <entry>1</entry>
                  <entry><literal>OBJECT_PATH</literal></entry>
                  <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
                  <entry>The object to send a call to,
                    or the object a signal is emitted from.
                    The special path
                    <literal>/org/freedesktop/DBus/Local</literal> is reserved;
                    implementations should not send messages with this path,
                    and the reference implementation of the bus daemon will
                    disconnect any application that attempts to do so.
                    This header field is controlled by the message sender.
                  </entry>
                </row>
                <row>
                  <entry><literal>INTERFACE</literal></entry>
                  <entry>2</entry>
                  <entry><literal>STRING</literal></entry>
                  <entry><literal>SIGNAL</literal></entry>
                  <entry>
                    The interface to invoke a method call on, or
                    that a signal is emitted from. Optional for
                    method calls, required for signals.
                    The special interface
                    <literal>org.freedesktop.DBus.Local</literal> is reserved;
                    implementations should not send messages with this
                    interface, and the reference implementation of the bus
                    daemon will disconnect any application that attempts to
                    do so.
                    This header field is controlled by the message sender.
                  </entry>
                </row>
                <row>
                  <entry><literal>MEMBER</literal></entry>
                  <entry>3</entry>
                  <entry><literal>STRING</literal></entry>
                  <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
                  <entry>
                    The member, either the method name or signal name.
                    This header field is controlled by the message sender.
                  </entry>
                </row>
                <row>
                  <entry><literal>ERROR_NAME</literal></entry>
                  <entry>4</entry>
                  <entry><literal>STRING</literal></entry>
                  <entry><literal>ERROR</literal></entry>
                  <entry>The name of the error that occurred, for errors</entry>
                </row>
                <row>
                  <entry><literal>REPLY_SERIAL</literal></entry>
                  <entry>5</entry>
                  <entry><literal>UINT32</literal></entry>
                  <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
                  <entry>
                    The serial number of the message this message is a reply
                    to. (The serial number is the second <literal>UINT32</literal>
                    in the header.)
                    This header field is controlled by the message sender.
                  </entry>
                </row>
                <row>
                  <entry><literal>DESTINATION</literal></entry>
                  <entry>6</entry>
                  <entry><literal>STRING</literal></entry>
                  <entry>optional</entry>
                  <entry>
                    The name of the connection this message is intended for.
                    This field is usually only meaningful in combination with
                    the message bus (see <xref linkend="message-bus"/>),
                    but other servers may define their own meanings for it.
                    This header field is controlled by the message sender.
                  </entry>
                </row>
                <row>
                  <entry><literal>SENDER</literal></entry>
                  <entry>7</entry>
                  <entry><literal>STRING</literal></entry>
                  <entry>optional</entry>
                  <entry>Unique name of the sending connection.
                    This field is usually only meaningful in combination with
                    the message bus, but other servers may define their own
                    meanings for it.
                    On a message bus, this header field is controlled by
                    the message bus, so it is as reliable and trustworthy
                    as the message bus itself.
                    Otherwise, this header field is controlled by the message
                    sender, unless there is out-of-band information that
                    indicates otherwise.
                  </entry>
                </row>
                <row>
                  <entry><literal>SIGNATURE</literal></entry>
                  <entry>8</entry>
                  <entry><literal>SIGNATURE</literal></entry>
                  <entry>optional</entry>
                  <entry>
                    The signature of the message body.
                    If omitted, it is assumed to be the
                    empty signature "" (i.e. the body must be 0-length).
                    This header field is controlled by the message sender.
                  </entry>
                </row>
                <row>
                  <entry><literal>UNIX_FDS</literal></entry>
                  <entry>9</entry>
                  <entry><literal>UINT32</literal></entry>
                  <entry>optional</entry>
                  <entry>The number of Unix file descriptors that
                  accompany the message.  If omitted, it is assumed
                  that no Unix file descriptors accompany the
                  message. The actual file descriptors need to be
                  transferred via a platform specific mechanism
                  out-of-band. They must be sent at the same time as
                  part of the message itself. They may not be sent
                  before the first byte of the message itself is
                  transferred or after the last byte of the message
                  itself. All file descriptors must be sent together
                  as a single set. Subsequent sets of file descriptors
                  within a message are to be ignored.
                  This header field is controlled by the message sender.
                  </entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
      </sect3>
    </sect2>

    <sect2 id="message-protocol-names">
      <title>Valid Names</title>
      <para>
        The various names in D-Bus messages have some restrictions.
      </para>
      <para>
        There is a <firstterm>maximum name length</firstterm>
        of 255 which applies to bus names, interfaces, and members.
      </para>
      <sect3 id="message-protocol-names-interface">
        <title>Interface names</title>
        <para>
          Interfaces have names with type <literal>STRING</literal>, meaning that
          they must be valid UTF-8. However, there are also some
          additional restrictions that apply to interface names
          specifically:
          <itemizedlist>
            <listitem><para>Interface names are composed of 2 or more elements separated by
                a period ('.') character. All elements must contain at least
                one character.
                </para>
            </listitem>
            <listitem><para>Each element must only contain the ASCII characters
                "[A-Z][a-z][0-9]_" and must not begin with a digit.
                </para>
            </listitem>

            <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
          </itemizedlist>
        </para>

        <para>
          Interface names should start with the reversed DNS domain name of
          the author of the interface (in lower-case), like interface names
          in Java. It is conventional for the rest of the interface name
          to consist of words run together, with initial capital letters
          on all words ("CamelCase"). Several levels of hierarchy can be used.
          It is also a good idea to include the major version of the interface
          in the name, and increment it if incompatible changes are made;
          this way, a single object can implement several versions of an
          interface in parallel, if necessary.
        </para>

        <para>
          For instance, if the owner of <literal>example.com</literal> is
          developing a D-Bus API for a music player, they might define
          interfaces called <literal>com.example.MusicPlayer1</literal>,
          <literal>com.example.MusicPlayer1.Track</literal> and
          <literal>com.example.MusicPlayer1.Seekable</literal>.
        </para>

        <para>
          If the author's DNS domain name contains hyphen/minus characters
          ('-'), which are not allowed in D-Bus interface names, they
          should be replaced by underscores. If the DNS domain name
          contains a digit immediately following a period ('.'), which
          is also not allowed in interface names), the interface name
          should add an underscore before that digit. For example, if
          the owner of 7-zip.org defined an interface for out-of-process
          plugins, it might be named <literal>org._7_zip.Plugin</literal>.
        </para>

        <para>
          If the author's DNS domain name is an internationalized domain
          name (IDN) such as <literal>δοκιμή.example</literal>,
          the ASCII encoding (known as ACE-encoding or Punycode) such as
          <literal>xn--jxalpdlp.example</literal>
          should be used as a basis for the reversed-domain-name form.
          As with any other name, hyphen/minus characters should be
          replaced by underscores in the reversed-domain-name form, for
          example <literal>example.xn__jxalpdlp.ExampleService1</literal>.
          For more information about internationalized domain names, see
          <ulink url="https://datatracker.ietf.org/doc/html/rfc5890">RFC
            5890 "Internationalized Domain Names for Applications (IDNA):
            Definitions and Document Framework"</ulink>.
        </para>

        <para>
          D-Bus does not distinguish between the concepts that would be
          called classes and interfaces in Java: either can be identified on
          D-Bus by an interface name.
        </para>
      </sect3>
      <sect3 id="message-protocol-names-bus">
        <title>Bus names</title>
        <para>
          Connections have one or more bus names associated with them.
          A connection has exactly one bus name that is a <firstterm>unique
            connection name</firstterm>. The unique connection name remains
          with the connection for its entire lifetime.
          A bus name is of type <literal>STRING</literal>,
          meaning that it must be valid UTF-8. However, there are also
          some additional restrictions that apply to bus names
          specifically:
          <itemizedlist>
            <listitem><para>Bus names that start with a colon (':')
                character are unique connection names. Other bus names
                are called <firstterm>well-known bus names</firstterm>.
                </para>
            </listitem>
            <listitem><para>Bus names are composed of 1 or more elements separated by
                a period ('.') character. All elements must contain at least
                one character.
                </para>
            </listitem>
            <listitem><para>Each element must only contain the ASCII characters
                "[A-Z][a-z][0-9]_-", with "-" discouraged in new bus
                names. Only elements that are part of a unique
                connection name may begin with a digit, elements in
                other bus names must not begin with a digit.
                </para>
            </listitem>

            <listitem><para>Bus names must contain at least one '.' (period)
              character (and thus at least two elements).
              </para></listitem>

            <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
            <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
          </itemizedlist>
        </para>
        <para>
          Note that the hyphen ('-') character is allowed in bus names
          but not in interface names. It is also problematic or not
          allowed in various specifications and APIs that refer to
          D-Bus, such as <ulink
            url="http://docs.flatpak.org/en/latest/introduction.html#identifiers"
            >Flatpak application IDs</ulink>,
          <ulink
            url="https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#dbus"
            >the <literal>DBusActivatable</literal> interface in the
            Desktop Entry Specification</ulink>, and the convention
          that an application's "main" interface and object path
          resemble its bus name. To avoid situations that require
          special-case handling, it is recommended that new D-Bus names
          consistently replace hyphens with underscores.
        </para>

        <para>
          Like <link linkend="message-protocol-names-interface">interface
            names</link>, well-known bus names should start with the
          reversed DNS domain name of the author of the interface (in
          lower-case), and it is conventional for the rest of the well-known
          bus name to consist of words run together, with initial
          capital letters. As with interface names, including a version
          number in well-known bus names is a good idea; it's possible to
          have the well-known bus name for more than one version
          simultaneously if backwards compatibility is required.
        </para>

        <para>
          As with <link linkend="message-protocol-names-interface">interface
            names</link>, if the author's DNS domain name
          contains hyphen/minus characters they should be replaced by
          underscores, if it contains leading digits they should be
          escaped by prepending an underscore, and internationalized
          domain names (IDN) need to be encoded in their ASCII form
          (ACE-encoding, Punycode) before replacing dashes with underscores.
          For example, if the owner
          of 7-zip.org used a D-Bus name for an archiving application,
          it might be named <literal>org._7_zip.Archiver</literal>,
          while the owner of <literal>δοκιμή.example</literal> might
          use the name <literal>example.xn__jxalpdlp.ExampleService1</literal>.
        </para>

        <para>
          If a well-known bus name implies the presence of a "main" interface,
          that "main" interface is often given the same name as
          the well-known bus name, and situated at the corresponding object
          path. For instance, if the owner of <literal>example.com</literal>
          is developing a D-Bus API for a music player, they might define
          that any application that takes the well-known name
          <literal>com.example.MusicPlayer1</literal> should have an object
          at the object path <literal>/com/example/MusicPlayer1</literal>
          which implements the interface
          <literal>com.example.MusicPlayer1</literal>.
        </para>
      </sect3>
      <sect3 id="message-protocol-names-member">
        <title>Member names</title>
        <para>
          Member (i.e. method or signal) names:
          <itemizedlist>
            <listitem><para>Must only contain the ASCII characters
                "[A-Z][a-z][0-9]_" and may not begin with a
                digit.</para></listitem>
            <listitem><para>Must not contain the '.' (period) character.</para></listitem>
            <listitem><para>Must not exceed the maximum name length.</para></listitem>
            <listitem><para>Must be at least 1 byte in length.</para></listitem>
          </itemizedlist>
        </para>

        <para>
          It is conventional for member names on D-Bus to consist of
          capitalized words with no punctuation ("camel-case").
          Method names should usually be verbs, such as
          <literal>GetItems</literal>, and signal names should usually be
          a description of an event, such as <literal>ItemsChanged</literal>.
        </para>
      </sect3>
      <sect3 id="message-protocol-names-error">
        <title>Error names</title>
        <para>
          Error names have the same restrictions as interface names.
        </para>

        <para>
          Error names have the same naming conventions as interface
          names, and often contain <literal>.Error.</literal>; for instance,
          the owner of <literal>example.com</literal> might define the
          errors <literal>com.example.MusicPlayer1.Error.FileNotFound</literal>
          and <literal>com.example.MusicPlayer1.Error.OutOfMemory</literal>.
          The errors defined by D-Bus itself, such as
          <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
          similar pattern.
        </para>
      </sect3>
    </sect2>

    <sect2 id="message-protocol-types">
      <title>Message Types</title>
      <para>
        Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
        <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
        This section describes these conventions.
      </para>
      <sect3 id="message-protocol-types-method">
        <title>Method Calls</title>
        <para>
          Some messages invoke an operation on a remote object.  These are
          called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
          messages map naturally to methods on objects in a typical program.
        </para>
        <para>
          A method call message is required to have a <literal>MEMBER</literal> header field
          indicating the name of the method. Optionally, the message has an
          <literal>INTERFACE</literal> field giving the interface the method is a part of.
          Including the <literal>INTERFACE</literal> in all method call
          messages is strongly recommended.
        </para>
        <para>
          In the absence of an <literal>INTERFACE</literal> field, if two
          or more interfaces on the same object have a method with the same
          name, it is undefined which of those methods will be invoked.
          Implementations may choose to either return an error, or deliver the
          message as though it had an arbitrary one of those interfaces.
        </para>
        <para>
          In some situations (such as the well-known system bus), messages
          are filtered through an access-control list external to the
          remote object implementation. If that filter rejects certain
          messages by matching their interface, or accepts only messages
          to specific interfaces, it must also reject messages that have no
          <literal>INTERFACE</literal>: otherwise, malicious
          applications could use this to bypass the filter.
        </para>
        <para>
          Method call messages also include a <literal>PATH</literal> field
          indicating the object to invoke the method on. If the call is passing
          through a message bus, the message will also have a
          <literal>DESTINATION</literal> field giving the name of the connection
          to receive the message.
        </para>
        <para>
          When an application handles a method call message, it is required to
          return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
          indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
          reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
        </para>
        <para>
          If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
          are the return value(s) or "out parameters" of the method call.
          If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
          and the call fails; no return value will be provided. It makes
          no sense to send multiple replies to the same method call.
        </para>
        <para>
          Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
          reply is required, so the caller will know the method
          was successfully processed.
        </para>
        <para>
          The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
          header field.
        </para>
        <para>
          If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
          then the application receiving the method should not send the reply message (regardless of
          whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
        </para>
        <para>
          Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
          destination name does not exist then a program to own the destination
          name will be started (activated) before the message is delivered. See
          <xref linkend="message-bus-starting-services"/>.
          The message
          will be held until the new program is successfully started or has
          failed to start; in case of failure, an error will be returned. This
          flag is only relevant in the context of a message bus, it is ignored
          during one-to-one communication with no intermediate bus.
        </para>
        <sect4 id="message-protocol-types-method-apis">
          <title>Mapping method calls to native APIs</title>
          <para>
            APIs for D-Bus may map method calls to a method call in a specific
            programming language, such as C++, or may map a method call written
            in an IDL to a D-Bus message.
          </para>
          <para>
            In APIs of this nature, arguments to a method are often termed "in"
            (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
            returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
            "inout" arguments, which are both sent and received, i.e. the caller
            passes in a value which is modified. Mapped to D-Bus, an "inout"
            argument is equivalent to an "in" argument, followed by an "out"
            argument. You can't pass things "by reference" over the wire, so
            "inout" is purely an illusion of the in-process API.
          </para>
          <para>
            Given a method with zero or one return values, followed by zero or more
            arguments, where each argument may be "in", "out", or "inout", the
            caller constructs a message by appending each "in" or "inout" argument,
            in order. "out" arguments are not represented in the caller's message.
          </para>
          <para>
            The recipient constructs a reply by appending first the return value
            if any, then each "out" or "inout" argument, in order.
            "in" arguments are not represented in the reply message.
          </para>
          <para>
            Error replies are normally mapped to exceptions in languages that have
            exceptions.
          </para>
          <para>
            In converting from native APIs to D-Bus, it is perhaps nice to
            map D-Bus naming conventions ("FooBar") to native conventions
            such as "fooBar" or "foo_bar" automatically. This is OK
            as long as you can say that the native API is one that
            was specifically written for D-Bus. It makes the most sense
            when writing object implementations that will be exported
            over the bus. Object proxies used to invoke remote D-Bus
            objects probably need the ability to call any D-Bus method,
            and thus a magic name mapping like this could be a problem.
          </para>
          <para>
            This specification doesn't require anything of native API bindings;
            the preceding is only a suggested convention for consistency
            among bindings.
          </para>
        </sect4>
      </sect3>

      <sect3 id="message-protocol-types-signal">
        <title>Signal Emission</title>
        <para>
          Unlike method calls, signal emissions have no replies.
          A signal emission is simply a single message of type <literal>SIGNAL</literal>.
          It must have three header fields: <literal>PATH</literal> giving the object
          the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
          the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
          for signals, though it is optional for method calls.
        </para>
      </sect3>

      <sect3 id="message-protocol-types-errors">
        <title>Errors</title>
        <para>
          Messages of type <literal>ERROR</literal> are most commonly replies
          to a <literal>METHOD_CALL</literal>, but may be returned in reply
          to any kind of message. The message bus for example
          will return an <literal>ERROR</literal> in reply to a signal emission if
          the bus does not have enough memory to send the signal.
        </para>
        <para>
          An <literal>ERROR</literal> may have any arguments, but if the first
          argument is a <literal>STRING</literal>, it must be an error message.
          The error message may be logged or shown to the user
          in some way.
        </para>
      </sect3>

      <sect3 id="message-protocol-types-notation">
        <title>Notation in this document</title>
        <para>
          This document uses a simple pseudo-IDL to describe particular method
          calls and signals. Here is an example of a method call:
          <programlisting>
            org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
                                                     out UINT32 resultcode)
          </programlisting>
          This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
          <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
          is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
          characters so it's known that the last part of the name in
          the "IDL" is the member name.
        </para>
        <para>
          In C++ that might end up looking like this:
          <programlisting>
            unsigned int org::freedesktop::DBus::StartServiceByName (const char  *name,
                                                                     unsigned int flags);
          </programlisting>
          or equally valid, the return value could be done as an argument:
          <programlisting>
            void org::freedesktop::DBus::StartServiceByName (const char   *name,
                                                             unsigned int  flags,
                                                             unsigned int *resultcode);
          </programlisting>
          It's really up to the API designer how they want to make
          this look. You could design an API where the namespace wasn't used
          in C++, using STL or Qt, using varargs, or whatever you wanted.
        </para>
        <para>
          Signals are written as follows:
          <programlisting>
            org.freedesktop.DBus.NameLost (STRING name)
          </programlisting>
          Signals don't specify "in" vs. "out" because only
          a single direction is possible.
        </para>
        <para>
          It isn't especially encouraged to use this lame pseudo-IDL in actual
          API implementations; you might use the native notation for the
          language you're using, or you might use COM or CORBA IDL, for example.
        </para>
      </sect3>
    </sect2>

    <sect2 id="message-protocol-handling-invalid">
      <title>Invalid Protocol and Spec Extensions</title>

      <para>
        For security reasons, the D-Bus protocol should be strictly parsed and
        validated, with the exception of defined extension points. Any invalid
        protocol or spec violations should result in immediately dropping the
        connection without notice to the other end. Exceptions should be
        carefully considered, e.g. an exception may be warranted for a
        well-understood idiosyncrasy of a widely-deployed implementation.  In
        cases where the other end of a connection is 100% trusted and known to
        be friendly, skipping validation for performance reasons could also make
        sense in certain cases.
      </para>

      <para>
        Generally speaking violations of the "must" requirements in this spec
        should be considered possible attempts to exploit security, and violations
        of the "should" suggestions should be considered legitimate (though perhaps
        they should generate an error in some cases).
      </para>

      <para>
        The following extension points are built in to D-Bus on purpose and must
        not be treated as invalid protocol. The extension points are intended
        for use by future versions of this spec, they are not intended for third
        parties.  At the moment, the only way a third party could extend D-Bus
        without breaking interoperability would be to introduce a way to negotiate new
        feature support as part of the auth protocol, using EXTENSION_-prefixed
        commands. There is not yet a standard way to negotiate features.
        <itemizedlist>
          <listitem>
            <para>
              In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
                commands result in an ERROR rather than a disconnect. This enables
                future extensions to the protocol. Commands starting with EXTENSION_ are
                reserved for third parties.
            </para>
          </listitem>
          <listitem>
            <para>
              The authentication protocol supports pluggable auth mechanisms.
            </para>
          </listitem>
          <listitem>
            <para>
              The address format (see <xref linkend="addresses"/>) supports new
              kinds of transport.
            </para>
          </listitem>
          <listitem>
            <para>
              Messages with an unknown type (something other than
              <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
              <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
              Unknown-type messages must still be well-formed in the same way
              as the known messages, however. They still have the normal
              header and body.
            </para>
          </listitem>
          <listitem>
            <para>
              Header fields with an unknown or unexpected field code must be ignored,
              though again they must still be well-formed.
            </para>
          </listitem>
          <listitem>
            <para>
              New standard interfaces (with new methods and signals) can of course be added.
            </para>
          </listitem>
        </itemizedlist>
      </para>

    </sect2>

  </sect1>

  <sect1 id="auth-protocol">
    <title>Authentication Protocol</title>
    <para>
      Before the flow of messages begins, two applications must
      authenticate. A simple plain-text protocol is used for
      authentication; this protocol is a SASL profile, and maps fairly
      directly from the SASL specification. The message encoding is
      NOT used here, only plain text messages.
    </para>
    <para>
      Using SASL in D-Bus requires that we define the meaning of non-empty
      authorization identity strings. When D-Bus is used on Unix platforms,
      a non-empty SASL authorization identity represents a Unix user. An
      authorization identity consisting entirely of ASCII decimal digits
      represents a numeric user ID as defined by POSIX, for example
      <literal>0</literal> for the root user or <literal>1000</literal>
      for the first user created on many systems. Non-numeric authorization
      identities are not required to be accepted or supported, but if used,
      they must be interpreted as a login name as found in the
      <literal>pw_name</literal> field of POSIX
      <literal>struct passwd</literal>, for example
      <literal>root</literal>, and normalized to the corresponding
      numeric user ID. For best interoperability, clients and servers
      should use numeric user IDs.
    </para>
    <para>
      When D-Bus is used on Windows platforms, a non-empty SASL
      authorization identity represents a Windows security identifier
      (SID) in its string form, for example
      <literal>S-1-5-21-3623811015-3361044348-30300820-1013</literal> for
      a domain or local computer user or <literal>S-1-5-18</literal> for
      the LOCAL_SYSTEM user. The user-facing usernames such as
      <literal>Administrator</literal> or <literal>LOCAL_SYSTEM</literal>
      are not used in the D-Bus protocol.
    </para>
    <para>
      In examples, "C:" and "S:" indicate lines sent by the client and
      server respectively. The client sends the first line, and the
      server must respond to each line from the client with a single-line
      reply, with one exception: there is no reply to the BEGIN command.
    </para>
    <sect2 id="auth-protocol-overview">
      <title>Protocol Overview</title>
      <para>
        The protocol is a line-based protocol, where each line ends with
        \r\n. Each line begins with an all-caps ASCII command name containing
        only the character range [A-Z_], a space, then any arguments for the
        command, then the \r\n ending the line. The protocol is
        case-sensitive. All bytes must be in the ASCII character set.

        Commands from the client to the server are as follows:

        <itemizedlist>
          <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
          <listitem><para>CANCEL</para></listitem>
          <listitem><para>BEGIN</para></listitem>
          <listitem><para>DATA &lt;data in hex encoding&gt;</para></listitem>
          <listitem><para>ERROR [human-readable error explanation]</para></listitem>
          <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
        </itemizedlist>

        From server to client are as follows:

        <itemizedlist>
          <listitem><para>REJECTED &lt;space-separated list of mechanism names&gt;</para></listitem>
          <listitem><para>OK &lt;GUID in hex&gt;</para></listitem>
          <listitem><para>DATA &lt;data in hex encoding&gt;</para></listitem>
          <listitem><para>ERROR [human-readable error explanation]</para></listitem>
          <listitem><para>AGREE_UNIX_FD</para></listitem>
        </itemizedlist>
      </para>
      <para>
        Unofficial extensions to the command set must begin with the letters
        "EXTENSION_", to avoid conflicts with future official commands.
        For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
      </para>
    </sect2>
    <sect2 id="auth-nul-byte">
      <title>Special credentials-passing nul byte</title>
      <para>
        Immediately after connecting to the server, the client must send a
        single nul byte. This byte may be accompanied by credentials
        information on some operating systems that use sendmsg() with
        SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
        sockets. However, the nul byte must be sent even on other kinds of
        socket, and even on operating systems that do not require a byte to be
        sent in order to transmit credentials. The text protocol described in
        this document begins after the single nul byte. If the first byte
        received from the client is not a nul byte, the server may disconnect
        that client.
      </para>
      <para>
        A nul byte in any context other than the initial byte is an error;
        the protocol is ASCII-only.
      </para>
      <para>
        The credentials sent along with the nul byte may be used with the
        SASL mechanism EXTERNAL.
      </para>
    </sect2>
    <sect2 id="auth-command-auth">
      <title>AUTH command</title>
      <para>
        The AUTH command is sent by the client to the server.
        The server replies with DATA, OK or REJECTED.
      </para>
      <para>
        If an AUTH command has no arguments, it is a request to list
        available mechanisms. The server must respond with a REJECTED
        command listing the mechanisms it understands, or with an error.
      </para>
      <para>
        If an AUTH command specifies a mechanism, and the server supports
        said mechanism, the server should begin exchanging SASL
        challenge-response data with the client using DATA commands.
      </para>
      <para>
        If the server does not support the mechanism given in the AUTH
        command, it must send either a REJECTED command listing the mechanisms
        it does support, or an error.
      </para>
      <para>
        If the [initial-response] argument is provided, it is intended for use
        with mechanisms that have no initial challenge (or an empty initial
        challenge), as if it were the argument to an initial DATA command. If
        the selected mechanism has an initial challenge and [initial-response]
        was provided, the server should reject authentication by sending
        REJECTED.
      </para>
      <para>
        If authentication succeeds after exchanging DATA commands,
        an OK command must be sent to the client.
      </para>
    </sect2>
    <sect2 id="auth-command-cancel">
      <title>CANCEL Command</title>
      <para>
        The CANCEL command is sent by the client to the server.
        The server replies with REJECTED.
      </para>
      <para>
        At any time up to sending the BEGIN command, the client may send a
        CANCEL command. On receiving the CANCEL command, the server must
        send a REJECTED command and abort the current authentication
        exchange.
      </para>
    </sect2>
    <sect2 id="auth-command-data">
      <title>DATA Command</title>
      <para>
        The DATA command may come from either client or server, and simply
        contains a hex-encoded block of data to be interpreted
        according to the SASL mechanism in use.
        If sent by the client, the server replies with DATA, OK or REJECTED.
      </para>
      <para>
        Some SASL mechanisms support sending an "empty string";
        FIXME we need some way to do this.
      </para>
    </sect2>
    <sect2 id="auth-command-begin">
      <title>BEGIN Command</title>
      <para>
        The BEGIN command is sent by the client to the server.
        The server does not reply.
      </para>
      <para>
        The BEGIN command acknowledges that the client has received an
        OK command from the server and completed any feature negotiation
        that it wishes to do, and declares that the stream of messages
        is about to begin.
      </para>
      <para>
        The first octet received by the server after the \r\n of the BEGIN
        command from the client must be the first octet of the
        authenticated/encrypted stream of D-Bus messages.
      </para>
      <para>
        Unlike all other commands, the server does not reply to
        the BEGIN command with an authentication command of its own.
        After the \r\n of the reply to the command before BEGIN,
        the next octet received by the client must be the first octet
        of the authenticated/encrypted stream of D-Bus messages.
      </para>
    </sect2>
    <sect2 id="auth-command-rejected">
      <title>REJECTED Command</title>
      <para>
        The REJECTED command is sent by the server to the client.
      </para>
      <para>
        The REJECTED command indicates that the current authentication
        exchange has failed, and further exchange of DATA is inappropriate.
        The client would normally try another mechanism, or try providing
        different responses to challenges.
      </para><para>
        Optionally, the REJECTED command has a space-separated list of
        available auth mechanisms as arguments. If a server ever provides
        a list of supported mechanisms, it must provide the same list
        each time it sends a REJECTED message. Clients are free to
        ignore all lists received after the first.
      </para>
    </sect2>
    <sect2 id="auth-command-ok">
      <title>OK Command</title>
      <para>
        The OK command is sent by the server to the client.
      </para>
      <para>
        The OK command indicates that the client has been
        authenticated. The client may now proceed with negotiating
        Unix file descriptor passing. To do that it shall send
        NEGOTIATE_UNIX_FD to the server.
      </para>
      <para>
        Otherwise, the client must respond to the OK command by
        sending a BEGIN command, followed by its stream of messages,
        or by disconnecting.  The server must not accept additional
        commands using this protocol after the BEGIN command has been
        received. Further communication will be a stream of D-Bus
        messages (optionally encrypted, as negotiated) rather than
        this protocol.
      </para>
      <para>
        If there is no negotiation, the first octet received by the client
        after the \r\n of the OK command must be the first octet of
        the authenticated/encrypted stream of D-Bus messages. If the
        client negotiates Unix file descriptor passing, the first
        octet received by the client after the \r\n of the AGREE_UNIX_FD
        or ERROR reply must be the first octet of the
        authenticated/encrypted stream.
      </para>
      <para>
        The OK command has one argument, which is the GUID of the server.
        See <xref linkend="addresses"/> for more on server GUIDs.
      </para>
    </sect2>
    <sect2 id="auth-command-error">
      <title>ERROR Command</title>
      <para>
        The ERROR command can be sent in either direction.
        If sent by the client, the server replies with REJECTED.
      </para>
      <para>
        The ERROR command indicates that either server or client did not
        know a command, does not accept the given command in the current
        context, or did not understand the arguments to the command. This
        allows the protocol to be extended; a client or server can send a
        command present or permitted only in new protocol versions, and if
        an ERROR is received instead of an appropriate response, fall back
        to using some other technique.
      </para>
      <para>
        If an ERROR is sent, the server or client that sent the
        error must continue as if the command causing the ERROR had never been
        received. However, the the server or client receiving the error
        should try something other than whatever caused the error;
        if only canceling/rejecting the authentication.
      </para>
      <para>
        If the D-Bus protocol changes incompatibly at some future time,
        applications implementing the new protocol would probably be able to
        check for support of the new protocol by sending a new command and
        receiving an ERROR from applications that don't understand it. Thus the
        ERROR feature of the auth protocol is an escape hatch that lets us
        negotiate extensions or changes to the D-Bus protocol in the future.
      </para>
    </sect2>
    <sect2 id="auth-command-negotiate-unix-fd">
      <title>NEGOTIATE_UNIX_FD Command</title>
      <para>
        The NEGOTIATE_UNIX_FD command is sent by the client to the server.
        The server replies with AGREE_UNIX_FD or ERROR.
      </para>
      <para>
        The NEGOTIATE_UNIX_FD command indicates that the client
        supports Unix file descriptor passing. This command may only
        be sent after the connection is authenticated, i.e. after OK
        was received by the client. This command may only be sent on
        transports that support Unix file descriptor passing.
      </para>
      <para>
        On receiving NEGOTIATE_UNIX_FD the server must respond with
        either AGREE_UNIX_FD or ERROR. It shall respond the former if
        the transport chosen supports Unix file descriptor passing and
        the server supports this feature. It shall respond the latter
        if the transport does not support Unix file descriptor
        passing, the server does not support this feature, or the
        server decides not to enable file descriptor passing due to
        security or other reasons.
      </para>
    </sect2>
    <sect2 id="auth-command-agree-unix-fd">
      <title>AGREE_UNIX_FD Command</title>
      <para>
        The AGREE_UNIX_FD command is sent by the server to the client.
      </para>
      <para>
        The AGREE_UNIX_FD command indicates that the server supports
        Unix file descriptor passing. This command may only be sent
        after the connection is authenticated, and the client sent
        NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
        command may only be sent on transports that support Unix file
        descriptor passing.
      </para>
      <para>
        On receiving AGREE_UNIX_FD the client must respond with BEGIN,
        followed by its stream of messages, or by disconnecting.  The
        server must not accept additional commands using this protocol
        after the BEGIN command has been received. Further
        communication will be a stream of D-Bus messages (optionally
        encrypted, as negotiated) rather than this protocol.
      </para>
    </sect2>
    <sect2 id="auth-command-future">
      <title>Future Extensions</title>
      <para>
        Future extensions to the authentication and negotiation
        protocol are possible. For that new commands may be
        introduced. If a client or server receives an unknown command
        it shall respond with ERROR and not consider this fatal. New
        commands may be introduced both before, and after
        authentication, i.e. both before and after the OK command.
      </para>
    </sect2>
    <sect2 id="auth-examples">
      <title>Authentication examples</title>

      <para>
        <figure>
          <title>Example of successful EXTERNAL authentication</title>
          <programlisting>
            31303030 is ASCII decimal "1000" represented in hex, so
            the client is authenticating as Unix uid 1000 in this example.

            C: AUTH EXTERNAL 31303030
            S: OK 1234deadbeef
            C: BEGIN
          </programlisting>
        </figure>
        <figure>
          <title>Example of finding out mechanisms then picking one</title>
          <programlisting>
            C: AUTH
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY 7ab83f32ee
            S: DATA 8799cabb2ea93e
            C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
            S: OK 1234deadbeef
            C: BEGIN
          </programlisting>
        </figure>
        <figure>
          <title>Example of client sends unknown command then falls back to regular auth</title>
          <programlisting>
            532d312d352d3138 is the Windows SID "S-1-5-18" in hex,
            so the client is authenticating as Windows SID S-1-5-18
            in this example.

            C: FOOBAR
            S: ERROR
            C: AUTH EXTERNAL 532d312d352d3138
            S: OK 1234deadbeef
            C: BEGIN
          </programlisting>
        </figure>
        <figure>
          <title>Example of server doesn't support initial auth mechanism</title>
          <programlisting>
            C: AUTH EXTERNAL
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY 7ab83f32ee
            S: DATA 8799cabb2ea93e
            C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
            S: OK 1234deadbeef
            C: BEGIN
          </programlisting>
        </figure>
        <figure>
          <title>Example of wrong password or the like followed by successful retry</title>
          <programlisting>
            C: AUTH EXTERNAL 736d6376
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY 7ab83f32ee
            S: DATA 8799cabb2ea93e
            C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
            S: REJECTED
            C: AUTH SKEY 7ab83f32ee
            S: DATA 8799cabb2ea93e
            C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
            S: OK 1234deadbeef
            C: BEGIN
          </programlisting>
        </figure>
        <figure>
          <title>Example of skey cancelled and restarted</title>
          <programlisting>
            C: AUTH EXTERNAL 32303438
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY 7ab83f32ee
            S: DATA 8799cabb2ea93e
            C: CANCEL
            S: REJECTED
            C: AUTH SKEY 7ab83f32ee
            S: DATA 8799cabb2ea93e
            C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
            S: OK 1234deadbeef
            C: BEGIN
          </programlisting>
        </figure>
        <figure>
          <title>Example of successful EXTERNAL authentication with successful negotiation of Unix FD passing</title>
          <programlisting>
            C: AUTH EXTERNAL 31303030
            S: OK 1234deadbeef
            C: NEGOTIATE_UNIX_FD
            S: AGREE_UNIX_FD
            C: BEGIN
          </programlisting>
        </figure>
        <figure>
          <title>Example of successful EXTERNAL authentication with unsuccessful negotiation of Unix FD passing</title>
          <programlisting>
            C: AUTH EXTERNAL 31303030
            S: OK 1234deadbeef
            C: NEGOTIATE_UNIX_FD
            S: ERROR Not supported on this OS
            C: BEGIN
          </programlisting>
        </figure>
      </para>
    </sect2>
    <sect2 id="auth-states">
      <title>Authentication state diagrams</title>

      <para>
        This section documents the auth protocol in terms of
        a state machine for the client and the server. This is
        probably the most robust way to implement the protocol.
      </para>

      <sect3 id="auth-states-client">
        <title>Client states</title>

        <para>
          To more precisely describe the interaction between the
          protocol state machine and the authentication mechanisms the
          following notation is used: MECH(CHALL) means that the
          server challenge CHALL was fed to the mechanism MECH, which
          returns one of

          <itemizedlist>
            <listitem>
              <para>
                CONTINUE(RESP) means continue the auth conversation
                and send RESP as the response to the server;
              </para>
            </listitem>

            <listitem>
              <para>
                OK(RESP) means that after sending RESP to the server
                the client side of the auth conversation is finished
                and the server should return "OK";
              </para>
            </listitem>

            <listitem>
              <para>
                ERROR means that CHALL was invalid and could not be
                processed.
              </para>
            </listitem>
          </itemizedlist>

          Both RESP and CHALL may be empty.
        </para>

        <para>
          The Client starts by getting an initial response from the
          default mechanism and sends AUTH MECH RESP, or AUTH MECH if
          the mechanism did not provide an initial response.  If the
          mechanism returns CONTINUE, the client starts in state
          <emphasis>WaitingForData</emphasis>, if the mechanism
          returns OK the client starts in state
          <emphasis>WaitingForOK</emphasis>.
        </para>

        <para>
          The client should keep track of available mechanisms and
          which it mechanisms it has already attempted. This list is
          used to decide which AUTH command to send. When the list is
          exhausted, the client should give up and close the
          connection.
        </para>

        <formalpara>
          <title><emphasis>WaitingForData</emphasis></title>
          <para>
            <itemizedlist>
              <listitem>
                <para>
                  Receive DATA CHALL
                  <simplelist>
                    <member>
                      MECH(CHALL) returns CONTINUE(RESP) &rarr; send
                      DATA RESP, goto
                      <emphasis>WaitingForData</emphasis>
                    </member>

                    <member>
                      MECH(CHALL) returns OK(RESP) &rarr; send DATA
                      RESP, goto <emphasis>WaitingForOK</emphasis>
                    </member>

                    <member>
                      MECH(CHALL) returns ERROR &rarr; send ERROR
                      [msg], goto <emphasis>WaitingForData</emphasis>
                    </member>
                  </simplelist>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive REJECTED [mechs] &rarr;
                  send AUTH [next mech], goto
                  WaitingForData or <emphasis>WaitingForOK</emphasis>
                </para>
              </listitem>
              <listitem>
                <para>
                  Receive ERROR &rarr; send
                  CANCEL, goto
                  <emphasis>WaitingForReject</emphasis>
                </para>
              </listitem>
              <listitem>
                <para>
                  Receive OK &rarr; <emphasis>authenticated</emphasis>,
                  choose one:

                  <simplelist>
                    <member>
                      send NEGOTIATE_UNIX_FD, goto
                      <emphasis>WaitingForAgreeUnixFD</emphasis>
                    </member>

                    <member>
                      send BEGIN, terminate auth conversation (successfully)
                    </member>
                  </simplelist>
                </para>
              </listitem>
              <listitem>
                <para>
                  Receive anything else &rarr; send
                  ERROR, goto
                  <emphasis>WaitingForData</emphasis>
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </formalpara>

        <formalpara>
          <title><emphasis>WaitingForOK</emphasis></title>
          <para>
            <itemizedlist>
              <listitem>
                <para>
                  Receive OK &rarr; <emphasis>authenticated</emphasis>,
                  choose one:

                  <simplelist>
                    <member>
                      send NEGOTIATE_UNIX_FD, goto
                      <emphasis>WaitingForAgreeUnixFD</emphasis>
                    </member>

                    <member>
                      send BEGIN, terminate auth conversation (successfully)
                    </member>
                  </simplelist>
                </para>
              </listitem>
              <listitem>
                <para>
                  Receive REJECTED [mechs] &rarr; send AUTH [next mech],
                  goto <emphasis>WaitingForData</emphasis> or
                  <emphasis>WaitingForOK</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive DATA &rarr; send CANCEL, goto
                  <emphasis>WaitingForReject</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive ERROR &rarr; send CANCEL, goto
                  <emphasis>WaitingForReject</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive anything else &rarr; send ERROR, goto
                  <emphasis>WaitingForOK</emphasis>
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </formalpara>

        <formalpara>
          <title><emphasis>WaitingForReject</emphasis></title>
          <para>
            <itemizedlist>
              <listitem>
                <para>
                  Receive REJECTED [mechs] &rarr; send AUTH [next mech],
                  goto <emphasis>WaitingForData</emphasis> or
                  <emphasis>WaitingForOK</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive anything else &rarr; terminate auth
                  conversation, disconnect
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </formalpara>

        <formalpara>
          <title><emphasis>WaitingForAgreeUnixFD</emphasis></title>
          <para>
            By the time this state is reached, the client has already
            been authenticated.
            <itemizedlist>
              <listitem>
                <para>
                  Receive AGREE_UNIX_FD &rarr; enable Unix fd passing,
                  send BEGIN, terminate auth conversation (successfully)
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive ERROR &rarr; disable Unix fd passing,
                  send BEGIN, terminate auth conversation (successfully)
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive anything else &rarr; terminate auth
                  conversation, disconnect
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </formalpara>

      </sect3>

      <sect3 id="auth-states-server">
        <title>Server states</title>

        <para>
          For the server MECH(RESP) means that the client response
          RESP was fed to the the mechanism MECH, which returns one of

          <itemizedlist>
            <listitem>
              <para>
                CONTINUE(CHALL) means continue the auth conversation and
                send CHALL as the challenge to the client;
              </para>
            </listitem>

            <listitem>
              <para>
                OK means that the client has been successfully
                authenticated;
              </para>
            </listitem>

            <listitem>
              <para>
                REJECTED means that the client failed to authenticate or
                there was an error in RESP.
              </para>
            </listitem>
          </itemizedlist>

          The server starts out in state
          <emphasis>WaitingForAuth</emphasis>.  If the client is
          rejected too many times the server must disconnect the
          client.
        </para>

        <formalpara>
          <title><emphasis>WaitingForAuth</emphasis></title>
          <para>
            <itemizedlist>

              <listitem>
                <para>
                  Receive AUTH &rarr; send REJECTED [mechs], goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive AUTH MECH RESP

                  <simplelist>
                    <member>
                      MECH not valid mechanism &rarr; send REJECTED
                      [mechs], goto
                      <emphasis>WaitingForAuth</emphasis>
                    </member>

                    <member>
                      MECH(RESP) returns CONTINUE(CHALL) &rarr; send
                      DATA CHALL, goto
                      <emphasis>WaitingForData</emphasis>
                    </member>

                    <member>
                      MECH(RESP) returns OK &rarr; send OK, goto
                      <emphasis>WaitingForBegin</emphasis>
                    </member>

                    <member>
                      MECH(RESP) returns REJECTED &rarr; send REJECTED
                      [mechs], goto
                      <emphasis>WaitingForAuth</emphasis>
                    </member>
                  </simplelist>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive BEGIN &rarr; terminate
                  auth conversation, disconnect
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive ERROR &rarr; send REJECTED [mechs], goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive anything else &rarr; send
                  ERROR, goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </formalpara>


        <formalpara>
          <title><emphasis>WaitingForData</emphasis></title>
          <para>
            <itemizedlist>
              <listitem>
                <para>
                  Receive DATA RESP
                  <simplelist>
                    <member>
                      MECH(RESP) returns CONTINUE(CHALL) &rarr; send
                      DATA CHALL, goto
                      <emphasis>WaitingForData</emphasis>
                    </member>

                    <member>
                      MECH(RESP) returns OK &rarr; send OK, goto
                      <emphasis>WaitingForBegin</emphasis>
                    </member>

                    <member>
                      MECH(RESP) returns REJECTED &rarr; send REJECTED
                      [mechs], goto
                      <emphasis>WaitingForAuth</emphasis>
                    </member>
                  </simplelist>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive BEGIN &rarr; terminate auth conversation,
                  disconnect
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive CANCEL &rarr; send REJECTED [mechs], goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive ERROR &rarr; send REJECTED [mechs], goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive anything else &rarr; send ERROR, goto
                  <emphasis>WaitingForData</emphasis>
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </formalpara>

        <formalpara>
          <title><emphasis>WaitingForBegin</emphasis></title>
          <para>
            <itemizedlist>
              <listitem>
                <para>
                  Receive BEGIN &rarr; terminate auth conversation,
                  client authenticated
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive NEGOTIATE_UNIX_FD &rarr; send AGREE_UNIX_FD
                  or ERROR, goto <emphasis>WaitingForBegin</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive CANCEL &rarr; send REJECTED [mechs], goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive ERROR &rarr; send REJECTED [mechs], goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive anything else &rarr; send ERROR, goto
                  <emphasis>WaitingForBegin</emphasis>
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </formalpara>

      </sect3>

    </sect2>
    <sect2 id="auth-mechanisms">
      <title>Authentication mechanisms</title>
      <para>
        This section describes some authentication mechanisms that are
        often supported by practical D-Bus implementations. The D-Bus
        protocol also allows any other standard SASL mechanism, although
        implementations of D-Bus often do not.
      </para>
      <sect3 id="auth-mechanisms-external">
        <title>EXTERNAL</title>
        <para>
          The EXTERNAL mechanism is defined in <ulink
            url="https://tools.ietf.org/html/rfc4422#appendix-A">RFC 4422
            "Simple Authentication and Security Layer (SASL)", appendix A
            "The SASL EXTERNAL Mechanism"</ulink>. This is the recommended
          authentication mechanism on platforms where credentials can be
          transferred out-of-band, in particular Unix platforms that can
          perform credentials-passing over the
          <link linkend="transports-unix-domain-sockets-addresses"
            >unix: transport</link>.
        </para>
        <para>
          On Unix platforms, interoperable clients should prefer to send
          the ASCII decimal string form of the integer Unix user ID as
          the authorization identity, for example 1000. When encoded in
          hex by the authentication protocol, this will typically result
          in a line like <literal>AUTH EXTERNAL 31303030</literal> followed
          by \r\n.
        </para>
        <para>
          On Windows platforms, clients that use the EXTERNAL mechanism
          should use the Windows security identifier in its string form
          as the authorization identity, for example
          <literal>S-1-5-21-3623811015-3361044348-30300820-1013</literal> for
          a domain or local computer user or <literal>S-1-5-18</literal> for
          the LOCAL_SYSTEM user. When encoded in hex by the authentication
          protocol, this will typically result in a line like
          <literal>AUTH EXTERNAL 532d312d352d3138</literal> followed by
          \r\n.
        </para>
      </sect3>
      <sect3 id="auth-mechanisms-sha">
        <title>DBUS_COOKIE_SHA1</title>
        <para>
          DBUS_COOKIE_SHA1 is a D-Bus-specific SASL mechanism. Its
          reference implementation is part of the reference implementation
          of D-Bus.
        </para>
        <para>
          This mechanism is designed to establish that a client
          has the ability to read a private file owned by the user being
          authenticated. If the client can prove that it has access to a secret
          cookie stored in this file, then the client is authenticated.
          Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
          directory. This is the recommended authentication mechanism
          for platforms and configurations where EXTERNAL cannot be used.
        </para>
        <para>
          Throughout this description, "hex encoding" must output the digits
          from a to f in lower-case; the digits A to F must not be used
          in the DBUS_COOKIE_SHA1 mechanism.
        </para>
        <para>
          Authentication proceeds as follows:
          <itemizedlist>
            <listitem>
              <para>
                The client sends the username it would like to authenticate
                as, hex-encoded.
              </para>
            </listitem>
            <listitem>
              <para>
                The server sends the name of its "cookie context" (see below); a
                space character; the integer ID of the secret cookie the client
                must demonstrate knowledge of; a space character; then a
                randomly-generated challenge string, all of this hex-encoded into
                one, single string.
              </para>
            </listitem>
            <listitem>
              <para>
                The client locates the cookie and generates its own
                randomly-generated challenge string. The client then concatenates
                the server's decoded challenge, a ":" character, its own challenge,
                another ":" character, and the cookie. It computes the SHA-1 hash
                of this composite string as a hex digest. It concatenates the
                client's challenge string, a space character, and the SHA-1 hex
                digest, hex-encodes the result and sends it back to the server.
              </para>
            </listitem>
            <listitem>
              <para>
                The server generates the same concatenated string used by the
                client and computes its SHA-1 hash. It compares the hash with
                the hash received from the client; if the two hashes match, the
                client is authenticated.
              </para>
            </listitem>
          </itemizedlist>
        </para>
        <para>
          Each server has a "cookie context," which is a name that identifies a
          set of cookies that apply to that server. A sample context might be
          "org_freedesktop_session_bus". Context names must be valid ASCII,
          nonzero length, and may not contain the characters slash ("/"),
          backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
          tab ("\t"), or period ("."). There is a default context,
          "org_freedesktop_general" that's used by servers that do not specify
          otherwise.
        </para>
        <para>
          Cookies are stored in a user's home directory, in the directory
          <filename>~/.dbus-keyrings/</filename>. This directory must
          not be readable or writable by other users. If it is,
          clients and servers must ignore it. The directory
          contains cookie files named after the cookie context.
        </para>
        <para>
          A cookie file contains one cookie per line. Each line
          has three space-separated fields:
          <itemizedlist>
            <listitem>
              <para>
                The cookie ID number, which must be a non-negative integer and
                may not be used twice in the same file.
              </para>
            </listitem>
            <listitem>
              <para>
                The cookie's creation time, in UNIX seconds-since-the-epoch
                format.
              </para>
            </listitem>
            <listitem>
              <para>
                The cookie itself, a hex-encoded random block of bytes. The cookie
                may be of any length, though obviously security increases
                as the length increases.
              </para>
            </listitem>
          </itemizedlist>
        </para>
        <para>
          Only server processes modify the cookie file.
          They must do so with this procedure:
          <itemizedlist>
            <listitem>
              <para>
                Create a lockfile name by appending ".lock" to the name of the
                cookie file.  The server should attempt to create this file
                using <literal>O_CREAT | O_EXCL</literal>.  If file creation
                fails, the lock fails. Servers should retry for a reasonable
                period of time, then they may choose to delete an existing lock
                to keep users from having to manually delete a stale
                lock. <footnote><para>Lockfiles are used instead of real file
                locking <literal>fcntl()</literal> because real locking
                implementations are still flaky on network
                filesystems.</para></footnote>
              </para>
            </listitem>
            <listitem>
              <para>
                Once the lockfile has been created, the server loads the cookie
                file. It should then delete any cookies that are old (the
                timeout can be fairly short), or more than a reasonable
                time in the future (so that cookies never accidentally
                become permanent, if the clock was set far into the future
                at some point). The reference implementation deletes cookies
                that are more than 5 minutes into the future, or more than
                7 minutes in the past. For interoperability, using the
                same arbitrary times in other implementations is suggested.
              </para>
            </listitem>
            <listitem>
              <para>
                If no sufficiently recent cookies remain, the server
                generates a new cookie. To avoid spurious authentication
                failures, cookies that are close to their deletion time
                should not be used for new authentication operations.
                For example, this avoids a client starting to use a cookie
                whose age is 6m59s, and having authentication subsequently
                fail because it takes 2 seconds, during which time the
                cookie's age became 7m01s, greater than 7 minutes, causing
                the server to delete it. The reference implementation
                generates a new cookie whenever the most recent cookie is
                older than 5 minutes, giving clients at least 2 minutes
                to finish authentication. For interoperability, using the
                same arbitrary time in other implementations is suggested.
              </para>
            </listitem>
            <listitem>
              <para>
                The pruned and possibly added-to cookie file
                must be resaved atomically (using a temporary
                file which is rename()'d).
              </para>
            </listitem>
            <listitem>
              <para>
                The lock must be dropped by deleting the lockfile.
              </para>
            </listitem>
          </itemizedlist>
        </para>
        <para>
          Clients need not lock the file in order to load it,
          because servers are required to save the file atomically.
        </para>
      </sect3>
      <sect3 id="auth-mechanisms-anonymous">
        <title>ANONYMOUS</title>
        <para>
          The ANONYMOUS mechanism is defined in <ulink
            url="https://tools.ietf.org/html/rfc4505">RFC 4505 "Anonymous
            Simple Authentication and Security Layer (SASL)
            Mechanism"</ulink>.
          It does not perform any authentication at all, and should not
          be accepted by message buses. However, it might sometimes be
          useful for non-message-bus uses of D-Bus.
        </para>
      </sect3>
    </sect2>
  </sect1>
  <sect1 id="addresses">
    <title>Server Addresses</title>
    <para>
      Server addresses consist of a transport name followed by a colon, and
      then an optional, comma-separated list of keys and values in the form key=value.
      Each value is escaped.
    </para>
    <para>
      For example:
      <programlisting>unix:path=/tmp/dbus-test</programlisting>
      Which is the address to a unix socket with the path /tmp/dbus-test.
    </para>
    <para>
      Value escaping is similar to URI escaping but simpler.
      <itemizedlist>
        <listitem>
          <para>
            The set of optionally-escaped bytes is:
            <literal>[-0-9A-Za-z_/.\*]</literal>. To escape, each
            <emphasis>byte</emphasis> (note, not character) which is not in the
            set of optionally-escaped bytes must be replaced with an ASCII
            percent (<literal>%</literal>) and the value of the byte in hex.
            The hex value must always be two digits, even if the first digit is
            zero. The optionally-escaped bytes may be escaped if desired.
          </para>
        </listitem>
        <listitem>
          <para>
            To unescape, append each byte in the value; if a byte is an ASCII
            percent (<literal>%</literal>) character then append the following
            hex value instead. It is an error if a <literal>%</literal> byte
            does not have two hex digits following. It is an error if a
            non-optionally-escaped byte is seen unescaped.
          </para>
        </listitem>
      </itemizedlist>
      The set of optionally-escaped bytes is intended to preserve address
      readability and convenience.
    </para>

    <para>
      A server may specify a key-value pair with the key <literal>guid</literal>
      and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
      describes the format of the <literal>guid</literal> field.  If present,
      this UUID may be used to distinguish one server address from another. A
      server should use a different UUID for each address it listens on. For
      example, if a message bus daemon offers both UNIX domain socket and TCP
      connections, but treats clients the same regardless of how they connect,
      those two connections are equivalent post-connection but should have
      distinct UUIDs to distinguish the kinds of connection.
    </para>

    <para>
      The intent of the address UUID feature is to allow a client to avoid
      opening multiple identical connections to the same server, by allowing the
      client to check whether an address corresponds to an already-existing
      connection.  Comparing two addresses is insufficient, because addresses
      can be recycled by distinct servers, and equivalent addresses may look
      different if simply compared as strings (for example, the host in a TCP
      address can be given as an IP address or as a hostname).
    </para>

    <para>
      Note that the address key is <literal>guid</literal> even though the
      rest of the API and documentation says "UUID," for historical reasons.
    </para>

    <para>
      [FIXME clarify if attempting to connect to each is a requirement
      or just a suggestion]
      When connecting to a server, multiple server addresses can be
      separated by a semi-colon. The library will then try to connect
      to the first address and if that fails, it'll try to connect to
      the next one specified, and so forth. For example
      <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
    </para>

    <para>
      Some addresses are <firstterm>connectable</firstterm>. A connectable
      address is one containing enough information for a client to connect
      to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
      is a connectable address. It is not necessarily possible to listen
      on every connectable address: for instance, it is not possible to
      listen on a <literal>unixexec:</literal> address.
    </para>

    <para>
      Some addresses are <firstterm>listenable</firstterm>. A listenable
      address is one containing enough information for a server to listen on
      it, producing a connectable address (which may differ from the
      original address). Many listenable addresses are not connectable:
      for instance, <literal>tcp:host=127.0.0.1</literal>
      is listenable, but not connectable (because it does not specify
      a port number).
    </para>

    <para>
      Listening on an address that is not connectable will result in a
      connectable address that is not the same as the listenable address.
      For instance, listening on <literal>tcp:host=127.0.0.1</literal>
      might result in the connectable address
      <literal>tcp:host=127.0.0.1,port=30958</literal>,
      listening on <literal>unix:tmpdir=/tmp</literal>
      might result in the connectable address
      <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>, or
      listening on <literal>unix:runtime=yes</literal>
      might result in the connectable address
      <literal>unix:path=/run/user/1234/bus</literal>.
    </para>
  </sect1>

  <sect1 id="transports">
    <title>Transports</title>
    <para>
      [FIXME we need to specify in detail each transport and its possible arguments]

      Current transports include: unix domain sockets (including
      abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
      using in-process pipes. Future possible transports include one that
      tunnels over X11 protocol.
    </para>

    <sect2 id="transports-unix-domain-sockets">
      <title>Unix Domain Sockets</title>
      <para>
        Unix domain sockets can be either paths in the file system or on Linux
        kernels, they can be abstract which are similar to paths but
        do not show up in the file system.
      </para>

      <para>
        When a Unix socket is opened by the D-Bus library, the socket address
        length does not include the whole <literal>struct sockaddr_un</literal>,
        but only the length of the pathname or abstract string (beside other
        fields).
      </para>
      <para>
        They are the recommended transport for D-Bus, either used alone or in
        conjunction with <link linkend="transports-systemd">systemd</link> or
        <link linkend="transports-launchd">launchd</link> addresses.
      </para>
      <para>
        Unix addresses that specify <literal>path</literal> or
        <literal>abstract</literal> are both listenable and connectable.
        Unix addresses that specify <literal>tmpdir</literal>
        or <literal>dir</literal> are only
        listenable: the corresponding connectable address will specify
        either <literal>path</literal> or <literal>abstract</literal>.
        Similarly, Unix addresses that specify <literal>runtime</literal>
        are only listenable, and the corresponding connectable address
        will specify <literal>path</literal>.
      </para>
      <sect3 id="transports-unix-domain-sockets-addresses">
        <title>Server Address Format</title>
        <para>
          Unix domain socket addresses are identified by the "unix:" prefix
          and support the following key/value pairs:
        </para>
        <informaltable>
         <tgroup cols="3">
          <thead>
           <row>
            <entry>Name</entry>
            <entry>Values</entry>
            <entry>Description</entry>
           </row>
          </thead>
          <tbody>
           <row>
            <entry>path</entry>
            <entry>(path)</entry>
            <entry>
              Path of the unix domain socket.
            </entry>
          </row>
          <row>
            <entry>dir</entry>
            <entry>(path)</entry>
            <entry>
              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;
              the resulting client address will have the "path" key instead.
              be set.
            </entry>
          </row>
          <row>
            <entry>tmpdir</entry>
            <entry>(path)</entry>
            <entry>
              The same as "dir", except that on platforms with
              abstract sockets, the server may attempt to create an
              abstract socket whose name starts with this directory instead
              of a path-based socket.
              The server is not required to create an abstract socket, even
              on platforms that support it: treating "tmpdir" as being
              equivalent to "dir" is a valid implementation, and recent
              versions of the reference implementation of D-Bus do this.
              See "abstract", below, for more details of the consequences
              of using abstract sockets.
              Like "dir", this key can only be used in server
              addresses, not in client addresses; the resulting client address
              will have the "abstract" or "path" key instead.
            </entry>
          </row>
          <row>
            <entry>abstract</entry>
            <entry>(string)</entry>
            <entry>
              Unique string in the abstract namespace, often syntactically
              resembling a path but unconnected to the filesystem namespace.
              This key is only supported on platforms with abstract Unix
              sockets, of which Linux is the only known example.
              Implementors should note that on Linux, abstract sockets are
              namespaced according to
              <ulink url="https://man7.org/linux/man-pages/man7/network_namespaces.7.html"
                >network namespaces</ulink>
              rather than being part of the filesystem.
              This means that abstract sockets are unaffected by mechanisms
              like
              <ulink url="https://man7.org/linux/man-pages/man2/chroot.2.html"
                >chroot(2)</ulink>
              and
              <ulink url="https://man7.org/linux/man-pages/man7/mount_namespaces.7.html"
                >mount namespaces</ulink>,
              which can lead to a sandbox escape if a sandboxing
              implementation alters the sandboxed process's view of the
              filesystem but shares the network namespace with the host.
            </entry>
          </row>
          <row>
            <entry>runtime</entry>
            <entry><literal>yes</literal></entry>
            <entry>If given, This key can only be used in server addresses, not in client addresses. If set, its value must be <literal>yes</literal>. This is typically used in an address string like <literal>unix:runtime=yes;unix:tmpdir=/tmp</literal> so that there can be a fallback if <literal>XDG_RUNTIME_DIR</literal> is not set.</entry>
          </row>
        </tbody>
        </tgroup>
       </informaltable>
       <para>
         Exactly one of the keys <literal>path</literal>,
         <literal>abstract</literal>, <literal>runtime</literal>,
         <literal>dir</literal> or <literal>tmpdir</literal> must be provided.
       </para>
      </sect3>
    </sect2>
    <sect2 id="transports-launchd">
      <title>launchd</title>
      <para>
        launchd is a server management system that replaces init, inetd
        and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
        bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
      </para>

      <para>
        launchd allocates a socket and provides it with the unix path through the
        DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
        spawned by launchd (or dbus-daemon, if it was started by launchd) can access
        it through its environment.
        Other processes can query for the launchd socket by executing:
        $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
        This is normally done by the D-Bus client library so doesn't have to be done
        manually.
      </para>
      <para>
        launchd is not available on Microsoft Windows.
      </para>
      <para>
        launchd addresses are listenable and connectable.
      </para>
      <sect3 id="transports-launchd-addresses">
        <title>Server Address Format</title>
        <para>
          launchd addresses are identified by the "launchd:" prefix
          and support the following key/value pairs:
        </para>
        <informaltable>
         <tgroup cols="3">
          <thead>
           <row>
            <entry>Name</entry>
            <entry>Values</entry>
            <entry>Description</entry>
           </row>
          </thead>
          <tbody>
           <row>
            <entry>env</entry>
            <entry>(environment variable)</entry>
            <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
          </row>
        </tbody>
        </tgroup>
       </informaltable>
       <para>
         The <literal>env</literal> key is required.
       </para>
      </sect3>
    </sect2>
    <sect2 id="transports-systemd">
      <title>systemd</title>
      <para>
        systemd is an open-source server management system that
        replaces init and inetd on newer Linux systems. It supports
        socket activation. The D-Bus systemd transport is used to acquire
        socket activation file descriptors from systemd and use them
        as D-Bus transport when the current process is spawned by
        socket activation from it.
      </para>
      <para>
        The systemd transport accepts only one or more Unix domain or
        TCP streams sockets passed in via socket activation. Using
        Unix domain sockets is strongly recommended.
      </para>
      <para>
        The systemd transport is not available on non-Linux operating systems.
      </para>
      <para>
        The systemd transport defines no parameter keys.
      </para>
      <para>
        systemd addresses are listenable, but not connectable. The
        corresponding connectable address is the <literal>unix</literal>
        or <literal>tcp</literal> address of the socket.
      </para>
    </sect2>
    <sect2 id="transports-tcp-sockets">
      <title>TCP Sockets</title>
      <para>
        The tcp transport provides TCP/IP based connections between clients
        located on the same or different hosts.
      </para>
      <para>
        Similar to remote X11, the TCP transport has no integrity or
        confidentiality protection, so it should normally only be
        used across the local loopback interface, for example using an
        address like <literal>tcp:host=127.0.0.1</literal> or
        <literal>tcp:host=localhost</literal>. In particular,
        configuring the well-known system bus or the well-known session
        bus to listen on a non-loopback TCP address is insecure.
      </para>
      <para>
        On Windows and most Unix platforms, the TCP stack is unable to transfer
        credentials over a TCP connection, so the
        <link linkend="auth-mechanisms-external">EXTERNAL</link>
        authentication mechanism does not normally work for this transport
        (although the reference implementation of D-Bus is able to identify
        loopback TCPv4 connections on Windows by their port number,
        partially enabling the EXTERNAL mechanism).
        The <link linkend="auth-mechanisms-sha">DBUS_COOKIE_SHA1</link>
        mechanism is normally used instead.
      </para>
      <para>
        Developers are sometimes tempted to use remote TCP as a debugging
        tool. However, if this functionality is left enabled in finished
        products, the result will be dangerously insecure. Instead of
        using remote TCP, developers should <ulink
          url="https://lists.freedesktop.org/archives/dbus/2018-April/017447.html"
          >relay connections via Secure Shell or a similar protocol</ulink>.
        <!-- TODO: Ideally someone would write a more formal guide to
             remote D-Bus debugging, and we could link to that instead -->
      </para>
      <para>
        Remote TCP connections were historically sometimes used to share
        a single session bus between login sessions of the same user on
        different machines within a trusted local area network, in
        conjunction with unencrypted remote X11, a NFS-shared home
        directory and NIS (YP) authentication. This is insecure against
        an attacker on the same LAN and should be considered strongly
        deprecated; more specifically, it is insecure in the same ways
        and for the same reasons as unencrypted remote X11 and NFSv2/NFSv3.
        The D-Bus maintainers
        recommend using a separate session bus per (user, machine) pair,
        only accessible from within that machine.
      </para>
      <para>
        All <literal>tcp</literal> addresses are listenable.
        <literal>tcp</literal> addresses in which both
        <literal>host</literal> and <literal>port</literal> are
        specified, and <literal>port</literal> is non-zero,
        are also connectable.
      </para>
      <sect3 id="transports-tcp-sockets-addresses">
        <title>Server Address Format</title>
        <para>
         TCP/IP socket addresses are identified by the "tcp:" prefix
         and support the following key/value pairs:
        </para>
        <informaltable>
         <tgroup cols="3">
          <thead>
           <row>
            <entry>Name</entry>
            <entry>Values</entry>
            <entry>Description</entry>
           </row>
          </thead>
          <tbody>
           <row>
            <entry>host</entry>
            <entry>(string)</entry>
            <entry>DNS name or IP address</entry>
          </row>
          <row>
           <entry>bind</entry>
           <entry>(string)</entry>
           <entry>Used in a listenable address to configure the interface
            on which the server will listen: either the IP address of one of
            the local machine's interfaces (most commonly <literal>127.0.0.1
            </literal>), or a DNS name that resolves to one of those IP
            addresses, or '*' to listen on all interfaces simultaneously.
            If not specified, the default is the same value as "host".
           </entry>
          </row>
          <row>
           <entry>port</entry>
           <entry>(number)</entry>
           <entry>The tcp port the server will open. A zero value let the server
            choose a free port provided from the underlaying operating system.
            libdbus is able to retrieve the real used port from the server.
           </entry>
          </row>
          <row>
           <entry>family</entry>
           <entry>(string)</entry>
           <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
          </row>
         </tbody>
        </tgroup>
       </informaltable>
      </sect3>
    </sect2>
    <sect2 id="transports-nonce-tcp-sockets">
      <title>Nonce-authenticated TCP Sockets</title>
      <para>
        The nonce-tcp transport provides a modified TCP transport using a
        simple authentication mechanism, to ensure that only clients with read
        access to a certain location in the filesystem can connect to the server.
        The server writes a secret, the nonce, to a file and an incoming client
        connection is only accepted if the client sends the nonce right after
        the connect. The nonce mechanism requires no setup and is orthogonal to
        the higher-level authentication mechanisms described in the
        Authentication section.
      </para>
      <para>
        The nonce-tcp transport is conceptually similar to a combination
        of the <link linkend="auth-mechanisms-sha">DBUS_COOKIE_SHA1</link>
        authentication mechanism and the
        <link linkend="transports-tcp-sockets">tcp</link> transport,
        and appears to have originally been implemented as a result of
        a misunderstanding of the SASL authentication mechanisms.
      </para>
      <para>
        Like the ordinary tcp transport, the nonce-tcp transport has no
        integrity or confidentiality protection, so it should normally
        only be used across the local loopback interface, for example
        using an address like <literal>tcp:host=127.0.0.1</literal> or
        <literal>tcp:host=localhost</literal>. Other uses are insecure.
        See <xref linkend="transports-tcp-sockets"/> for more
        information on situations where these transports have been used,
        and alternatives to these transports.
      </para>

      <para>
        On start, the server generates a random 16 byte nonce and writes it
        to a file in the user's temporary directory. The nonce file location
        is published as part of the server's D-Bus address using the
        "noncefile" key-value pair.

        After an accept, the server reads 16 bytes from the socket. If the
        read bytes do not match the nonce stored in the nonce file, the
        server MUST immediately drop the connection.
        If the nonce match the received byte sequence, the client is accepted
        and the transport behaves like an ordinary tcp transport.
      </para>
      <para>
        After a successful connect to the server socket, the client MUST read
        the nonce from the file published by the server via the noncefile=
        key-value pair and send it over the socket. After that, the
        transport behaves like an ordinary tcp transport.
      </para>
      <para>
        All nonce-tcp addresses are listenable. nonce-tcp addresses in which
        <literal>host</literal>, <literal>port</literal> and
        <literal>noncefile</literal> are all specified,
        and <literal>port</literal> is nonzero, are also connectable.
      </para>
      <sect3 id="transports-nonce-tcp-sockets-addresses">
        <title>Server Address Format</title>
        <para>
         Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
         and support the following key/value pairs:
        </para>
        <informaltable>
         <tgroup cols="3">
          <thead>
           <row>
            <entry>Name</entry>
            <entry>Values</entry>
            <entry>Description</entry>
           </row>
          </thead>
          <tbody>
           <row>
            <entry>host</entry>
            <entry>(string)</entry>
            <entry>DNS name or IP address</entry>
          </row>
          <row>
           <entry>bind</entry>
           <entry>(string)</entry>
           <entry>The same as for tcp: addresses
           </entry>
          </row>
          <row>
           <entry>port</entry>
           <entry>(number)</entry>
           <entry>The tcp port the server will open. A zero value let the server
            choose a free port provided from the underlaying operating system.
            libdbus is able to retrieve the real used port from the server.
           </entry>
          </row>
          <row>
           <entry>family</entry>
           <entry>(string)</entry>
           <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
          </row>
          <row>
           <entry>noncefile</entry>
           <entry>(path)</entry>
           <entry>File location containing the secret.
             This is only meaningful in connectable addresses:
             a listening D-Bus server that offers this transport
             will always create a new nonce file.</entry>
          </row>
         </tbody>
        </tgroup>
       </informaltable>
      </sect3>
    </sect2>
    <sect2 id="transports-exec">
      <title>Executed Subprocesses on Unix</title>
      <para>
        This transport forks off a process and connects its standard
        input and standard output with an anonymous Unix domain
        socket. This socket is then used for communication by the
        transport. This transport may be used to use out-of-process
        forwarder programs as basis for the D-Bus protocol.
      </para>
      <para>
        The forked process will inherit the standard error output and
        process group from the parent process.
      </para>
      <para>
        Executed subprocesses are not available on Windows.
      </para>
      <para>
        <literal>unixexec</literal> addresses are connectable, but are not
        listenable.
      </para>
      <sect3 id="transports-exec-addresses">
        <title>Server Address Format</title>
        <para>
          Executed subprocess addresses are identified by the "unixexec:" prefix
          and support the following key/value pairs:
        </para>
        <informaltable>
         <tgroup cols="3">
          <thead>
           <row>
            <entry>Name</entry>
            <entry>Values</entry>
            <entry>Description</entry>
           </row>
          </thead>
          <tbody>
           <row>
            <entry>path</entry>
            <entry>(path)</entry>
            <entry>Path of the binary to execute, either an absolute
            path or a binary name that is searched for in the default
            search path of the OS. This corresponds to the first
            argument of execlp(). This key is mandatory.</entry>
          </row>
          <row>
            <entry>argv0</entry>
            <entry>(string)</entry>
            <entry>The program name to use when executing the
            binary. If omitted the same value as specified for path=
            will be used. This corresponds to the second argument of
            execlp().</entry>
          </row>
          <row>
            <entry>argv1, argv2, ...</entry>
            <entry>(string)</entry>
            <entry>Arguments to pass to the binary. This corresponds
            to the third and later arguments of execlp(). If a
            specific argvX is not specified no further argvY for Y > X
            are taken into account.</entry>
          </row>
        </tbody>
        </tgroup>
       </informaltable>
      </sect3>
    </sect2>
   </sect1>
   <sect1 id="meta-transports">
    <title>Meta Transports</title>
    <para>
      Meta transports are a kind of transport with special enhancements or
      behavior. Currently available meta transports include: autolaunch
    </para>

    <sect2 id="meta-transports-autolaunch">
     <title>Autolaunch</title>
     <para>The autolaunch transport provides a way for dbus clients to autodetect
       a running dbus session bus and to autolaunch a session bus if not present.
     </para>
      <para>
        On Unix, <literal>autolaunch</literal> addresses are connectable,
        but not listenable.
      </para>
      <para>
        On Windows, <literal>autolaunch</literal> addresses are both
        connectable and listenable.
      </para>

     <sect3 id="meta-transports-autolaunch-addresses">
       <title>Server Address Format</title>
       <para>
         Autolaunch addresses uses the "autolaunch:" prefix and support the
         following key/value pairs:
       </para>
       <informaltable>
        <tgroup cols="3">
         <thead>
          <row>
           <entry>Name</entry>
           <entry>Values</entry>
           <entry>Description</entry>
          </row>
         </thead>
         <tbody>
          <row>
           <entry>scope</entry>
           <entry>(string)</entry>
           <entry>scope of autolaunch (Windows only)
            <itemizedlist>
             <listitem>
              <para>
               "*install-path" - limit session bus to dbus installation path.
               The dbus installation path is determined from the location of
               the shared dbus library. If the library is located in a 'bin'
               subdirectory the installation root is the directory above,
               otherwise the directory where the library lives is taken as
               installation root.
               <programlisting>
                   &lt;install-root&gt;/bin/[lib]dbus-1.dll
                   &lt;install-root&gt;/[lib]dbus-1.dll
               </programlisting>
              </para>
             </listitem>
             <listitem>
              <para>
               "*user" - limit session bus to the recent user.
              </para>
             </listitem>
             <listitem>
              <para>
               other values - specify dedicated session bus like "release",
               "debug" or other
              </para>
             </listitem>
            </itemizedlist>
           </entry>
         </row>
        </tbody>
       </tgroup>
      </informaltable>
     </sect3>

     <sect3 id="meta-transports-autolaunch-windows-implementation">
      <title>Windows implementation</title>
      <para>
        On start, the server opens a platform specific transport, creates a mutex
        and a shared memory section containing the related session bus address.
        This mutex will be inspected by the dbus client library to detect a
        running dbus session bus. The access to the mutex and the shared memory
        section are protected by global locks.
      </para>
      <para>
       In the recent implementation the autolaunch transport uses a tcp transport
       on localhost with a port choosen from the operating system. This detail may
       change in the future.
      </para>
      <para>
        Disclaimer: The recent implementation is in an early state and may not
        work in all cirumstances and/or may have security issues. Because of this
        the implementation is not documentated yet.
      </para>
     </sect3>
    </sect2>
   </sect1>

  <sect1 id="uuids">
    <title>UUIDs</title>
    <para>
      A working D-Bus implementation uses universally-unique IDs in two places.
      First, each server address has a UUID identifying the address,
      as described in <xref linkend="addresses"/>. Second, each operating
      system kernel instance running a D-Bus client or server has a UUID
      identifying that kernel, retrieved by invoking the method
      org.freedesktop.DBus.Peer.GetMachineId() (see <xref
      linkend="standard-interfaces-peer"/>).
    </para>
    <para>
      The term "UUID" in this document is intended literally, i.e. an
      identifier that is universally unique. It is not intended to refer to
      RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
    </para>
    <para>
      The UUID must contain 128 bits of data and be hex-encoded.  The
      hex-encoded string may not contain hyphens or other non-hex-digit
      characters, and it must be exactly 32 characters long.  To generate a
      UUID, the current reference implementation concatenates 96 bits of random
      data followed by the 32-bit time in seconds since the UNIX epoch (in big
      endian byte order).
    </para>
    <para>
      It would also be acceptable and probably better to simply generate 128
      bits of random data, as long as the random number generator is of high
      quality. The timestamp could conceivably help if the random bits are not
      very random. With a quality random number generator, collisions are
      extremely unlikely even with only 96 bits, so it's somewhat academic.
    </para>
    <para>
      Implementations should, however, stick to random data for the first 96 bits
      of the UUID.
    </para>
  </sect1>

  <sect1 id="standard-interfaces">
    <title>Standard Interfaces</title>
    <para>
      See <xref linkend="message-protocol-types-notation"/> for details on
       the notation used in this section. There are some standard interfaces
      that may be useful across various D-Bus applications.
    </para>
    <sect2 id="standard-interfaces-peer">
      <title><literal>org.freedesktop.DBus.Peer</literal></title>
      <para>
        The <literal>org.freedesktop.DBus.Peer</literal> interface
        has two methods:
        <programlisting>
          org.freedesktop.DBus.Peer.Ping ()
          org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
        </programlisting>
      </para>
      <para>
        On receipt of the <literal>METHOD_CALL</literal> message
        <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
        nothing other than reply with a <literal>METHOD_RETURN</literal> as
        usual.  It does not matter which object path a ping is sent to.  The
        reference implementation handles this method automatically.
      </para>
      <para>
        On receipt of the <literal>METHOD_CALL</literal> message
        <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
        reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
        UUID representing the identity of the machine the process is running on.
        This UUID must be the same for all processes on a single system at least
        until that system next reboots. It should be the same across reboots
        if possible, but this is not always possible to implement and is not
        guaranteed.
        It does not matter which object path a GetMachineId is sent to.  The
        reference implementation handles this method automatically.
      </para>
      <para>
        On Unix, implementations should try to read the machine ID from
        <literal>/var/lib/dbus/machine-id</literal> and
        <literal>/etc/machine-id</literal>. The latter is
        <ulink url='https://www.freedesktop.org/software/systemd/man/machine-id.html'>
        defined by systemd</ulink>, but systems not using systemd may provide
        an equivalent file.
        If both exist, they are expected to have the same contents, and if they
        differ, the spec does not define which takes precedence (the reference
        implementation prefers <literal>/var/lib/dbus/machine-id</literal>,
        but sd-bus does not).
      </para>
      <para>
        On Windows, the hardware profile GUID is used as the machine ID,
        with the punctuation removed. This can be obtained with the
        <ulink url='https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-getcurrenthwprofilea'>
        <function>GetCurrentHwProfile</function></ulink> function.
      </para>
      <para>
        The UUID is intended to be per-instance-of-the-operating-system, so may represent
        a virtual machine running on a hypervisor, rather than a physical machine.
        Basically if two processes see the same UUID, they should also see the same
        shared memory, UNIX domain sockets, process IDs, and other features that require
        a running OS kernel in common between the processes.
      </para>
      <para>
        The UUID is often used where other programs might use a hostname. Hostnames
        can change without rebooting, however, or just be "localhost" - so the UUID
        is more robust.
      </para>
      <para>
        <xref linkend="uuids"/> explains the format of the UUID.
      </para>
    </sect2>

    <sect2 id="standard-interfaces-introspectable">
      <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
      <para>
        This interface has one method:
        <programlisting>
          org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
        </programlisting>
      </para>
      <para>
        Objects instances may implement
        <literal>Introspect</literal> which returns an XML description of
        the object, including its interfaces (with signals and methods), objects
        below it in the object path tree, and its properties.
      </para>
      <para>
        <xref linkend="introspection-format"/> describes the format of this XML string.
      </para>
    </sect2>
    <sect2 id="standard-interfaces-properties">
      <title><literal>org.freedesktop.DBus.Properties</literal></title>
      <para>
        Many native APIs will have a concept of object <firstterm>properties</firstterm>
        or <firstterm>attributes</firstterm>. These can be exposed via the
        <literal>org.freedesktop.DBus.Properties</literal> interface.
      </para>
      <para>
        <programlisting>
              org.freedesktop.DBus.Properties.Get (in STRING interface_name,
                                                   in STRING property_name,
                                                   out VARIANT value);
              org.freedesktop.DBus.Properties.Set (in STRING interface_name,
                                                   in STRING property_name,
                                                   in VARIANT value);
              org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
                                                      out ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt; props);
        </programlisting>
      </para>
      <para>
        It is conventional to give D-Bus properties names consisting of
        capitalized words without punctuation ("CamelCase"), like
        <link linkend="message-protocol-names-member">member names</link>.
        For instance, the GObject property
        <literal>connection-status</literal> or the Qt property
        <literal>connectionStatus</literal> could be represented on D-Bus
        as <literal>ConnectionStatus</literal>.
      </para>
      <para>
        Strictly speaking, D-Bus property names are not required to follow
        the same naming restrictions as member names, but D-Bus property
        names that would not be valid member names (in particular,
        GObject-style dash-separated property names) can cause interoperability
        problems and should be avoided.
      </para>
      <para>
        The available properties and whether they are writable can be determined
        by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
        see <xref linkend="standard-interfaces-introspectable"/>.
      </para>
      <para>
        An empty string may be provided for the interface name; in this case,
        if there are multiple properties on an object with the same name,
        the results are undefined (picking one by according to an arbitrary
        deterministic rule, or returning an error, are the reasonable
        possibilities).
      </para>
      <para>
        If <literal>org.freedesktop.DBus.Properties.GetAll</literal> is called
        with a valid interface name which contains no properties, an empty array
        should be returned. If it is called with a valid interface name for
        which some properties are not accessible to the caller (for example, due
        to per-property access control implemented in the service), those
        properties should be silently omitted from the result array.
        If <literal>org.freedesktop.DBus.Properties.Get</literal> is called for
        any such properties, an appropriate access control error should be
        returned.
      </para>
      <para>
        If one or more properties change on an object, the
        <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
        signal may be emitted (this signal was added in 0.14):
      </para>
      <para>
        <programlisting>
              org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
                                                                 ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt; changed_properties,
                                                                 ARRAY&lt;STRING&gt; invalidated_properties);
        </programlisting>
      </para>
      <para>
        where <literal>changed_properties</literal> is a dictionary
        containing the changed properties with the new values and
        <literal>invalidated_properties</literal> is an array of
        properties that changed but the value is not conveyed.
      </para>
      <para>
        Whether the <literal>PropertiesChanged</literal> signal is
        supported can be determined by calling
        <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
        that the signal may be supported for an object but it may
        differ how whether and how it is used on a per-property basis
        (for e.g. performance or security reasons). Each property (or
        the parent interface) must be annotated with the
        <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
        annotation to convey this (usually the default value
        <literal>true</literal> is sufficient meaning that the
        annotation does not need to be used). See <xref
        linkend="introspection-format"/> for details on this
        annotation.
      </para>
    </sect2>

    <sect2 id="standard-interfaces-objectmanager">
      <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
      <para>
        An API can optionally make use of this interface for one or
        more sub-trees of objects. The root of each sub-tree implements
        this interface so other applications can get all objects,
        interfaces and properties in a single method call.  It is
        appropriate to use this interface if users of the tree of
        objects are expected to be interested in all interfaces of all
        objects in the tree; a more granular API should be used if
        users of the objects are expected to be interested in a small
        subset of the objects, a small subset of their interfaces, or
        both.
      </para>
      <para>
        The method that applications can use to get all objects and
        properties is <literal>GetManagedObjects</literal>:
      </para>
      <para>
        <programlisting>
          org.freedesktop.DBus.ObjectManager.GetManagedObjects (out ARRAY of DICT_ENTRY&lt;OBJPATH,ARRAY of DICT_ENTRY&lt;STRING,ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt;&gt;&gt; objpath_interfaces_and_properties);
        </programlisting>
      </para>
      <para>
        The return value of this method is a dict whose keys are
        object paths. All returned object paths are children of the
        object path implementing this interface, i.e. their object
        paths start with the ObjectManager's object path plus '/'.
      </para>
      <para>
        Each value is a dict whose keys are interfaces names.  Each
        value in this inner dict is the same dict that would be
        returned by the <link
        linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
        method for that combination of object path and interface. If
        an interface has no properties, the empty dict is returned.
      </para>
      <para>
        Changes are emitted using the following two signals:
      </para>
      <para>
        <programlisting>
          org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
                                                              ARRAY of DICT_ENTRY&lt;STRING,ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt;&gt; interfaces_and_properties);
          org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
                                                                ARRAY&lt;STRING&gt; interfaces);
        </programlisting>
      </para>
      <para>
        The <literal>InterfacesAdded</literal> signal is emitted when
        either a new object is added or when an existing object gains
        one or more interfaces. The
        <literal>InterfacesRemoved</literal> signal is emitted
        whenever an object is removed or it loses one or more
        interfaces. The second parameter of the
        <literal>InterfacesAdded</literal> signal contains a dict with
        the interfaces and properties (if any) that have been added to
        the given object path. Similarly, the second parameter of the
        <literal>InterfacesRemoved</literal> signal contains an array
        of the interfaces that were removed. Note that changes on
        properties on existing interfaces are not reported using this
        interface - an application should also monitor the existing <link
        linkend="standard-interfaces-properties">PropertiesChanged</link>
        signal on each object.
      </para>
      <para>
        Applications SHOULD NOT export objects that are children of an
        object (directly or otherwise) implementing this interface but
        which are not returned in the reply from the
        <literal>GetManagedObjects()</literal> method of this
        interface on the given object.
      </para>
      <para>
        The intent of the <literal>ObjectManager</literal> interface
        is to make it easy to write a robust client
        implementation. The trivial client implementation only needs
        to make two method calls:
      </para>
      <para>
        <programlisting>
          org.freedesktop.DBus.AddMatch (bus_proxy,
                                         "type='signal',sender='org.example.App2',path_namespace='/org/example/App2'");
          objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
        </programlisting>
      </para>
      <para>
        on the message bus and the remote application's
        <literal>ObjectManager</literal>, respectively. Whenever a new
        remote object is created (or an existing object gains a new
        interface), the <literal>InterfacesAdded</literal> signal is
        emitted, and since this signal contains all properties for the
        interfaces, no calls to the
        <literal>org.freedesktop.Properties</literal> interface on the
        remote object are needed. Additionally, since the initial
        <literal>AddMatch()</literal> rule already includes signal
        messages from the newly created child object, no new
        <literal>AddMatch()</literal> call is needed.
      </para>

      <para>
        <emphasis>
          The <literal>org.freedesktop.DBus.ObjectManager</literal>
          interface was added in version 0.17 of the D-Bus
          specification.
        </emphasis>
      </para>
    </sect2>
  </sect1>

  <sect1 id="introspection-format">
    <title>Introspection Data Format</title>
    <para>
      As described in <xref linkend="standard-interfaces-introspectable"/>,
      objects may be introspected at runtime, returning an XML string
      that describes the object. The same XML format may be used in
      other contexts as well, for example as an "IDL" for generating
      static language bindings.
    </para>
    <para>
      Here is an example of introspection data:
      <programlisting>
        &lt;!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
         "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd"&gt;
        &lt;node name="/com/example/sample_object0"&gt;
          &lt;interface name="com.example.SampleInterface0"&gt;
            &lt;method name="Frobate"&gt;
              &lt;arg name="foo" type="i" direction="in"/&gt;
              &lt;arg name="bar" type="s" direction="out"/&gt;
              &lt;arg name="baz" type="a{us}" direction="out"/&gt;
              &lt;annotation name="org.freedesktop.DBus.Deprecated" value="true"/&gt;
            &lt;/method&gt;
            &lt;method name="Bazify"&gt;
              &lt;arg name="bar" type="(iiu)" direction="in"/&gt;
              &lt;arg name="bar" type="v" direction="out"/&gt;
            &lt;/method&gt;
            &lt;method name="Mogrify"&gt;
              &lt;arg name="bar" type="(iiav)" direction="in"/&gt;
            &lt;/method&gt;
            &lt;signal name="Changed"&gt;
              &lt;arg name="new_value" type="b"/&gt;
            &lt;/signal&gt;
            &lt;property name="Bar" type="y" access="readwrite"/&gt;
          &lt;/interface&gt;
          &lt;node name="child_of_sample_object"/&gt;
          &lt;node name="another_child_of_sample_object"/&gt;
       &lt;/node&gt;
      </programlisting>
    </para>
    <para>
      A more formal DTD and spec needs writing, but here are some quick notes.
      <itemizedlist>
        <listitem>
          <para>
            Only the root &lt;node&gt; element can omit the node name, as it's
            known to be the object that was introspected.  If the root
            &lt;node&gt; does have a name attribute, it must be an absolute
            object path. If child &lt;node&gt; have object paths, they must be
            relative.
          </para>
        </listitem>
        <listitem>
          <para>
            If a child &lt;node&gt; has any sub-elements, then they
            must represent a complete introspection of the child.
            If a child &lt;node&gt; is empty, then it may or may
            not have sub-elements; the child must be introspected
            in order to find out. The intent is that if an object
            knows that its children are "fast" to introspect
            it can go ahead and return their information, but
            otherwise it can omit it.
          </para>
        </listitem>
        <listitem>
          <para>
            The direction element on &lt;arg&gt; may be omitted,
            in which case it defaults to "in" for method calls
            and "out" for signals. Signals only allow "out"
            so while direction may be specified, it's pointless.
          </para>
        </listitem>
        <listitem>
          <para>
            The possible directions are "in" and "out",
            unlike CORBA there is no "inout"
          </para>
        </listitem>
        <listitem>
          <para>
            The possible property access flags are
            "readwrite", "read", and "write"
          </para>
        </listitem>
        <listitem>
          <para>
            Multiple interfaces can of course be listed for
            one &lt;node&gt;.
          </para>
        </listitem>
        <listitem>
          <para>
            The "name" attribute on arguments is optional.
          </para>
        </listitem>
      </itemizedlist>
    </para>
    <para>
        Method, interface, property, signal, and argument elements may have
        "annotations", which are generic key/value pairs of metadata.
        They are similar conceptually to Java's annotations and C# attributes.
        Well-known annotations:
     </para>
     <informaltable>
       <tgroup cols="3">
         <thead>
           <row>
             <entry>Name</entry>
             <entry>Values (separated by ,)</entry>
             <entry>Description</entry>
           </row>
         </thead>
         <tbody>
           <row>
             <entry>org.freedesktop.DBus.Deprecated</entry>
             <entry>true,false</entry>
             <entry>Whether or not the entity is deprecated; defaults to false</entry>
           </row>
           <row>
             <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
             <entry>(string)</entry>
             <entry>The C symbol; may be used for methods and interfaces</entry>
           </row>
           <row>
             <entry>org.freedesktop.DBus.Method.NoReply</entry>
             <entry>true,false</entry>
             <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
           </row>
           <row>
             <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
             <entry>true,invalidates,const,false</entry>
             <entry>
               <para>
                 If set to <literal>false</literal>, the
                 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
                 signal, see <xref
                 linkend="standard-interfaces-properties"/> is not
                 guaranteed to be emitted if the property changes.
               </para>
               <para>
                 If set to <literal>const</literal> the property never
                 changes value during the lifetime of the object it
                 belongs to, and hence the signal is never emitted for
                 it.
               </para>
               <para>
                 If set to <literal>invalidates</literal> the signal
                 is emitted but the value is not included in the
                 signal.
               </para>
               <para>
                 If set to <literal>true</literal> the signal is
                 emitted with the value included.
               </para>
               <para>
                 The value for the annotation defaults to
                 <literal>true</literal> if the enclosing interface
                 element does not specify the annotation. Otherwise it
                 defaults to the value specified in the enclosing
                 interface element.
               </para>
               <para>
                 This annotation is intended to be used by code
                 generators to implement client-side caching of
                 property values. For all properties for which the
                 annotation is set to <literal>const</literal>,
                 <literal>invalidates</literal> or
                 <literal>true</literal> the client may
                 unconditionally cache the values as the properties
                 don't change or notifications are generated for them
                 if they do.
               </para>
             </entry>
           </row>
         </tbody>
       </tgroup>
     </informaltable>
  </sect1>
  <sect1 id="message-bus">
    <title>Message Bus Specification</title>
    <sect2 id="message-bus-overview">
      <title>Message Bus Overview</title>
      <para>
        The message bus accepts connections from one or more applications.
        Once connected, applications can exchange messages with other
        applications that are also connected to the bus.
      </para>
      <para>
        In order to route messages among connections, the message bus keeps a
        mapping from names to connections. Each connection has one
        unique-for-the-lifetime-of-the-bus name automatically assigned.
        Applications may request additional names for a connection. Additional
        names are usually "well-known names" such as
        "com.example.TextEditor1". When a name is bound to a connection,
        that connection is said to <firstterm>own</firstterm> the name.
      </para>
      <para>
        The bus itself owns a special name,
        <literal>org.freedesktop.DBus</literal>, with an object
        located at <literal>/org/freedesktop/DBus</literal> that
        implements the <literal>org.freedesktop.DBus</literal>
        interface. This service allows applications to make
        administrative requests of the bus itself. For example,
        applications can ask the bus to assign a name to a connection.
      </para>
      <para>
        Each name may have <firstterm>queued owners</firstterm>.  When an
        application requests a name for a connection and the name is already in
        use, the bus will optionally add the connection to a queue waiting for
        the name. If the current owner of the name disconnects or releases
        the name, the next connection in the queue will become the new owner.
      </para>

      <para>
        This feature causes the right thing to happen if you start two text
        editors for example; the first one may request "com.example.TextEditor1",
        and the second will be queued as a possible owner of that name. When
        the first exits, the second will take over.
      </para>

      <para>
        Applications may send <firstterm>unicast messages</firstterm> to
        a specific recipient or to the message bus itself, or
        <firstterm>broadcast messages</firstterm> to all interested recipients.
        See <xref linkend="message-bus-routing"/> for details.
      </para>
    </sect2>

    <sect2 id="message-bus-names">
      <title>Message Bus Names</title>
      <para>
        Each connection has at least one name, assigned at connection time and
        returned in response to the
        <literal>org.freedesktop.DBus.Hello</literal> method call.  This
        automatically-assigned name is called the connection's <firstterm>unique
        name</firstterm>.  Unique names are never reused for two different
        connections to the same bus.
      </para>
      <para>
        Ownership of a unique name is a prerequisite for interaction with
        the message bus. It logically follows that the unique name is always
        the first name that an application comes to own, and the last
        one that it loses ownership of.
      </para>
      <para>
        Unique connection names must begin with the character ':' (ASCII colon
        character); bus names that are not unique names must not begin
        with this character. (The bus must reject any attempt by an application
        to manually request a name beginning with ':'.) This restriction
        categorically prevents "spoofing"; messages sent to a unique name
        will always go to the expected connection.
      </para>
      <para>
        When a connection is closed, all the names that it owns are deleted (or
        transferred to the next connection in the queue if any).
      </para>
      <para>
        A connection can request additional names to be associated with it using
        the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
        linkend="message-protocol-names-bus"/> describes the format of a valid
        name. These names can be released again using the
        <literal>org.freedesktop.DBus.ReleaseName</literal> message.
      </para>
    </sect2>

    <sect2 id="message-bus-routing">
      <title>Message Bus Message Routing</title>

      <para>
        Messages may have a <literal>DESTINATION</literal> field (see <xref
          linkend="message-protocol-header-fields"/>), resulting in a
        <firstterm>unicast message</firstterm>.  If the
        <literal>DESTINATION</literal> field is present, it specifies a message
        recipient by name. Method calls and replies normally specify this field.
        The message bus must send messages (of any type) with the
        <literal>DESTINATION</literal> field set to the specified recipient,
        regardless of whether the recipient has set up a match rule matching
        the message.
      </para>

      <para>
        When the message bus receives a signal, if the
        <literal>DESTINATION</literal> field is absent, it is considered to
        be a <firstterm>broadcast signal</firstterm>, and is sent to all
        applications with <firstterm>message matching rules</firstterm> that
        match the message. Most signal messages are broadcasts, and
        no other message types currently defined in this specification
        may be broadcast.
      </para>

      <para>
        Unicast signal messages (those with a <literal>DESTINATION</literal>
        field) are not commonly used, but they are treated like any unicast
        message: they are delivered to the specified receipient,
        regardless of its match rules.  One use for unicast signals is to
        avoid a race condition in which a signal is emitted before the intended
        recipient can call <link linkend="bus-messages-add-match">AddMatch</link> to
        receive that signal: if the signal is sent directly to that recipient
        using a unicast message, it does not need to add a match rule at all,
        and there is no race condition.  Another use for unicast signals,
        on message buses whose security policy prevents eavesdropping, is to
        send sensitive information which should only be visible to one
        recipient.
      </para>

      <para>
        When the message bus receives a method call, if the
        <literal>DESTINATION</literal> field is absent, the call is taken to be
        a standard one-to-one message and interpreted by the message bus
        itself. For example, sending an
        <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
        <literal>DESTINATION</literal> will cause the message bus itself to
        reply to the ping immediately; the message bus will not make this
        message visible to other applications.
      </para>

      <para>
        Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
        the ping message were sent with a <literal>DESTINATION</literal> name of
        <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
        forwarded, and the Yoyodyne Corporation screensaver application would be
        expected to reply to the ping.
      </para>

      <para>
        Message bus implementations may impose a security policy which
        prevents certain messages from being sent or received.
        When a method call message cannot be sent or received due to a security
        policy, the message bus should send an error reply, unless the
        original message had the <literal>NO_REPLY</literal> flag.
      </para>

      <sect3 id="message-bus-routing-eavesdropping">
        <title>Eavesdropping</title>
        <para>
          Receiving a unicast message whose <literal>DESTINATION</literal>
          indicates a different recipient is called
          <firstterm>eavesdropping</firstterm>. On a message bus which acts as
          a security boundary (like the standard system bus), the security
          policy should usually prevent eavesdropping, since unicast messages
          are normally kept private and may contain security-sensitive
          information.
        </para>

        <para>
          Eavesdropping interacts poorly with buses with non-trivial
          access control restrictions, and is deprecated. The
          <literal>BecomeMonitor</literal> method (see
          <xref linkend="bus-messages-become-monitor"/>) provides
          a preferable way to monitor buses.
        </para>

        <para>
          Eavesdropping is mainly useful for debugging tools, such as
          the <literal>dbus-monitor</literal> tool in the reference
          implementation of D-Bus. Tools which eavesdrop on the message bus
          should be careful to avoid sending a reply or error in response to
          messages intended for a different client.
        </para>

        <para>
          Clients may attempt to eavesdrop by adding match rules
          (see <xref linkend="message-bus-routing-match-rules"/>) containing
          the <literal>eavesdrop='true'</literal> match. For
          compatibility with older message bus implementations, if adding such
          a match rule results in an error reply, the client may fall back to
          adding the same rule with the <literal>eavesdrop</literal> match
          omitted.
        </para>
      </sect3>

      <sect3 id="message-bus-routing-match-rules">
        <title>Match Rules</title>
        <para>
          An important part of the message bus routing protocol is match
          rules. Match rules describe the messages that should be sent to a
          client, based on the contents of the message.  Broadcast signals
          are only sent to clients which have a suitable match rule: this
          avoids waking up client processes to deal with signals that are
          not relevant to that client.
        </para>
        <para>
          Messages that list a client as their <literal>DESTINATION</literal>
          do not need to match the client's match rules, and are sent to that
          client regardless. As a result, match rules are mainly used to
          receive a subset of broadcast signals.
        </para>
        <para>
          Match rules can also be used for eavesdropping
          (see <xref linkend="message-bus-routing-eavesdropping"/>),
          if the security policy of the message bus allows it, but this
          usage is deprecated in favour of the <literal>BecomeMonitor</literal>
          method (see <xref linkend="bus-messages-become-monitor"/>).
        </para>
        <para>
          Match rules are added using the AddMatch bus method
          (see <xref linkend="bus-messages-add-match"/>).  Rules are
          specified as a string of comma separated key/value pairs.
          Excluding a key from the rule indicates a wildcard match.
          For instance excluding the the member from a match rule but
          adding a sender would let all messages from that sender through.
          An example of a complete rule would be
          "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
        </para>
        <para>
          Within single quotes (ASCII apostrophe, U+0027), a backslash
          (U+005C) represents itself, and an apostrophe ends the quoted
          section. Outside single quotes, \' (backslash, apostrophe)
          represents an apostrophe, and any backslash not followed by
          an apostrophe represents itself. For instance, the match rules
          <literal>arg0=''\''',arg1='\',arg2=',',arg3='\\'</literal> and
          <literal>arg0=\',arg1=\,arg2=',',arg3=\\</literal>
          both match messages where the arguments are a 1-character string
          containing an apostrophe, a 1-character string containing a
          backslash, a 1-character string containing a comma, and a
          2-character string containing two backslashes<footnote>
            <para>
              This idiosyncratic quoting style is based on the rules for
              escaping items to appear inside single-quoted strings
              in POSIX <literal>/bin/sh</literal>, but please
              note that backslashes that are not inside single quotes have
              different behaviour. This syntax does not offer any way to
              represent an apostrophe inside single quotes (it is necessary
              to leave the single-quoted section, backslash-escape the
              apostrophe and re-enter single quotes), or to represent a
              comma outside single quotes (it is necessary to wrap it in
              a single-quoted section).
            </para>
          </footnote>.
        </para>
        <para>
          The following table describes the keys that can be used to create
          a match rule.
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Key</entry>
                  <entry>Possible Values</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry><literal>type</literal></entry>
                  <entry>'signal', 'method_call', 'method_return', 'error'</entry>
                  <entry>Match on the message type.  An example of a type match is type='signal'</entry>
                </row>
                <row>
                  <entry><literal>sender</literal></entry>
                  <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
                  and <xref linkend="term-unique-name"/> respectively)
                  </entry>
                  <entry>Match messages sent by a particular sender.  An example of a sender match
                  is sender='org.freedesktop.Hal'</entry>
                </row>
                <row>
                  <entry><literal>interface</literal></entry>
                  <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
                  <entry>Match messages sent over or to a particular interface.  An example of an
                  interface match is interface='org.freedesktop.Hal.Manager'.
                  If a message omits the interface header, it must not match any rule
                  that specifies this key.</entry>
                </row>
                <row>
                  <entry><literal>member</literal></entry>
                  <entry>Any valid method or signal name</entry>
                  <entry>Matches messages which have the give method or signal name. An example of
                  a member match is member='NameOwnerChanged'</entry>
                </row>
                <row>
                  <entry><literal>path</literal></entry>
                  <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
                  <entry>Matches messages which are sent from or to the given object. An example of a
                  path match is path='/org/freedesktop/Hal/Manager'</entry>
                </row>
                <row>
                  <entry><literal>path_namespace</literal></entry>
                  <entry>An object path</entry>
                  <entry>
                    <para>
                      Matches messages which are sent from or to an
                      object for which the object path is either the
                      given value, or that value followed by one or
                      more path components.
                    </para>

                    <para>
                      For example,
                      <literal>path_namespace='/com/example/foo'</literal>
                      would match signals sent by
                      <literal>/com/example/foo</literal>
                      or by
                      <literal>/com/example/foo/bar</literal>,
                      but not by
                      <literal>/com/example/foobar</literal>.
                    </para>

                    <para>
                      Using both <literal>path</literal> and
                      <literal>path_namespace</literal> in the same match
                      rule is not allowed.
                    </para>

                    <para>
                      <emphasis>
                        This match key was added in version 0.16 of the
                        D-Bus specification and implemented by the bus
                        daemon in dbus 1.5.0 and later.
                      </emphasis>
                    </para>
                </entry>
                </row>
                <row>
                  <entry><literal>destination</literal></entry>
                  <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
                  <entry>Matches messages which are being sent to the given unique name. An
                  example of a destination match is destination=':1.0'</entry>
                </row>
                <row>
                  <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
                  <entry>Any string</entry>
                  <entry>Arg matches are special and are used for further restricting the
                  match based on the arguments in the body of a message. Only arguments of type
                  STRING can be matched in this way. An example of an argument match
                  would be arg3='Foo'. Only argument indexes from 0 to 63 should be
                  accepted.</entry>
                </row>
                <row>
                  <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
                  <entry>Any string</entry>
                  <entry>
                    <para>Argument path matches provide a specialised form of wildcard matching for
                      path-like namespaces. They can match arguments whose type is either STRING or
                      OBJECT_PATH. As with normal argument matches,
                      if the argument is exactly equal to the string given in the match
                      rule then the rule is satisfied. Additionally, there is also a
                      match when either the string given in the match rule or the
                      appropriate message argument ends with '/' and is a prefix of the
                      other. An example argument path match is arg0path='/aa/bb/'. This
                      would match messages with first arguments of '/', '/aa/',
                      '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
                      messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>

                    <para>This is intended for monitoring “directories” in file system-like
                      hierarchies, as used in the <citetitle>dconf</citetitle> configuration
                      system. An application interested in all nodes in a particular hierarchy would
                      monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
                      emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
                      represent a modification to the “bar” property, or a signal with zeroth
                      argument <literal>"/ca/example/"</literal> to represent atomic modification of
                      many properties within that directory, and the interested application would be
                      notified in both cases.</para>
                    <para>
                      <emphasis>
                        This match key was added in version 0.12 of the
                        D-Bus specification, implemented for STRING
                        arguments by the bus daemon in dbus 1.2.0 and later,
                        and implemented for OBJECT_PATH arguments in dbus 1.5.0
                        and later.
                      </emphasis>
                    </para>
                  </entry>
                </row>
                <row>
                  <entry><literal>arg0namespace</literal></entry>
                  <entry>Like a bus name, except that the string is not
                    required to contain a '.' (period)</entry>
                  <entry>
                    <para>Match messages whose first argument is of type STRING, and is a bus name
                      or interface name within the specified namespace. This is primarily intended
                      for watching name owner changes for a group of related bus names, rather than
                      for a single name or all name changes.</para>

                    <para>Because every valid interface name is also a valid
                      bus name, this can also be used for messages whose
                      first argument is an interface name.</para>

                    <para>For example, the match rule
                      <literal>member='NameOwnerChanged',arg0namespace='com.example.backend1'</literal>
                      matches name owner changes for bus names such as
                      <literal>com.example.backend1.foo</literal>,
                      <literal>com.example.backend1.foo.bar</literal>, and
                      <literal>com.example.backend1</literal> itself.</para>

                    <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
                    <para>
                      <emphasis>
                        This match key was added in version 0.16 of the
                        D-Bus specification and implemented by the bus
                        daemon in dbus 1.5.0 and later.
                      </emphasis>
                    </para>
                  </entry>
                </row>
                <row>
                  <entry><literal>eavesdrop</literal></entry>
                  <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
                  <entry>
                    <para>
                      Since D-Bus 1.5.6, match rules do not
                      match messages which have a <literal>DESTINATION</literal>
                      field unless the match rule specifically
                      requests this
                      (see <xref linkend="message-bus-routing-eavesdropping"/>)
                      by specifying <literal>eavesdrop='true'</literal>
                      in the match rule.  <literal>eavesdrop='false'</literal>
                      restores the default behaviour. Messages are
                      delivered to their <literal>DESTINATION</literal>
                      regardless of match rules, so this match does not
                      affect normal delivery of unicast messages.
                      In older versions of D-Bus, this match was not allowed
                      in match rules, and all match rules behaved as if
                      <literal>eavesdrop='true'</literal> had been used.
                    </para>
                    <para>
                      Use of <literal>eavesdrop='true'</literal> is
                      deprecated. Monitors should prefer to use the
                      <literal>BecomeMonitor</literal> method (see
                      <xref linkend="bus-messages-become-monitor"/>),
                      which was introduced in version 0.26 of the D-Bus
                      specification and version 1.9.10 of the reference
                      dbus-daemon.
                    </para>
                    <para>
                      Message bus implementations may restrict match rules
                      with <literal>eavesdrop='true'</literal> so that they
                      can only be added by privileged connections.
                    </para>
                    <para>
                      <emphasis>
                        This match key was added in version 0.18 of the
                        D-Bus specification and implemented by the bus
                        daemon in dbus 1.5.6 and later.
                      </emphasis>
                    </para>
                  </entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
      </sect3>
    </sect2>
    <sect2 id="message-bus-starting-services">
      <title>Message Bus Starting Services (Activation)</title>
      <para>
        The message bus can start applications on behalf of other applications.
        This is referred to as <firstterm>service activation</firstterm> or
        <firstterm>activation</firstterm>.
        An application that can be started in this way is called a
        <firstterm>service</firstterm> or an
        <firstterm>activatable service</firstterm>.
      </para>

      <para>
        <firstterm>Starting a service</firstterm> should be read as synonymous
        with service activation.
      </para>

      <para>
        In D-Bus, service activation is normally done by
        <firstterm>auto-starting</firstterm>.
        In auto-starting, applications send a
        message to a particular well-known name, such as
        <literal>com.example.TextEditor1</literal>, without specifying the
        <literal>NO_AUTO_START</literal> flag in the message header.
        If no application on the bus owns the requested name, but the bus
        daemon does know how to start an activatable service for that name,
        then the bus daemon will start that service, wait for it to request
        that name, and deliver the message to it.
      </para>

      <para>
        It is also possible for applications to send an explicit request to
        start a service: this is another form of activation, distinct from
        auto-starting. See
        <xref linkend="bus-messages-start-service-by-name"/> for details.
      </para>

      <para>
        In either case, this implies a contract documented along with the name
        <literal>com.example.TextEditor1</literal> for which object
        the owner of that name will provide, and what interfaces those
        objects will have.
      </para>

      <para>
        To find an executable corresponding to a particular name, the bus daemon
        looks for <firstterm>service description files</firstterm>.  Service
        description files define a mapping from names to executables. Different
        kinds of message bus will look for these files in different places, see
        <xref linkend="message-bus-types"/>.
      </para>
      <para>
        Service description files have the ".service" file
        extension. The message bus will only load service description files
        ending with .service; all other files will be ignored.  The file format
        is similar to that of <ulink
        url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
        entries</ulink>. All service description files must be in UTF-8
        encoding. To ensure that there will be no name collisions, service files
        must be namespaced using the same mechanism as messages and service
        names.
      </para>

      <para>
        On the well-known system bus, the name of a service description file
        must be its well-known name plus <literal>.service</literal>,
        for instance
        <literal>com.example.ConfigurationDatabase1.service</literal>.
      </para>

      <para>
        On the well-known session bus, services should follow the same
        service description file naming convention as on the system bus,
        but for backwards compatibility they are not required to do so.
      </para>

      <para>
        [FIXME the file format should be much better specified than "similar to
        .desktop entries" esp. since desktop entries are already
        badly-specified. ;-)]
        These sections from the specification apply to service files as well:

        <itemizedlist>
          <listitem><para>General syntax</para></listitem>
          <listitem><para>Comment format</para></listitem>
        </itemizedlist>

        Service description files must contain a
        <literal>D-BUS Service</literal> group with at least the keys
        <literal>Name</literal> (the well-known name of the service)
        and <literal>Exec</literal> (the command to be executed).

        <figure>
          <title>Example service description file</title>
          <programlisting>
            # Sample service description file
            [D-BUS Service]
            Name=com.example.ConfigurationDatabase1
            Exec=/usr/bin/sample-configd
          </programlisting>
        </figure>
      </para>

      <para>
        Additionally, service description files for the well-known system
        bus on Unix must contain a <literal>User</literal> key, whose value
        is the name of a user account (e.g. <literal>root</literal>).
        The system service will be run as that user.
      </para>

      <para>
        When an application asks to start a service by name, the bus daemon tries to
        find a service that will own that name. It then tries to spawn the
        executable associated with it. If this fails, it will report an
        error.
      </para>

      <para>
        On the well-known system bus, it is not possible for two .service files
        in the same directory to offer the same service, because they are
        constrained to have names that match the service name.
      </para>

      <para>
        On the well-known session bus, if two .service files in the same
        directory offer the same service name, the result is undefined.
        Distributors should avoid this situation, for instance by naming
        session services' .service files according to their service name.
      </para>

      <para>
        If two .service files in different directories offer the same
        service name, the one in the higher-priority directory is used:
        for instance, on the system bus, .service files in
        /etc/dbus-1/system-services takes precedence over those in
        /run/dbus-1/system-services, which in turn take precedence over
        those in /usr/local/share/dbus-1/system-services, which in turn take
        precedence over those in /usr/share/dbus-1/system-services.
      </para>
      <para>
        The executable launched will have the environment variable
        <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
        message bus so it can connect and request the appropriate names.
      </para>
      <para>
        The executable being launched may want to know whether the message bus
        starting it is one of the well-known message buses (see <xref
        linkend="message-bus-types"/>). To facilitate this, the bus must also set
        the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
        of the well-known buses. The currently-defined values for this variable
        are <literal>system</literal> for the systemwide message bus,
        and <literal>session</literal> for the per-login-session message
        bus. The new executable must still connect to the address given
        in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
        resulting connection is to the well-known bus.
      </para>
      <para>
        [FIXME there should be a timeout somewhere, either specified
        in the .service file, by the client, or just a global value
        and if the client being activated fails to connect within that
        timeout, an error should be sent back.]
      </para>

      <sect3 id="message-bus-starting-services-scope">
        <title>Message Bus Service Scope</title>
        <para>
          The "scope" of a service is its "per-", such as per-session,
          per-machine, per-home-directory, or per-display. The reference
          implementation doesn't yet support starting services in a different
          scope from the message bus itself. So e.g. if you start a service
          on the session bus its scope is per-session.
        </para>
        <para>
          We could add an optional scope to a bus name. For example, for
          per-(display,session pair), we could have a unique ID for each display
          generated automatically at login and set on screen 0 by executing a
          special "set display ID" binary. The ID would be stored in a
          <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
          random bytes. This ID would then be used to scope names.
          Starting/locating a service could be done by ID-name pair rather than
          only by name.
        </para>
        <para>
          Contrast this with a per-display scope. To achieve that, we would
          want a single bus spanning all sessions using a given display.
          So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
          property on screen 0 of the display, pointing to this bus.
        </para>
      </sect3>

      <sect3 id="message-bus-starting-services-systemd">
        <title>systemd Activation</title>

        <para>
          Service description files may contain a
          <literal>SystemdService</literal> key. Its value is the name of a
          <ulink
            url="https://www.freedesktop.org/wiki/Software/systemd/">systemd</ulink>
          service, for example
          <literal>dbus-com.example.MyDaemon.service</literal>.
        </para>

        <para>
          If this key is present, the bus daemon may carry out activation for
          this D-Bus service by sending a request to systemd asking it to
          start the systemd service whose name is the value of
          <literal>SystemdService</literal>. For example, the reference
          <literal>dbus-daemon</literal> has a
          <literal>--systemd-activation</literal> option that enables this
          feature, and that option is given when it is started by systemd.
        </para>

        <para>
          On the well-known system bus, it is a common practice to set
          <literal>SystemdService</literal> to <literal>dbus-</literal>,
          followed by the well-known bus name, followed by
          <literal>.service</literal>, then register that name as an alias
          for the real systemd service. This allows D-Bus activation of a
          service to be enabled or disabled independently of whether the
          service is started by systemd during boot.
        </para>
      </sect3>

      <sect3 id="message-bus-starting-services-apparmor">
        <title>Mediating Activation with AppArmor</title>

        <para>
          Please refer to
          <ulink url="http://wiki.apparmor.net/index.php/Documentation">AppArmor documentation</ulink>
          for general information on AppArmor, and how it mediates D-Bus
          messages when used in conjunction with a kernel and
          <literal>dbus-daemon</literal> that support this.
        </para>

        <para>
          In recent versions of the reference <literal>dbus-daemon</literal>,
          AppArmor policy rules of type <literal>dbus send</literal>
          are also used to control auto-starting: if a message is sent to
          the well-known name of an activatable service, the
          <literal>dbus-daemon</literal> will attempt to determine whether
          it would deliver the message to that service
          <emphasis>before</emphasis>auto-starting it, by making some
          assumptions about the resulting process's credentials.
        </para>

        <para>
          If it does proceed with auto-starting, when the service appears, the
          <literal>dbus-daemon</literal> repeats the policy check (with
          the service's true credentials, which might not be identical)
          before delivering the message. In practice, this second check will
          usually be more strict than the first; the first check would only
          be more strict if there are "blacklist"-style rules like
          <literal>deny dbus send peer=(label=/usr/bin/protected)</literal>
          that match on the peer's specific credentials, but AppArmor is
          normally used in a "whitelist" style where this does not apply.
        </para>

        <para>
          To support this process, service description files may contain a
          <literal>AssumedAppArmorLabel</literal> key. Its value is the name
          of an AppArmor label, for example
          <literal>/usr/sbin/mydaemon</literal>.
          If present, AppArmor mediation of messages that auto-start a
          service will decide whether to allow auto-starting to occur based
          on the assumption that the activated service will be confined
          under the specified label; in particular, rules of the form
          <literal>dbus send peer=(label=/usr/sbin/mydaemon)</literal> or
          <literal>deny dbus send peer=(label=/usr/sbin/mydaemon)</literal>
          will match it, allowing or denying as appropriate
          (even if there is in fact no profile of that name loaded).
        </para>

        <para>
          Otherwise, AppArmor mediation of messages that auto-start a
          service will decide whether to allow auto-starting to occur
          without specifying any particular label. In particular, any rule of
          the form <literal>dbus send peer=(label=X)</literal> or
          <literal>deny dbus send peer=(label=X)</literal>
          (for any value of X, including the special label
          <literal>unconfined</literal>) will not influence whether the
          auto-start is allowed.
        </para>

        <para>
          Rules of type <literal>dbus receive</literal> are not checked
          when deciding whether to allow auto-starting; they are only checked
          against the service's profile after the service has started, when
          deciding whether to deliver the message that caused the auto-starting
          operation.
        </para>

        <para>
          Explicit activation via the
          <link linkend="bus-messages-start-service-by-name">StartServiceByName</link>
          method call is not currently
          affected by this mediation: if a confined process is to be prevented
          from starting arbitrary services, then it must not be allowed to call
          that method.
        </para>
      </sect3>
    </sect2>

    <sect2 id="message-bus-types">
      <title>Well-known Message Bus Instances</title>
      <para>
        Two standard message bus instances are defined here, along with how
        to locate them and where their service files live.
      </para>
      <sect3 id="message-bus-types-login">
        <title>Login session message bus</title>
        <para>
          Each time a user logs in, a <firstterm>login session message
            bus</firstterm> may be started. All applications in the user's login
          session may interact with one another using this message bus.
        </para>
        <para>
          The address of the login session message bus is given
          in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
          variable. If that variable is not set, applications may
          also try to read the address from the X Window System root
          window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
          The root window property must have type <literal>STRING</literal>.
          The environment variable should have precedence over the
          root window property.
        </para>
        <para>The address of the login session message bus is given in the
        <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
        DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
        "autolaunch:", the system should use platform-specific methods of
        locating a running D-Bus session server, or starting one if a running
        instance cannot be found. Note that this mechanism is not recommended
        for attempting to determine if a daemon is running. It is inherently
        racy to attempt to make this determination, since the bus daemon may
        be started just before or just after the determination is made.
        Therefore, it is recommended that applications do not try to make this
        determination for their functionality purposes, and instead they
        should attempt to start the server.</para>

        <sect4 id="message-bus-types-login-x-windows">
          <title>X Windowing System</title>
          <para>
            For the X Windowing System, the application must locate the
            window owner of the selection represented by the atom formed by
            concatenating:
            <itemizedlist>
              <listitem>
                <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
              </listitem>

              <listitem>
                <para>the current user's username</para>
              </listitem>

              <listitem>
                <para>the literal character '_' (underscore)</para>
              </listitem>

              <listitem>
                <para>the machine's ID</para>
              </listitem>
            </itemizedlist>
          </para>

          <para>
            The following properties are defined for the window that owns
            this X selection:
            <informaltable frame="all">
              <tgroup cols="2">
                <tbody>
                  <row>
                    <entry>
                      <para>Atom</para>
                    </entry>

                    <entry>
                      <para>meaning</para>
                    </entry>
                  </row>

                  <row>
                    <entry>
                      <para>_DBUS_SESSION_BUS_ADDRESS</para>
                    </entry>

                    <entry>
                      <para>the actual address of the server socket</para>
                    </entry>
                  </row>

                  <row>
                    <entry>
                      <para>_DBUS_SESSION_BUS_PID</para>
                    </entry>

                    <entry>
                      <para>the PID of the server process</para>
                    </entry>
                  </row>
                </tbody>
              </tgroup>
            </informaltable>
          </para>

          <para>
            At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
            present in this window.
          </para>

          <para>
            If the X selection cannot be located or if reading the
            properties from the window fails, the implementation MUST conclude
            that there is no D-Bus server running and proceed to start a new
            server. (See below on concurrency issues)
          </para>

          <para>
            Failure to connect to the D-Bus server address thus obtained
            MUST be treated as a fatal connection error and should be reported
            to the application.
          </para>

          <para>
            As an alternative, an implementation MAY find the information
            in the following file located in the current user's home directory,
            in subdirectory .dbus/session-bus/:
            <itemizedlist>
              <listitem>
                <para>the machine's ID</para>
              </listitem>

              <listitem>
                <para>the literal character '-' (dash)</para>
              </listitem>

              <listitem>
                <para>the X display without the screen number, with the
                following prefixes removed, if present: ":", "localhost:"
                ."localhost.localdomain:". That is, a display of
                "localhost:10.0" produces just the number "10"</para>
              </listitem>
            </itemizedlist>
          </para>

          <para>
            The contents of this file NAME=value assignment pairs and
            lines starting with # are comments (no comments are allowed
            otherwise). The following variable names are defined:
            <informaltable
              frame="all">
              <tgroup cols="2">
                <tbody>
                  <row>
                    <entry>
                      <para>Variable</para>
                    </entry>

                    <entry>
                      <para>meaning</para>
                    </entry>
                  </row>

                  <row>
                    <entry>
                      <para>DBUS_SESSION_BUS_ADDRESS</para>
                    </entry>

                    <entry>
                      <para>the actual address of the server socket</para>
                    </entry>
                  </row>

                  <row>
                    <entry>
                      <para>DBUS_SESSION_BUS_PID</para>
                    </entry>

                    <entry>
                      <para>the PID of the server process</para>
                    </entry>
                  </row>

                  <row>
                    <entry>
                      <para>DBUS_SESSION_BUS_WINDOWID</para>
                    </entry>

                    <entry>
                      <para>the window ID</para>
                    </entry>
                  </row>
                </tbody>
              </tgroup>
            </informaltable>
          </para>

          <para>
            At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
            in this file.
          </para>

          <para>
            Failure to open this file MUST be interpreted as absence of a
            running server. Therefore, the implementation MUST proceed to
            attempting to launch a new bus server if the file cannot be
            opened.
          </para>

          <para>
            However, success in opening this file MUST NOT lead to the
            conclusion that the server is running. Thus, a failure to connect to
            the bus address obtained by the alternative method MUST NOT be
            considered a fatal error. If the connection cannot be established,
            the implementation MUST proceed to check the X selection settings or
            to start the server on its own.
          </para>

          <para>
            If the implementation concludes that the D-Bus server is not
            running it MUST attempt to start a new server and it MUST also
            ensure that the daemon started as an effect of the "autolaunch"
            mechanism provides the lookup mechanisms described above, so
            subsequent calls can locate the newly started server. The
            implementation MUST also ensure that if two or more concurrent
            initiations happen, only one server remains running and all other
            initiations are able to obtain the address of this server and
            connect to it. In other words, the implementation MUST ensure that
            the X selection is not present when it attempts to set it, without
            allowing another process to set the selection between the
            verification and the setting (e.g., by using XGrabServer /
            XungrabServer).
          </para>
        </sect4>
        <sect4>
          <title>Finding session services</title>
          <para>
            On Unix systems, the session bus should search for .service files
            in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
            by the
            <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
            Implementations may also search additional locations,
            with a higher or lower priority than the XDG directories.
          </para>
          <para>
            As described in the XDG Base Directory Specification, software
            packages should install their session .service files to their
            configured <literal>${datadir}/dbus-1/services</literal>,
            where <literal>${datadir}</literal> is as defined by the GNU
            coding standards. System administrators or users can arrange
            for these service files to be read by setting XDG_DATA_DIRS or by
            symlinking them into the default locations.
          </para>
        </sect4>
      </sect3>
      <sect3 id="message-bus-types-system">
        <title>System message bus</title>
        <para>
          A computer may have a <firstterm>system message bus</firstterm>,
          accessible to all applications on the system. This message bus may be
          used to broadcast system events, such as adding new hardware devices,
          changes in the printer queue, and so forth.
        </para>
        <para>
          The address of the system message bus is given
          in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
          variable. If that variable is not set, applications should try
          to connect to the well-known address
          <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
          Implementations of the well-known system bus should listen on
          an address that will result in that connection being successful.
        </para>
        <para>
          On systems where <filename>/var/run/</filename> is known to be
          synonymous with <filename>/run/</filename> (such as most Linux
          operating system distributions), implementations might prefer
          to make use of that knowledge to connect to or listen on
          <literal>unix:path=/run/dbus/system_bus_socket</literal> instead,
          which has some minor technical advantages, particularly during
          early startup and late shutdown.
        </para>
        <para>
          In practice, implementations of D-Bus often have build-time
          configuration options for the system bus address, whose defaults
          often depend on other build-time options such as the installation
          prefix (in particular, this is the case for dbus, the reference
          implementation of D-Bus).
          Distributors intending to provide access to the well-known
          system bus should verify that they are using an interoperable
          address.
        </para>
        <para>
          On Unix systems, the system bus should default to searching
          for .service files in
          <literal>/etc/dbus-1/system-services</literal>,
          <literal>/run/dbus-1/system-services</literal>,
          <literal>/usr/local/share/dbus-1/system-services</literal>,
          <literal>/usr/share/dbus-1/system-services</literal> and
          <literal>/lib/dbus-1/system-services</literal>, with that order
          of precedence. It may also search other implementation-specific
          locations, but should not vary these locations based on environment
          variables.
          The directory in <literal>/etc/</literal> is reserved for use by
          either the local system administrator,
          or an asset manager deploying services that are not part of the OS.
          The directory in <literal>/run/</literal> is reserved for ephemeral
          services that will disappear after a reboot.
          <footnote>
            <para>
              The system bus is security-sensitive and is typically executed
              by an init system with a clean environment. Its launch helper
              process is particularly security-sensitive, and specifically
              clears its own environment.
            </para>
          </footnote>
        </para>
        <para>
          Software packages should install their system .service
          files to their configured
          <literal>${datadir}/dbus-1/system-services</literal>,
          where <literal>${datadir}</literal> is as defined by the GNU
          coding standards. System administrators can arrange
          for these service files to be read by editing the system bus'
          configuration file or by symlinking them into the default
          locations.
        </para>
      </sect3>
    </sect2>

    <sect2 id="message-bus-messages">
      <title>Message Bus Interface: <literal>org.freedesktop.DBus</literal></title>
      <para>
        The special message bus name <literal>org.freedesktop.DBus</literal>
        responds to a number of additional messages at the object path
        <literal>/org/freedesktop/DBus</literal>,
        implementing the <literal>org.freedesktop.DBus</literal> interface.
        That object path is also used when emitting signals such as
        <link linkend='bus-messages-name-owner-changed'>NameOwnerChanged</link>.
      </para>

      <para>
        For historical reasons, some of the methods in the
        <literal>org.freedesktop.DBus</literal> interface are available
        on multiple object paths. Message bus implementations should
        accept method calls that were added before specification version
        0.26 on any object path. Message bus implementations should
        not accept newer method calls on unexpected object paths,
        and as a security hardening measure, older method calls
        that are security-sensitive may be rejected with the error
        <literal>org.freedesktop.DBus.Error.AccessDenied</literal> when
        called on an unexpected object path. Client software should send
        all method calls to <literal>/org/freedesktop/DBus</literal>
        instead of relying on this.
      </para>

      <sect3 id="bus-messages-hello">
        <title>Method: <literal>org.freedesktop.DBus.Hello</literal></title>
        <para>
          As a method:
          <programlisting>
            STRING Hello ()
          </programlisting>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Unique name assigned to the connection</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Before an application is able to send messages to other applications
          it must send the <literal>org.freedesktop.DBus.Hello</literal> message
          to the message bus to obtain a unique name. If an application without
          a unique name tries to send a message to another application, or a
          message to the message bus itself that isn't the
          <literal>org.freedesktop.DBus.Hello</literal> message, it will be
          disconnected from the bus.
        </para>
        <para>
          There is no corresponding "disconnect" request; if a client wishes to
          disconnect from the bus, it simply closes the socket (or other
          communication channel).
        </para>
      </sect3>

      <sect3 id="bus-messages-request-name">
        <title>Method: <literal>org.freedesktop.DBus.RequestName</literal></title>
        <para>
          As a method:
          <programlisting>
            UINT32 RequestName (in STRING name, in UINT32 flags)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name to request</entry>
                </row>
                <row>
                  <entry>1</entry>
                  <entry>UINT32</entry>
                  <entry>Flags</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>UINT32</entry>
                  <entry>Return value</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Ask the message bus to assign the given name to the method caller. Each
          name maintains a queue of possible owners, where the head of the queue is
          the primary or current owner of the name. Each potential owner in the
          queue maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
          DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName call.
          When RequestName is invoked the following occurs:
          <itemizedlist>
            <listitem>
              <para>
                If the method caller is currently the primary owner of the name,
                the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
                values are updated with the values from the new RequestName call,
                and nothing further happens.
              </para>
            </listitem>

            <listitem>
              <para>
                If the current primary owner (head of the queue) has
                DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
                invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
                the caller of RequestName replaces the current primary owner at
                the head of the queue and the current primary owner moves to the
                second position in the queue. If the caller of RequestName was
                in the queue previously its flags are updated with the values from
                the new RequestName in addition to moving it to the head of the queue.
              </para>
            </listitem>

            <listitem>
              <para>
                If replacement is not possible, and the method caller is
                currently in the queue but not the primary owner, its flags are
                updated with the values from the new RequestName call.
              </para>
            </listitem>

            <listitem>
              <para>
                If replacement is not possible, and the method caller is
                currently not in the queue, the method caller is appended to the
                queue.
              </para>
            </listitem>

            <listitem>
              <para>
                If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
                set and is not the primary owner, it is removed from the
                queue. This can apply to the previous primary owner (if it
                was replaced) or the method caller (if it updated the
                DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
                queue, or if it was just added to the queue with that flag set).
              </para>
            </listitem>
          </itemizedlist>
        </para>
        <para>
          Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
          queue," even if another application already in the queue had specified
          DBUS_NAME_FLAG_REPLACE_EXISTING.  This comes up if a primary owner
          that does not allow replacement goes away, and the next primary owner
          does allow replacement. In this case, queued items that specified
          DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
          automatically replace the new primary owner. In other words,
          DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
          time RequestName is called. This is deliberate to avoid an infinite loop
          anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
          and DBUS_NAME_FLAG_REPLACE_EXISTING.
        </para>
        <para>
          The flags argument contains any of the following values logically ORed
          together:

          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Conventional Name</entry>
                  <entry>Value</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
                  <entry>0x1</entry>
                  <entry>

                    If an application A specifies this flag and succeeds in
                    becoming the owner of the name, and another application B
                    later calls RequestName with the
                    DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
                    will lose ownership and receive a
                    <literal>org.freedesktop.DBus.NameLost</literal> signal, and
                    application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
                    is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
                    is not specified by application B, then application B will not replace
                    application A as the owner.

                  </entry>
                </row>
                <row>
                  <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
                  <entry>0x2</entry>
                  <entry>

                    Try to replace the current owner if there is one. If this
                    flag is not set the application will only become the owner of
                    the name if there is no current owner. If this flag is set,
                    the application will replace the current owner if
                    the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.

                  </entry>
                </row>
                <row>
                  <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
                  <entry>0x4</entry>
                  <entry>

                    Without this flag, if an application requests a name that is
                    already owned, the application will be placed in a queue to
                    own the name when the current owner gives it up. If this
                    flag is given, the application will not be placed in the
                    queue, the request for the name will simply fail.  This flag
                    also affects behavior when an application is replaced as
                    name owner; by default the application moves back into the
                    waiting queue, unless this flag was provided when the application
                    became the name owner.

                  </entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>

          The return code can be one of the following values:

          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Conventional Name</entry>
                  <entry>Value</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
                  <entry>1</entry> <entry>The caller is now the primary owner of
                  the name, replacing any previous owner. Either the name had no
                  owner before, or the caller specified
                  DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
                  DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
                </row>
                <row>
                  <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
                  <entry>2</entry>

                  <entry>The name already had an owner,
                    DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
                    the current owner did not specify
                    DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
                    application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
                    </entry>
                </row>
                <row>
                  <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
                  <entry>The name already has an owner,
                  DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
                  DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
                  current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
                  specified by the requesting application.</entry>
                </row>
                <row>
                  <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
                  <entry>4</entry>
                  <entry>The application trying to request ownership of a name is already the owner of it.</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
       </sect3>

       <sect3 id="bus-messages-release-name">
        <title>Method: <literal>org.freedesktop.DBus.ReleaseName</literal></title>
        <para>
          As a method:
          <programlisting>
            UINT32 ReleaseName (in STRING name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name to release</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>UINT32</entry>
                  <entry>Return value</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Ask the message bus to release the method caller's claim to the given
          name. If the caller is the primary owner, a new primary owner will be
          selected from the queue if any other owners are waiting. If the
          caller is waiting in the queue for the name, the caller will removed
          from the queue and will not be made an owner of the name if it later
          becomes available. If there are no other owners in the queue for the
          name, it will be removed from the bus entirely.

          The return code can be one of the following values:

          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Conventional Name</entry>
                  <entry>Value</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
                  <entry>1</entry> <entry>The caller has released his claim on
                  the given name. Either the caller was the primary owner of
                  the name, and the name is now unused or taken by somebody
                  waiting in the queue for the name, or the caller was waiting
                  in the queue for the name and has now been removed from the
                  queue.</entry>
                </row>
                <row>
                  <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
                  <entry>2</entry>
                  <entry>The given name does not exist on this bus.</entry>
                </row>
                <row>
                  <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
                  <entry>3</entry>
                  <entry>The caller was not the primary owner of this name,
                  and was also not waiting in the queue to own this name.</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
       </sect3>

       <sect3 id="bus-messages-list-queued-owners">
        <title>Method: <literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
        <para>
          As a method:
          <programlisting>
            ARRAY of STRING ListQueuedOwners (in STRING name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>The well-known bus name to query, such as
                    <literal>com.example.cappuccino</literal></entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>ARRAY of STRING</entry>
                  <entry>The unique bus names of connections currently queued
                    for the name</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          List the connections currently queued for a bus name (see
          <xref linkend="term-queued-owner"/>).
        </para>
      </sect3>

      <sect3 id="bus-messages-list-names">
        <title>Method: <literal>org.freedesktop.DBus.ListNames</literal></title>
        <para>
          As a method:
          <programlisting>
            ARRAY of STRING ListNames ()
          </programlisting>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>ARRAY of STRING</entry>
                  <entry>Array of strings where each string is a bus name</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Returns a list of all currently-owned names on the bus.
        </para>
      </sect3>
      <sect3 id="bus-messages-list-activatable-names">
        <title>Method: <literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
        <para>
          As a method:
          <programlisting>
            ARRAY of STRING ListActivatableNames ()
          </programlisting>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>ARRAY of STRING</entry>
                  <entry>Array of strings where each string is a bus name</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Returns a list of all names that can be activated on the bus.
        </para>
      </sect3>
      <sect3 id="bus-messages-name-exists">
        <title>Method: <literal>org.freedesktop.DBus.NameHasOwner</literal></title>
        <para>
          As a method:
          <programlisting>
            BOOLEAN NameHasOwner (in STRING name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name to check</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>BOOLEAN</entry>
                  <entry>Return value, true if the name exists</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Checks if the specified name exists (currently has an owner).
        </para>
      </sect3>

      <sect3 id="bus-messages-name-owner-changed">
        <title>Signal: <literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
        <para>
          This is a signal:
          <programlisting>
            NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name with a new owner</entry>
                </row>
                <row>
                  <entry>1</entry>
                  <entry>STRING</entry>
                  <entry>Old owner or empty string if none</entry>
                </row>
                <row>
                  <entry>2</entry>
                  <entry>STRING</entry>
                  <entry>New owner or empty string if none</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This signal indicates that the owner of a name has changed.
          It's also the signal to use to detect the appearance of
          new names on the bus.
        </para>
      </sect3>
      <sect3 id="bus-messages-name-lost">
        <title>Signal: <literal>org.freedesktop.DBus.NameLost</literal></title>
        <para>
          This is a signal:
          <programlisting>
            NameLost (STRING name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name which was lost</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This signal is sent to a specific application when it loses
          ownership of a name.
        </para>
      </sect3>

      <sect3 id="bus-messages-name-acquired">
        <title>Signal: <literal>org.freedesktop.DBus.NameAcquired</literal></title>
        <para>
          This is a signal:
          <programlisting>
            NameAcquired (STRING name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name which was acquired</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This signal is sent to a specific application when it gains
          ownership of a name.
        </para>
      </sect3>

      <sect3 id="bus-messages-activatable-services-changed">
        <title>Signal: <literal>org.freedesktop.DBus.ActivatableServicesChanged</literal></title>
        <para>
          This is a signal:
          <programlisting>
            ActivatableServicesChanged ()
          </programlisting>
        </para>
        <para>
          This signal is sent when the list of activatable services,
          as returned by ListActivatableNames(), might have changed
          (see <xref linkend="bus-messages-list-activatable-names"/>).
          Clients that have cached information about the activatable
          services should call ListActivatableNames() again to update
          their cache.
        </para>
        <para>
          The presence of this signal is indicated by a
          bus feature property (for details see
          <xref linkend="message-bus-properties-features"/>).
          In older implementations that do not have this feature,
          there is no way to be informed when the list of activatable
          names has changed.
        </para>
      </sect3>

      <sect3 id="bus-messages-start-service-by-name">
        <title>Method: <literal>org.freedesktop.DBus.StartServiceByName</literal></title>
        <para>
          As a method:
          <programlisting>
            UINT32 StartServiceByName (in STRING name, in UINT32 flags)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name of the service to start</entry>
                </row>
                <row>
                  <entry>1</entry>
                  <entry>UINT32</entry>
                  <entry>Flags (currently not used)</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        Reply arguments:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Argument</entry>
                <entry>Type</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>0</entry>
                <entry>UINT32</entry>
                <entry>Return value</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
        Tries to launch the executable associated with a name (service
        activation), as an explicit request. This is an alternative to
        relying on auto-starting. For more information on how services
        are activated and the difference between auto-starting and explicit
        activation, see
        <xref linkend="message-bus-starting-services"/>.
        </para>
        <para>
          It is often preferable to carry out auto-starting
          instead of calling this method. This is because calling this method
          is subject to a
          <ulink url="https://en.wikipedia.org/wiki/Time_of_check_to_time_of_use">time-of-check/time-of-use</ulink>
          issue: if a caller asks the message bus to start a service so that
          the same caller can make follow-up method calls to that service,
          the fact that the message bus was able to start the required
          service is no guarantee that it will not have crashed or otherwise
          exited by the time the caller makes those follow-up method calls.
          As a result, calling this method does not remove the need for
          the caller to handle errors from method calls. Given that fact,
          it is usually simpler to rely on auto-starting, in which the
          required service starts as a side-effect of the first method call.
        </para>
        <para>
          The return value can be one of the following values:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Identifier</entry>
                  <entry>Value</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>DBUS_START_REPLY_SUCCESS</entry>
                  <entry>1</entry>
                  <entry>The service was successfully started.</entry>
                </row>
                <row>
                  <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
                  <entry>2</entry>
                  <entry>A connection already owns the given name.</entry>
                </row>
              </tbody>
             </tgroup>
           </informaltable>
        </para>

      </sect3>

      <sect3 id="bus-messages-update-activation-environment">
        <title>Method: <literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
        <para>
          As a method:
          <programlisting>
            UpdateActivationEnvironment (in ARRAY of DICT_ENTRY&lt;STRING,STRING&gt; environment)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>ARRAY of DICT_ENTRY&lt;STRING,STRING&gt;</entry>
                  <entry>Environment to add or update</entry>
                </row>
              </tbody>
            </tgroup>
            </informaltable>
            Normally, session bus activated services inherit the environment of the bus daemon.  This method adds to or modifies that environment when activating services.
        </para>
        <para>
          Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
        </para>
        <para>
          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.
        </para>

      </sect3>

      <sect3 id="bus-messages-get-name-owner">
        <title>Method: <literal>org.freedesktop.DBus.GetNameOwner</literal></title>
        <para>
          As a method:
          <programlisting>
            STRING GetNameOwner (in STRING name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name to get the owner of</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        Reply arguments:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Argument</entry>
                <entry>Type</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>0</entry>
                <entry>STRING</entry>
                <entry>Return value, a unique connection name</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
        Returns the unique connection name of the primary owner of the name
        given. If the requested name doesn't have an owner, returns a
        <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
       </para>
      </sect3>

      <sect3 id="bus-messages-get-connection-unix-user">
        <title>Method: <literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
        <para>
          As a method:
          <programlisting>
            UINT32 GetConnectionUnixUser (in STRING bus_name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Unique or well-known bus name of the connection to
                    query, such as <literal>:12.34</literal> or
                    <literal>com.example.tea</literal></entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        Reply arguments:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Argument</entry>
                <entry>Type</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>0</entry>
                <entry>UINT32</entry>
                <entry>Unix user ID</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
        Returns the Unix user ID of the process connected to the server. If
        unable to determine it (for instance, because the process is not on the
        same machine as the bus daemon), an error is returned.
       </para>
      </sect3>

      <sect3 id="bus-messages-get-connection-unix-process-id">
        <title>Method: <literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
        <para>
          As a method:
          <programlisting>
            UINT32 GetConnectionUnixProcessID (in STRING bus_name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Unique or well-known bus name of the connection to
                    query, such as <literal>:12.34</literal> or
                    <literal>com.example.tea</literal></entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        Reply arguments:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Argument</entry>
                <entry>Type</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>0</entry>
                <entry>UINT32</entry>
                <entry>Unix process id</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
        Returns the Unix process ID of the process connected to the server. If
        unable to determine it (for instance, because the process is not on the
        same machine as the bus daemon), an error is returned.
       </para>
      </sect3>

      <sect3 id="bus-messages-get-connection-credentials">
        <title>Method: <literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
        <para>
          As a method:
          <programlisting>
            ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt; GetConnectionCredentials (in STRING bus_name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Unique or well-known bus name of the connection to
                    query, such as <literal>:12.34</literal> or
                    <literal>com.example.tea</literal></entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        Reply arguments:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Argument</entry>
                <entry>Type</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>0</entry>
                <entry>ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt;</entry>
                <entry>Credentials</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>

      <para>
        Returns as many credentials as possible for the process connected to
        the server. If unable to determine certain credentials (for instance,
        because the process is not on the same machine as the bus daemon,
        or because this version of the bus daemon does not support a
        particular security framework), or if the values of those credentials
        cannot be represented as documented here, then those credentials
        are omitted.
      </para>

      <para>
        Keys in the returned dictionary not containing "." are defined
        by this specification. Bus daemon implementors supporting
        credentials frameworks not mentioned in this document should either
        contribute patches to this specification, or use keys containing
        "." and starting with a reversed domain name.
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Key</entry>
                <entry>Value type</entry>
                <entry>Value</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>UnixUserID</entry>
                <entry>UINT32</entry>
                <entry>The numeric Unix user ID, as defined by POSIX</entry>
              </row>
              <row>
                <entry>UnixGroupIDs</entry>
                <entry>ARRAY of UINT32</entry>
                <entry>The numeric Unix group IDs (including both the primary
                  group and the supplementary groups), as defined by POSIX, in
                  numerically sorted order. This array is either complete or
                  absent: if the message bus is able to determine some but not
                  all of the caller's groups, or if one of the groups is not
                  representable in a UINT32, it must not add this credential
                  to the dictionary.
                </entry>
              </row>
              <row>
                <entry>ProcessFD</entry>
                <entry>UNIX_FD</entry>
                <entry>A file descriptor pinning the process, on platforms
                  that have this concept. On Linux, the SO_PEERPIDFD socket
                  option is a suitable implementation. This is safer to use
                  to identify a process than the <literal>ProcessID</literal>,
                  as the latter is subject to re-use attacks, while the FD
                  cannot be recycled. If the original process no longer exists
                  the FD will no longer be resolvable.</entry>
              </row>
              <row>
                <entry>ProcessID</entry>
                <entry>UINT32</entry>
                <entry>The numeric process ID, on platforms that have
                  this concept. On Unix, this is the process ID defined by
                  POSIX.</entry>
              </row>
              <row>
                <entry>WindowsSID</entry>
                <entry>STRING</entry>
                <entry>The Windows security identifier in its string form,
                e.g. "S-1-5-21-3623811015-3361044348-30300820-1013" for
                a domain or local computer user or "S-1-5-18" for the
                LOCAL_SYSTEM user</entry>
              </row>

              <row>
                <entry>LinuxSecurityLabel</entry>
                <entry>ARRAY of BYTE</entry>
                <entry>
                  <para>On Linux systems, the security label that would result
                    from the SO_PEERSEC getsockopt call. The array contains
                    the non-zero bytes of the security label in an unspecified
                    ASCII-compatible encoding<footnote>
                      <para>It could be ASCII or UTF-8, but could also be
                        ISO Latin-1 or any other encoding.</para>
                    </footnote>, followed by a single zero byte.</para>
                  <para>
                    For example, the SELinux context
                    <literal>system_u:system_r:init_t:s0</literal>
                    (a string of length 27) would be encoded as 28 bytes
                    ending with ':', 's', '0', '\x00'.<footnote>
                      <para>Note that this is not the same as the older
                        GetConnectionSELinuxContext method, which does
                        not append the zero byte. Always appending the
                        zero byte allows callers to read the string
                        from the message payload without copying.</para>
                    </footnote>
                  </para>
                  <para>
                    On SELinux systems this is the SELinux context, as output
                    by <literal>ps -Z</literal> or <literal>ls -Z</literal>.
                    Typical values might include
                    <literal>system_u:system_r:init_t:s0</literal>,
                    <literal>unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023</literal>,
                    or
                    <literal>unconfined_u:unconfined_r:chrome_sandbox_t:s0-s0:c0.c1023</literal>.
                  </para>
                  <para>
                    On Smack systems, this is the Smack label.
                    Typical values might include
                    <literal>_</literal>, <literal>*</literal>,
                    <literal>User</literal>, <literal>System</literal>
                    or <literal>System::Shared</literal>.
                  </para>
                  <para>
                    On AppArmor systems, this is the AppArmor context,
                    a composite string encoding the AppArmor label (one or more
                    profiles) and the enforcement mode.
                    Typical values might include <literal>unconfined</literal>,
                    <literal>/usr/bin/firefox (enforce)</literal> or
                    <literal>user1 (complain)</literal>.
                  </para>
                </entry>
              </row>

            </tbody>
          </tgroup>
        </informaltable>
       </para>

        <para>
          This method was added in D-Bus 1.7 to reduce the round-trips
          required to list a process's credentials. In older versions, calling
          this method will fail: applications wishing to support obsolete
          message bus versions can recover by using the separate methods such as
          <link linkend="bus-messages-get-connection-unix-user">GetConnectionUnixUser</link>
          instead.
        </para>
      </sect3>

      <sect3 id="bus-messages-get-adt-audit-session-data">
        <title>Method: <literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
        <para>
          As a method:
          <programlisting>
            ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Unique or well-known bus name of the connection to
                    query, such as <literal>:12.34</literal> or
                    <literal>com.example.tea</literal></entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>ARRAY of BYTE</entry>
                  <entry>auditing data as returned by
                    adt_export_session_data()</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
          Returns auditing data used by Solaris ADT, in an unspecified
          binary format. If you know what this means, please contribute
          documentation via the D-Bus bug tracking system.
          This method is on the core DBus interface for historical reasons;
          the same information should be made available via the
          <link linkend="bus-messages-get-connection-credentials">GetConnectionCredentials</link>
          method in future.
        </para>
      </sect3>

      <sect3 id="bus-messages-get-connection-selinux-security-context">
        <title>Method: <literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
        <para>
          As a method:
          <programlisting>
            ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Unique or well-known bus name of the connection to
                    query, such as <literal>:12.34</literal> or
                    <literal>com.example.tea</literal></entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>ARRAY of BYTE</entry>
                  <entry>some sort of string of bytes, not necessarily UTF-8,
                    not including '\0'</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
          Returns the security context used by SELinux, in an unspecified
          format. If you know what this means, please contribute
          documentation via the D-Bus bug tracking system.
          This method is on the core DBus interface for historical reasons;
          the same information should be made available via the
          <link linkend="bus-messages-get-connection-credentials">GetConnectionCredentials</link>
          method in future.
        </para>
      </sect3>


      <sect3 id="bus-messages-add-match">
        <title>Method: <literal>org.freedesktop.DBus.AddMatch</literal></title>
        <para>
          As a method:
          <programlisting>
            AddMatch (in STRING rule)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Match rule to add to the connection</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
        If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
        error is returned.
       </para>
      </sect3>
      <sect3 id="bus-messages-remove-match">
        <title>Method: <literal>org.freedesktop.DBus.RemoveMatch</literal></title>
        <para>
          As a method:
          <programlisting>
            RemoveMatch (in STRING rule)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Match rule to remove from the connection</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
        If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
        error is returned.
       </para>
      </sect3>

      <sect3 id="bus-messages-get-id">
        <title>Method: <literal>org.freedesktop.DBus.GetId</literal></title>
        <para>
          As a method:
          <programlisting>
            GetId (out STRING id)
          </programlisting>
        Reply arguments:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Argument</entry>
                <entry>Type</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>0</entry>
                <entry>STRING</entry>
                <entry>Unique ID identifying the bus daemon</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
        Gets the unique ID of the bus. The unique ID here is shared among all addresses the
        bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
        <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
        ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
        There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
        by org.freedesktop.DBus.Peer.GetMachineId().
        For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
        </para>
      </sect3>

      <sect3 id="message-bus-properties-features">
        <title>Property: <literal>org.freedesktop.DBus.Features</literal></title>
        <para>
          As a property:
          <programlisting>
            Read-only constant ARRAY of STRING Features
          </programlisting>
          This property lists abstract “features” provided by the message
          bus, and can be used by clients to detect the capabilities
          of the message bus with which they are communicating.
          This property was added in version 1.11.x of the reference
          implementation of the message bus.
          Like all properties, it is accessed via method calls to the standard
          <link linkend="standard-interfaces-properties">Properties</link>
          interface.
        </para>

        <para>
          Items in the returned array not containing “.” are defined
          by this specification. Bus daemon implementors wishing to advertise
          features not mentioned in this document should either contribute
          patches to this specification, or use keys containing “.” and
          starting with their own reversed domain name, for example
          <literal>com.example.MyBus.SubliminalMessages</literal>.
        </para>

        <para>
          The features currently defined in this specification are as follows:
          <variablelist>

            <varlistentry>
              <term><literal>ActivatableServicesChanged</literal></term>
              <listitem>
                <para>
                  This message bus emits the <literal>ActivatableServicesChanged</literal>
                  signal whenever its list of activatable services might have changed
                  (for details see <xref linkend="bus-messages-activatable-services-changed"/>).
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><literal>AppArmor</literal></term>
              <listitem>
                <para>
                  This message bus filters messages via the
                  <ulink url="http://wiki.apparmor.net/">AppArmor</ulink>
                  security framework. This feature should only be
                  advertised if AppArmor mediation is enabled and
                  active at runtime; merely compiling in support
                  for AppArmor should not result in this feature being
                  advertised on message bus instances where it is disabled by
                  message bus or operating system configuration.
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><literal>HeaderFiltering</literal></term>
              <listitem>
                <para>
                  This message bus guarantees that it will remove
                  header fields that it does not understand when it
                  relays messages, so that a client receiving a
                  recently-defined header field that is specified to be
                  controlled by the message bus can safely assume that
                  it was in fact set by the message bus. This check is
                  needed because older message bus implementations did
                  not guarantee to filter headers in this way, so a
                  malicious client could send any recently-defined
                  header field with a crafted value of its choice
                  through an older message bus that did not understand
                  that header field.
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><literal>SELinux</literal></term>
              <listitem>
                <para>
                  This message bus filters messages via the
                  <ulink url="https://selinuxproject.org/">SELinux</ulink>
                  security framework. Similar to <literal>AppArmor</literal>,
                  this feature should only be advertised if SELinux mediation
                  is enabled and active at runtime (if SELinux is placed in
                  permissive mode, that is still considered to be active).
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><literal>SystemdActivation</literal></term>
              <listitem>
                <para>
                  When asked to activate a service that has the
                  <literal>SystemdService</literal> field in its
                  <filename>.service</filename> file, this message bus will
                  carry out systemd activation (for details see
                  <xref linkend="message-bus-starting-services-systemd"/>).
                </para>
              </listitem>
            </varlistentry>

          </variablelist>
        </para>
      </sect3>

      <sect3 id="message-bus-properties-interfaces">
        <title>Property: <literal>org.freedesktop.DBus.Interfaces</literal></title>
        <para>
          As a property:
          <programlisting>
            Read-only constant ARRAY of STRING Interfaces
          </programlisting>
          This property lists interfaces provided by the
          <literal>/org/freedesktop/DBus</literal> object,
          and can be used by clients to detect the capabilities
          of the message bus with which they are communicating.
          Unlike the standard Introspectable interface, querying this
          property does not require parsing XML.
          This property was added in version 1.11.x of the reference
          implementation of the message bus.
          Like all properties, it is accessed via method calls to the standard
          <link linkend="standard-interfaces-properties">Properties</link>
          interface.
        </para>

        <para>
          The standard <literal>org.freedesktop.DBus</literal> and
          <literal>org.freedesktop.DBus.Properties</literal> interfaces
          are not included in the value of this property, because their
          presence can be inferred from the fact that a method call on
          <literal>org.freedesktop.DBus.Properties</literal> asking for
          properties of <literal>org.freedesktop.DBus</literal> was
          successful. The standard <literal>org.freedesktop.DBus.Peer</literal>
          and <literal>org.freedesktop.DBus.Introspectable</literal>
          interfaces are not included in the value of this property either,
          because they do not indicate features of the message bus
          implementation.
        </para>
      </sect3>
    </sect2>

    <sect2>
      <title>Message Bus Introspection</title>
      <para>
        The message bus should implement the standard
        <link linkend="standard-interfaces-introspectable">Introspectable</link>
        interface.
      </para>
    </sect2>

    <sect2>
      <title>Message Bus as a Peer</title>
      <para>
        The message bus should implement the standard
        <link linkend="standard-interfaces-peer">Peer</link> interface.
        Support for the Peer interface was added in version
        1.11.x of the reference implementation of the message bus.
      </para>
    </sect2>

    <sect2 id="message-bus-properties">
      <title>Message Bus Properties</title>
      <para>
        The message bus should implement the standard
        <link linkend="standard-interfaces-properties">Properties</link>
        interface on the object path
        <literal>/org/freedesktop/DBus</literal>.
        The specific properties to be provided are documented as part of
        their respective interfaces.
        Support for the Properties interface was added in version
        1.11.x of the reference implementation of the message bus.
      </para>
    </sect2>

    <sect2 id="message-bus-monitoring-interface">
      <title>Monitoring Interface: <literal>org.freedesktop.DBus.Monitoring</literal></title>
      <para>
        The special message bus name <literal>org.freedesktop.DBus</literal>
        may optionally implement the
        <literal>org.freedesktop.DBus.Monitoring</literal> interface on
        the object path <literal>/org/freedesktop/DBus</literal>.
      </para>

      <sect3 id="bus-messages-become-monitor">
        <title>Method: <literal>org.freedesktop.DBus.Monitoring.BecomeMonitor</literal></title>
        <para>
          As a method:
          <programlisting>
            BecomeMonitor (in ARRAY of STRING rule, in UINT32 flags)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>ARRAY of STRING</entry>
                  <entry>Match rules to add to the connection</entry>
                </row>
                <row>
                  <entry>1</entry>
                  <entry>UINT32</entry>
                  <entry>Not used, must be 0</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>

        <para>
          Converts the connection into a <emphasis>monitor
            connection</emphasis> which can be used as a debugging/monitoring
          tool. Only a user who is privileged on this
          bus (by some implementation-specific definition) may create
          monitor connections<footnote>
            <para>
              In the reference implementation,
              the default configuration is that each user (identified by
              numeric user ID) may monitor their own session bus,
              and the root user (user ID zero) may monitor the
              system bus.
            </para>
          </footnote>.
       </para>

       <para>
         Monitor connections lose all their bus names, including the unique
         connection name, and all their match rules. Sending messages on a
         monitor connection is not allowed: applications should use a private
         connection for monitoring.
       </para>

       <para>
         Monitor connections may receive all messages, even messages that
         should only have gone to some other connection ("eavesdropping").
         The first argument is a list of match rules, which replace any
         match rules that were previously active for this connection.
         These match rules are always treated as if they contained the
         special <literal>eavesdrop='true'</literal> member.
       </para>

       <para>
         As a special case, an empty list of match rules (which would
         otherwise match nothing, making the monitor useless) is treated
         as a shorthand for matching all messages.
       </para>

       <para>
         The second argument might be used for flags to influence the
         behaviour of the monitor connection in future D-Bus versions.
       </para>

       <para>
         Message bus implementations should attempt to minimize the
         side-effects of monitoring — in particular, unlike ordinary
         eavesdropping, monitoring the system bus does not require the
         access control rules to be relaxed, which would change the set
         of messages that can be delivered to their (non-monitor)
         destinations. However, it is unavoidable that monitoring
         will increase the message bus's resource consumption. In
         edge cases where there was barely enough time or memory without
         monitoring, this might result in message deliveries failing
         when they would otherwise have succeeded.
       </para>
      </sect3>
    </sect2>

    <sect2 id="message-bus-debug-stats-interface">
      <title>Debug Statistics Interface: <literal>org.freedesktop.DBus.Debug.Stats</literal></title>
      <para>
        The special message bus name <literal>org.freedesktop.DBus</literal>
        may optionally implement the
        <literal>org.freedesktop.DBus.Debug.Stats</literal> interface on
        the object path <literal>/org/freedesktop/DBus</literal>.
        This interface provides statistics which might be useful to
        investigate performance bottlenecks.
      </para>
      <para>
        In the reference implementation, this interface is an optional
        feature, which can be enabled or disabled at compile-time.
        On the well-known system bus or other buses that serve more than
        one user, access to this interface should normally be restricted to
        administrative users, but can be opened up to developers via
        configuration if required.
      </para>

      <sect3 id="bus-messages-get-stats">
        <title>Method: <literal>org.freedesktop.DBus.Debug.Stats.GetStats</literal></title>
        <para>
          As a method:
          <programlisting>
            GetStats (out ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt; stats)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt;</entry>
                  <entry>Statistics</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
          Get statistics about the message bus itself.
        </para>
        <para>
          The following keys in the result dictionary are defined by
          this specification:

          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Key</entry>
                  <entry>Value type</entry>
                  <entry>Value</entry>
                </row>
              </thead>
              <tbody>

                <!-- Basic information -->
                <row>
                  <entry><literal>Serial</literal></entry>
                  <entry>UINT32</entry>
                  <entry>A serial number which is incremented with each
                    call to the <literal>GetStats</literal> method.</entry>
                </row>

                <!-- Connection count -->
                <row>
                  <entry><literal>ActiveConnections</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The number of active connections currently handled
                    by this message bus.
                    The exact meaning of an active connection is
                    implementation-defined: in the reference dbus-daemon,
                    a connection is considered to be active after it has
                    successfully called
                    <link xref="bus-messages-hello">the Hello
                      method</link>.</entry>
                </row>
                <row>
                  <entry><literal>IncompleteConnections</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The number of incomplete connections currently handled
                    by this message bus.
                    The exact meaning of an incomplete connection is
                    implementation-defined: in the reference dbus-daemon,
                    a connection is considered to be incomplete if it is
                    still carrying out the SASL handshake or if it has not
                    yet successfully called
                    <link xref="bus-messages-hello">the Hello
                      method</link>.</entry>
                </row>

                <!-- Match rules -->
                <row>
                  <entry><literal>MatchRules</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The total number of match rules that are
                    currently in use.</entry>
                </row>
                <row>
                  <entry><literal>PeakMatchRules</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The largest total number of match rules that have
                    been in use at any one time.</entry>
                </row>
                <row>
                  <entry><literal>PeakMatchRulesPerConnection</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The largest total number of match rules that have
                    been in use by a single connection at any one time.</entry>
                </row>

                <!-- Bus names -->
                <row>
                  <entry><literal>BusNames</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The total number of unique or well-known bus names
                    that are currently owned by all connections.</entry>
                </row>
                <row>
                  <entry><literal>PeakBusNames</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The largest total number of bus names
                    that have been owned by all connections at any
                    one time.</entry>
                </row>
                <row>
                  <entry><literal>PeakBusNamesPerConnection</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The largest total number of bus names
                    that have been owned by any single connection at any
                    one time.</entry>
                </row>

              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          All other keys are implementation-specific until or unless they
          are standardized in this specification.
          The reference dbus-daemon provides additional keys
          <literal>ListMemPoolUsedBytes</literal>,
          <literal>ListMemPoolCachedBytes</literal>,
          <literal>ListMemPoolAllocatedBytes</literal>
          with UINT32 values, all of which describe implementation details
          of libdbus’ memory management.
        </para>
      </sect3>

      <sect3 id="bus-messages-get-connection-stats">
        <title>Method: <literal>org.freedesktop.DBus.Debug.Stats.GetConnectionStats</literal></title>
        <para>
          As a method:
          <programlisting>
            GetConnectionStats (in STRING connection, out ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt; stats)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Unique or well-known name of a connection</entry>
                </row>
                <row>
                  <entry>1</entry>
                  <entry>ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt;</entry>
                  <entry>Statistics</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
          Get statistics about a connection, identified by its unique
          connection name or by any well-known bus name for which it is
          the primary owner.
          This method is not meaningful for the message bus
          <literal>org.freedesktop.DBus</literal> itself.
       </para>
        <para>
          The following keys in the result dictionary are defined by
          this specification:

          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Key</entry>
                  <entry>Value type</entry>
                  <entry>Value</entry>
                </row>
              </thead>
              <tbody>

                <!-- Basic information -->
                <row>
                  <entry><literal>Serial</literal></entry>
                  <entry>UINT32</entry>
                  <entry>A serial number which is incremented with each
                    call to the <literal>GetConnectionStats</literal>
                    method.</entry>
                </row>
                <row>
                  <entry><literal>UniqueName</literal></entry>
                  <entry>STRING</entry>
                  <entry>The connection's unique connection name.
                    If the first parameter to
                    <literal>GetConnectionStats</literal> was a unique
                    name, this will be the same unique name.
                    If the first parameter was a well-known name, this
                    is equivalent to the result of calling
                    <link xref="bus-messages-get-name-owner">GetNameOwner</link>
                    on that well-known name.</entry>
                </row>

                <!-- Match rules -->
                <row>
                  <entry><literal>MatchRules</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The total number of match rules that are
                    currently in use by this connection.</entry>
                </row>
                <row>
                  <entry><literal>PeakMatchRules</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The largest total number of match rules that have
                    been in use by this connection at any one time.</entry>
                </row>

                <!-- Bus names -->
                <row>
                  <entry><literal>BusNames</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The total number of bus names that are
                    currently owned by this connection.</entry>
                </row>
                <row>
                  <entry><literal>PeakBusNames</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The largest total number of bus names
                    that have been owned by this connection at any
                    one time.</entry>
                </row>

                <!-- Incoming queued messages -->
                <row>
                  <entry><literal>IncomingMessages</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The total number of messages queued in the message
                    bus that have been received from this connection and not
                    yet dispatched.</entry>
                </row>
                <row>
                  <entry><literal>IncomingBytes</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The total number of bytes in the incoming
                    messages.</entry>
                </row>
                <row>
                  <entry><literal>IncomingFDs</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The total number of Unix file descriptors
                    attached to the incoming messages.</entry>
                </row>
                <row>
                  <entry><literal>PeakIncomingBytes</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The highest total number of bytes in the incoming
                    messages at any one time.</entry>
                </row>
                <row>
                  <entry><literal>PeakIncomingFDs</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The highest total number of Unix file descriptors
                    attached to the incoming messages at any one time.</entry>
                </row>

                <!-- Outgoing queued messages -->
                <row>
                  <entry><literal>OutgoingMessages</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The total number of messages queued in the message
                    bus to be delivered to this connection.</entry>
                </row>
                <row>
                  <entry><literal>OutgoingBytes</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The total number of bytes in the outgoing
                    messages.</entry>
                </row>
                <row>
                  <entry><literal>OutgoingFDs</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The total number of Unix file descriptors
                    attached to the outgoing messages.</entry>
                </row>
                <row>
                  <entry><literal>PeakOutgoingBytes</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The highest total number of bytes in the outgoing
                    messages at any one time.</entry>
                </row>
                <row>
                  <entry><literal>PeakOutgoingFDs</literal></entry>
                  <entry>UINT32</entry>
                  <entry>The highest total number of Unix file descriptors
                    attached to the outgoing messages at any one time.</entry>
                </row>

              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          All other keys are implementation-specific until or unless they
          are standardized in this specification.
        </para>
      </sect3>

      <sect3 id="bus-messages-get-all-match-rules">
        <title>Method: <literal>org.freedesktop.DBus.Debug.Stats.GetAllMatchRules</literal></title>
        <para>
          As a method:
          <programlisting>
            GetAllMatchRules (out ARRAY of DICT_ENTRY&lt;STRING,ARRAY&lt;STRING&gt;&gt; rules)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>ARRAY of DICT_ENTRY&lt;STRING,ARRAY&lt;STRING&gt;&gt;</entry>
                  <entry>Match rules</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
          List all of the match rules that are active on this message bus.
          The keys in the result dictionary are unique connection names.
          The values are lists of match rules registered by that connection,
          in an unspecified order.
          If a connection has registered the same match rule more than once,
          it is unspecified whether duplicate entries appear in the list.
       </para>
      </sect3>

    </sect2>

    <sect2 id="message-bus-verbose-interface">
      <title>Verbose Interface: <literal>org.freedesktop.DBus.Verbose</literal></title>
      <para>
        The special message bus name <literal>org.freedesktop.DBus</literal>
        may optionally implement the
        <literal>org.freedesktop.DBus.Verbose</literal> interface on
        the object path <literal>/org/freedesktop/DBus</literal>.
        In the reference implementation, this interface is an optional
        feature, which can be enabled or disabled at compile-time.
        On the well-known system bus or other buses that serve more than
        one user, access to this interface should normally be restricted to
        administrative users, but can be opened up to developers via
        configuration if required.
      </para>

      <sect3 id="bus-messages-enable-verbose">
        <title>Method: <literal>org.freedesktop.DBus.Verbose.EnableVerbose</literal></title>
        <para>
          As a method:
          <programlisting>
            EnableVerbose ()
          </programlisting>
          Start producing verbose debugging messages in an
          implementation-specific way.
          For example, these messages might go to standard error or to a
          system event log.
       </para>
      </sect3>

      <sect3 id="bus-messages-disable-verbose">
        <title>Method: <literal>org.freedesktop.DBus.Verbose.DisableVerbose</literal></title>
        <para>
          As a method:
          <programlisting>
            DisableVerbose ()
          </programlisting>
          Stop producing verbose debugging messages.
       </para>
      </sect3>

    </sect2>
  </sect1>
<!--
  <appendix id="implementation-notes">
    <title>Implementation notes</title>
    <sect1 id="implementation-notes-subsection">
      <title></title>
      <para>
      </para>
    </sect1>
  </appendix>
-->

  <glossary><title>Glossary</title>
    <para>
      This glossary defines some of the terms used in this specification.
    </para>

    <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
      <glossdef>
        <para>
          The message bus maintains an association between names and
          connections. (Normally, there's one connection per application.)  A
          bus name is simply an identifier used to locate connections. For
          example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
          name might be used to send a message to a screensaver from Yoyodyne
          Corporation.  An application is said to <firstterm>own</firstterm> a
          name if the message bus has associated the application's connection
          with the name.  Names may also have <firstterm>queued
          owners</firstterm> (see <xref linkend="term-queued-owner"/>).
            The bus assigns a unique name to each connection,
            see <xref linkend="term-unique-name"/>. Other names
              can be thought of as "well-known names" and are
              used to find applications that offer specific functionality.
        </para>

        <para>
          See <xref linkend="message-protocol-names-bus"/> for details of
          the syntax and naming conventions for bus names.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-message"><glossterm>Message</glossterm>
      <glossdef>
        <para>
          A message is the atomic unit of communication via the D-Bus
          protocol. It consists of a <firstterm>header</firstterm> and a
          <firstterm>body</firstterm>; the body is made up of
          <firstterm>arguments</firstterm>.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
      <glossdef>
        <para>
          The message bus is a special application that forwards
          or routes messages between a group of applications
          connected to the message bus. It also manages
          <firstterm>names</firstterm> used for routing
          messages.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-name"><glossterm>Name</glossterm>
      <glossdef>
        <para>
          See <xref linkend="term-bus-name"/>. "Name" may
            also be used to refer to some of the other names
            in D-Bus, such as interface names.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="namespace"><glossterm>Namespace</glossterm>
      <glossdef>
        <para>
          Used to prevent collisions when defining new interfaces, bus names
          etc. The convention used is the same one Java uses for defining
          classes: a reversed domain name.
          See <xref linkend="message-protocol-names-bus"/>,
          <xref linkend="message-protocol-names-interface"/>,
          <xref linkend="message-protocol-names-error"/>,
          <xref linkend="message-protocol-marshaling-object-path"/>.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-object"><glossterm>Object</glossterm>
      <glossdef>
        <para>
          Each application contains <firstterm>objects</firstterm>, which have
          <firstterm>interfaces</firstterm> and
          <firstterm>methods</firstterm>. Objects are referred to by a name,
          called a <firstterm>path</firstterm>.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
      <glossdef>
        <para>
          An application talking directly to another application, without going
          through a message bus. One-to-one connections may be "peer to peer" or
          "client to server." The D-Bus protocol has no concept of client
          vs. server after a connection has authenticated; the flow of messages
          is symmetrical (full duplex).
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-path"><glossterm>Path</glossterm>
      <glossdef>
        <para>
          Object references (object names) in D-Bus are organized into a
          filesystem-style hierarchy, so each object is named by a path. As in
          LDAP, there's no difference between "files" and "directories"; a path
          can refer to an object, while still having child objects below it.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
      <glossdef>
        <para>
          Each bus name has a primary owner; messages sent to the name go to the
          primary owner. However, certain names also maintain a queue of
          secondary owners "waiting in the wings." If the primary owner releases
          the name, then the first secondary owner in the queue automatically
          becomes the new owner of the name.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-service"><glossterm>Service</glossterm>
      <glossdef>
        <para>
          A service is an executable that can be launched by the bus daemon.
          Services normally guarantee some particular features, for example they
          may guarantee that they will request a specific name such as
          "com.example.Screensaver1", have a singleton object
          "/com/example/Screensaver1", and that object will implement the
          interface "com.example.Screensaver1.Control".
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
      <glossdef>
        <para>
          ".service files" tell the bus about service applications that can be
          launched (see <xref linkend="term-service"/>). Most importantly they
          provide a mapping from bus names to services that will request those
            names when they start up.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
      <glossdef>
        <para>
          The special name automatically assigned to each connection by the
          message bus. This name will never change owner, and will be unique
          (never reused during the lifetime of the message bus).
          It will begin with a ':' character.
        </para>
      </glossdef>
    </glossentry>

  </glossary>
</article>