path: root/doc/dbus-specification.sgml

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<article id="index">
  <artheader>
    <title>D-BUS Specification</title>
    <releaseinfo>Version 0.8</releaseinfo>
    <date>06 September 2003</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>
    </authorgroup>
  </artheader>

  <sect1 id="introduction">
    <title>Introduction</title>
    <para>
      D-BUS is a system for low-latency, low-overhead, easy to use
      interprocess communication (IPC). In more detail:
      <itemizedlist>
        <listitem>
          <para>
            D-BUS is <emphasis>low-latency</emphasis> because it is designed 
            to avoid round trips and allow asynchronous operation, much like 
            the X protocol.
          </para>
        </listitem>
        <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.
          </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 peer-to-peer 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 D-BUS 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 interoperablity, for example a file
      monitoring service or a configuration service.
    </para>
  </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 inteprets 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>

    <sect2 id="message-protocol-header-encoding">
      <title>Header Encoding</title>
      <para>
        Following the mandatory fields, there are zero or more named fields (see
        <xref linkend="message-protocol-header-fields">), and then nul bytes
        padding the header such that its total length in bytes is a multiple of
        8.
      </para>
      <para>
        The header MUST begin with the following mandatory fields in the following
        order:
        <informaltable>
          <tgroup cols=2>
            <thead>
              <row>
                <entry>Size</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>1 byte</entry>
                <entry>Endianness flag; ASCII 'l' for little-endian 
                  or ASCII 'B' for big-endian.</entry>
              </row>
              <row>
                <entry>1 byte</entry>
                <entry>Type of message. Unknown types MUST be ignored. 
                  Currently-defined types are described below.
                </entry>
              </row>
              <row>
                <entry>1 byte</entry>
                <entry>Bitwise OR of flags. Unknown flags
                  MUST be ignored. Currently-defined flags are described below.
                </entry>
              </row>
              <row>
                <entry>1 byte</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 0.
                </entry>
              </row>
              <row>
                <entry>4 bytes</entry>
                <entry>An unsigned 32-bit integer in the
                  message's byte order, indicating the total length in bytes of
                  the header including named fields and any alignment padding.
                  MUST be a multiple of 8.
                </entry>
              </row>
              <row>
                <entry>4 bytes</entry>
                <entry>An unsigned 32-bit integer in the
                  message's byte order, indicating the total length in bytes of
                  the message body.
                </entry>
              </row>      
              <row>
                <entry>4 bytes</entry>
                <entry>The message's serial number, an unsigned 32-bit integer in
                  the message's byte order. The serial number is a cookie used to 
                  identify message replies; thus all outstanding unreplied-to messages 
                  from the same connection MUST have a different serial number.
                  Zero is not a valid serial number, but all other numbers are 
                  allowed.
                </entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
      <para>
        Types that can appear in the second byte of the header:
        <informaltable>
          <tgroup cols=3>
            <thead>
              <row>
                <entry>Conventional name</entry>
                <entry>Decimal value</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>INVALID</entry>
                <entry>0</entry>
                <entry>This is an invalid type, if seen in a message 
                  the connection should be dropped immediately.</entry>
              </row>
              <row>
                <entry>METHOD_CALL</entry>
                <entry>1</entry>
                <entry>Method call.</entry>
              </row>
              <row>
                <entry>METHOD_RETURN</entry>
                <entry>2</entry>
                <entry>Method reply with returned data.</entry>
              </row>
              <row>
                <entry>ERROR</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>SIGNAL</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>NO_REPLY_EXPECTED</entry>
                <entry>0x1</entry>
                <entry>This message does not expect method return replies or
                error replies; the reply can be omitted as an
                optimization. However, it is compliant with this specification
                to return the reply despite this flag.</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
    </sect2>

    <sect2 id="message-protocol-header-fields">
      <title>Header Fields</title>
      <para>
        In addition to the required header information mentioned 
        in <xref linkend="message-protocol-header-encoding">, 
          the header may contain zero or more named 
          header fields. Future versions of this protocol
          specification may add new fields. Implementations must
          ignore fields they do not understand. Implementations
          must not invent their own header fields; only changes to 
          this specification may introduce new header fields.
      </para>

      <para>
        Header fields MUST be aligned to a 4-byte boundary. Header field
        names MUST consist of a single byte, possible values of which are
        defined below. Following the name, the field MUST have a type code
        represented as a single unsigned byte, and then a properly-aligned
        value of that type.  See <xref
        linkend="message-protocol-arguments"> for a description of how each
        type is encoded. If an implementation sees a header field name that
        it does not understand, it MUST ignore that field.
      </para>

      <para>
        Here are the currently-defined named header fields:
        <informaltable>
          <tgroup cols=3>
            <thead>
              <row>
                <entry>Conventional Name</entry>
		<entry>Decimal Value</entry>
                <entry>Type</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>INVALID</entry>
		<entry>0</entry>
                <entry>INVALID</entry>
                <entry>Not a valid field name (error if it appears in a message)</entry>
              </row>
              <row>
                <entry>PATH</entry>
		<entry>1</entry>
                <entry>STRING</entry>
                <entry>The object to send the message to; objects are identified by 
                a path, "/foo/bar"</entry>
              </row>
              <row>
                <entry>INTERFACE</entry>
		<entry>2</entry>
                <entry>STRING</entry>
                <entry>The interface to invoke a method call on, or 
                that a signal is emitted from. e.g. "org.freedesktop.Introspectable"</entry>
              </row>
              <row>
                <entry>MEMBER</entry>
		<entry>3</entry>
                <entry>STRING</entry>
                <entry>The member, either the method name or signal name. 
                e.g. "Frobate"</entry>
              </row>
              <row>
                <entry>ERROR_NAME</entry>
		<entry>4</entry>
                <entry>STRING</entry>
                <entry>The name of the error that occurred, for errors</entry>
              </row>
              <row>
                <entry>REPLY_SERIAL</entry>
		<entry>5</entry>
                <entry>UINT32</entry>
                <entry>The serial number of the message this message is a reply
                to. (The serial number is one of the mandatory header fields,
                see <xref linkend="message-protocol-header-encoding">.)</entry>
              </row>
              <row>
                <entry>SERVICE</entry>
		<entry>6</entry>
                <entry>STRING</entry>
                <entry>The name of the service this message should be routed to. 
                Only used in combination with the message bus, see 
                <xref linkend="message-bus">.</entry>
              </row>
              <row>
                <entry>SENDER_SERVICE</entry>
		<entry>7</entry>
                <entry>STRING</entry>
                <entry>Sender service. The name of the base service that sent
                this message.  The message bus fills in this field; the field is
                only meaningful in combination with the message bus.</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
      
    </sect2>
    <sect2 id="message-protocol-header-padding">
      <title>Header Alignment Padding</title>
      <para>
        To allow implementations to keep the header and the body in a single
        buffer while keeping data types aligned, the total length of the header
        must be a multiple of 8 bytes.  To achieve this, the header MUST be padded
        with nul bytes to align its total length on an 8-byte boundary. 
        The minimum number of padding bytes MUST be used. Because all possible 
        named fields use at least 8 bytes, implementations can distinguish 
        padding (which must be less than 8 bytes) from additional named fields
        (which must be at least 8 bytes).
      </para>
    </sect2>

    <sect2 id="message-protocol-arguments">
      <title>Message Arguments</title>
      <para>
        The message body is made up of arguments. Each argument is a type code,
        represented by a single unsigned byte, followed by the aligned value of
        the argument in a type-dependent format. Alignment padding between the 
        typecode and the value is initialized to zero.
      </para>
      <para>
        <informaltable>
          <tgroup cols=3>
            <thead>
              <row>
                <entry>Type name</entry>
                <entry>Code</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>INVALID</entry>
                <entry>0</entry>
                <entry>Not a valid type code (error if it appears in a message)</entry>
              </row><row>
                <entry>NIL</entry>
                <entry>1</entry>
                <entry>Marks an "unset" or "nonexistent" argument</entry>
              </row><row>
		<entry>BYTE</entry>
		<entry>2</entry>
		<entry>8-bit unsigned integer</entry>
              </row><row>
		<entry>BOOLEAN</entry>
		<entry>3</entry>
		<entry>Boolean value, 0 is FALSE and 1 is TRUE. Everything else is invalid.</entry>
	      </row><row>
                <entry>INT32</entry>
                <entry>4</entry>
                <entry>32-bit signed integer</entry>
              </row><row>
                <entry>UINT32</entry>
                <entry>5</entry>
                <entry>32-bit unsigned integer</entry>
	      </row><row>
                <entry>INT64</entry>
                <entry>6</entry>
                <entry>64-bit signed integer</entry>
              </row><row>
                <entry>UINT64</entry>
                <entry>7</entry>
                <entry>64-bit unsigned integer</entry>
              </row><row>
                <entry>DOUBLE</entry>
                <entry>8</entry>
                <entry>IEEE 754 double</entry>
              </row><row>
                <entry>STRING</entry>
                <entry>9</entry>
                <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be zero terminated. </entry>
              </row><row>
		<entry>NAMED</entry>
		<entry>10</entry>
		<entry>A named byte array, used for custom types</entry>
	      </row><row>
                <entry>ARRAY</entry>
                <entry>11</entry>
                <entry>Array</entry>
              </row><row>
                <entry>DICT</entry>
                <entry>12</entry>
                <entry>A dictionary of key/value pairs</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
      <para>
        The types are encoded as follows:
        <informaltable>
          <tgroup cols=2>
            <thead>
              <row>
                <entry>Type name</entry>
                <entry>Encoding</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>INVALID</entry>
                <entry>Not applicable; cannot be encoded.</entry>
              </row><row>
                <entry>NIL</entry>
                <entry>No data is encoded; the type code is followed immediately 
                by the type code of the next argument.</entry>
              </row><row>
                <entry>BYTE</entry>
                <entry>A byte.</entry>
              </row><row>
                <entry>BOOLEAN</entry>
                <entry>A byte, with valid values 0 and 1.</entry>
              </row><row>
                <entry>INT32</entry>
                <entry>32-bit signed integer in the message's byte order, aligned to 4-byte boundary.</entry>
              </row><row>
                <entry>UINT32</entry>
                <entry>32-bit unsigned integer in the message's byte order, aligned to 4-byte boundary.</entry>
              </row><row>
                <entry>INT64</entry>
                <entry>64-bit signed integer in the message's byte order, aligned to 8-byte boundary.</entry>
              </row><row>
                <entry>UINT64</entry>
                <entry>64-bit unsigned integer in the message's byte order, aligned to 8-byte boundary.</entry>
              </row><row>
                <entry>DOUBLE</entry>
                <entry>64-bit IEEE 754 double in the message's byte order, aligned to 8-byte boundary.</entry>
              </row><row>
                <entry>STRING</entry>
                <entry>UINT32 aligned to 4-byte boundary indicating the string's 
                  length in bytes excluding its terminating nul, followed by 
                  string data of the given length, followed by a terminating nul 
                  byte.
                </entry>
              </row><row>
                <entry>NAMED</entry>
                <entry>A string (encoded as the STRING type above) giving the
                  name of the type followed by an UINT32 aligned to 4-byte boundary
                  indicating the data length in bytes, followed by the data.
                </entry>
              </row><row>
                <entry>ARRAY</entry>
                <entry>A sequence of bytes giving the element type of the array, terminated
                by a type different from ARRAY (just one byte for one-dimensional arrays, but
                larger for multi-dimensional arrays), followed by an UINT32 (aligned to 4 bytes)
                giving the length of the array data in bytes. This is followed by each array entry
                encoded the way it would normally be encoded, except arrays, which are encoded
                without the type information, since that is already declared above. Arrays containing
                NIL are not allowed.
                </entry>
              </row><row>
                <entry>DICT</entry>
                <entry>UINT32 giving the length of the dictionary data in bytes.
                This is followed by a number of keyname/value pairs, where the
                keyname is encoded as a STRING above, and the value is encoded
                as a byte with typecode and how that type normally would be encoded
                alone.
                </entry>
	      </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
    </sect2>

    <sect2 id="message-protocol-names">
      <title>Valid names</title>
      <para>
        The various header fields of type STRING have some restrictions 
        on the string's format.
      </para>
      <sect3 id="message-protocol-names-service">
        <title>Service names</title>
        <para>
          Services have names with type STRING, meaning that 
          they must be valid UTF-8. However, there are also some 
          additional restrictions that apply to service names 
          specifically:
          <itemizedlist>
	    <listitem><para>They must contain at least one '.' (period) character</para></listitem>
	    <listitem><para>They must not begin with a '.' (period) character</para></listitem>
	    <listitem><para>They must not exceed 256 bytes in length</para></listitem>
	    <listitem><para>They must be at least 1 byte in length</para></listitem>
          </itemizedlist>
          
          As a special exception, base service names (those beginning with a colon
          (':') character) need not contain a period.
        </para>
        <para>
          FIXME really, shouldn't we ban basically everything non-alphanumeric 
          so the name will work in all programming languages?
        </para>
      </sect3>
      <sect3 id="message-protocol-names-interface">
        <title>Interface names</title>
        <para>
          Interface names have the same restrictions as service names, 
          but do not have the special exception for names beginning with 
          a colon.
        </para>
        <para>
          FIXME really, shouldn't we ban basically everything non-alphanumeric 
          so the name will work in all programming languages?
        </para>
      </sect3>
      <sect3 id="message-protocol-names-method">
        <title>Method names</title>
        <para>
          Method names:
          <itemizedlist>
	    <listitem><para>May not contain the '.' (period) character</para></listitem>
	    <listitem><para>Must not exceed 256 bytes in length</para></listitem>
	    <listitem><para>Must be at least 1 byte in length</para></listitem>
          </itemizedlist>
        </para>
        <para>
          FIXME really, shouldn't we ban basically everything non-alphanumeric 
          so the name will work in all programming languages?
        </para>
      </sect3>
      <sect3 id="message-protocol-names-path">
        <title>Path names</title>
        <para>
          A path must begin with an ASCII '/' (slash) character. Paths may not
          end with a slash character unless the path is the one-byte string
          "/". Two slash characters may not appear adjacent to one another (the
          empty string is not a valid "subdirectory"). Paths may not exceed
          256 bytes in length.
        </para>
      </sect3>
      <sect3 id="message-protocol-names-error">
        <title>Error names</title>
        <para>
          Error names have the same restrictions as interface names.
        </para>
        <para>
          FIXME really, shouldn't we ban basically everything non-alphanumeric 
          so the name will work in all programming languages?
        </para>
      </sect3>
    </sect2>

    <sect2 id="message-protocol-types">
      <title>Message types</title>
      <para>
        Each of the message types (METHOD_CALL, METHOD_RETURN, ERROR, and
        SIGNAL) has its own expected usage conventions and header fields.
      </para>
      <sect3 id="message-protocol-types-method">
        <title>Method Calls, Returns, and Errors</title>
        <para>
          Some messages invoke an operation on a remote object.  These are
          called method call messages and have the type tag METHOD_CALL. Such
          messages map naturally to methods on objects in a typical program.
        </para>
        <para>
          A method call message is expected to have a MEMBER header field
          indicating the name of the method. Optionally, the message has an
          INTERFACE field giving the interface the method is a part of. In the
          absence of an INTERFACE field, if two interfaces on the same object have
          a method with the same name, it is undefined which of the two methods
          will be invoked. Implementations may also choose to return an error in
          this ambiguous case. However, if a method name is unique
          implementations should not require an interface field.
        </para>
        <para>
          Method call messages also include a PATH field indicating the 
          object to invoke the method on. If the call is passing through 
          a message bus, the message will also have a SERVICE field giving 
          the service to receive the message.
        </para>
        <para>
          When an application handles a method call message, it is expected to
          return a reply. The reply is identified by a REPLY_SERIAL header field
          indicating the serial number of the METHOD_CALL being replied to. The
          reply can have one of two types; either METHOD_RETURN or ERROR.
        </para>
        <para>
          If the reply has type METHOD_RETURN, the arguments to the reply message 
          are the return value(s) or "out parameters" of the method call. 
          If the reply has type ERROR, 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 METHOD_RETURN 
          reply is expected, so the caller will know the method 
          was successfully processed.
        </para>
        <para>
          If a METHOD_CALL message has the flag NO_REPLY_EXPECTED, 
          then as an optimization the application receiving the method 
          call may choose to omit the reply message (regardless of 
          whether the reply would have been METHOD_RETURN or ERROR). 
          However, it is also acceptable to ignore the NO_REPLY_EXPECTED
          flag and reply anyway.
        </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 METHOD_CALL), or "out" (which implies
            returned in the METHOD_RETURN). 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>
        </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 SIGNAL.
          It must have three header fields: PATH giving the object 
          the signal was emitted from, plus INTERFACE and MEMBER giving
          the fully-qualified name of the signal.
        </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.ActivateService (in STRING service_name, in UINT32 flags,
                                                  out UINT32 resultcode)
          </programlisting>
          This means INTERFACE = org.freedesktop.DBus, MEMBER = ActivateService, 
          METHOD_CALL arguments are STRING and UINT32, METHOD_RETURN argument
          is UINT32. Remember that the MEMBER 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::ActivateService (const char  *service_name, 
                                                                  unsigned int flags);
          </programlisting>
          or equally valid, the return value could be done as an argument:
          <programlisting>
            void org::freedesktop::DBus::ActivateService (const char   *service_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.ServiceLost (STRING service_name)
          </programlisting>
          Signals don't specify "in" vs. "out" because only 
          a single direction is possible.
        </para>
        <para>
          In this ad hoc notation, the special type name ANY means any type
          other than NIL, and the special type name ANY_OR_NIL means any valid
          type.
        </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>

  </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>
      In examples, "C:" and "S:" indicate lines sent by the client and
      server respectively.
    </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 base 64 encoding&gt;</para></listitem>
	                                  <listitem><para>ERROR [human-readable error explanation]</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</para></listitem>
	  <listitem><para>DATA &lt;data in base 64 encoding&gt;</para></listitem>
	  <listitem><para>ERROR</para></listitem>
	</itemizedlist>
      </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>
        If an AUTH command has no arguments, it is a request to list
        available mechanisms. The server SHOULD respond with a REJECTED
        command listing the mechanisms it understands.
      </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 SHOULD send a REJECTED command listing the mechanisms
        it does support.
      </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, the
        server should reject authentication by sending REJECTED.
      </para>
      <para>
        If authentication succeeds after exchanging DATA commands, 
        an OK command should be sent to the client. 
      </para>
      <para>
        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.
      </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>
    </sect2>
    <sect2 id="auth-command-cancel">
      <title>CANCEL Command</title>
      <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 base64-encoded block of data to be interpreted 
        according to the SASL mechanism in use.
      </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 acknowledges that the client has received an 
        OK command from the server, and 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>
    </sect2>
    <sect2 id="auth-command-rejected">
      <title>REJECTED Command</title>
      <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 indicates that the client has been authenticated,
        and that further communication will be a stream of D-BUS messages
        (optionally encrypted, as negotiated) rather than this protocol.
      </para>
      <para>
        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.
      </para>
      <para>
        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 OK command has been sent.
      </para>
    </sect2>
    <sect2 id="auth-command-error">
      <title>ERROR Command</title>
      <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>
    </sect2>
    <sect2 id="auth-examples">
      <title>Authentication examples</title>
      
      <para>
        <figure>
	  <title>Example of successful magic cookie authentication</title>
	  <programlisting>
            (MAGIC_COOKIE is a made up mechanism)

            C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
            S: OK
            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 bW9yZ2Fu
            S: DATA OTUgUWE1ODMwOA==
            C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
            S: OK
            C: BEGIN
          </programlisting>
	</figure>
        <figure>
	  <title>Example of client sends unknown command then falls back to regular auth</title>
	  <programlisting>
            C: FOOBAR
            S: ERROR
            C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
            S: OK
            C: BEGIN
          </programlisting>
	</figure>
        <figure>
	  <title>Example of server doesn't support initial auth mechanism</title>
	  <programlisting>
            C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY bW9yZ2Fu
            S: DATA OTUgUWE1ODMwOA==
            C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
            S: OK
            C: BEGIN
          </programlisting>
	</figure>
        <figure>
	  <title>Example of wrong password or the like followed by successful retry</title>
	  <programlisting>
            C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY bW9yZ2Fu
            S: DATA OTUgUWE1ODMwOA==
            C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
            S: REJECTED
            C: AUTH SKEY bW9yZ2Fu
            S: DATA OTUgUWE1ODMwOA==
            C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
            S: OK
            C: BEGIN
          </programlisting>
	</figure>
        <figure>
	  <title>Example of skey cancelled and restarted</title>
	  <programlisting>
            C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY bW9yZ2Fu
            S: DATA OTUgUWE1ODMwOA==
            C: CANCEL
            S: REJECTED
            C: AUTH SKEY bW9yZ2Fu
            S: DATA OTUgUWE1ODMwOA==
            C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
            S: OK
            C: BEGIN
          </programlisting>
	</figure>
      </para>
    </sect2>
    <sect2 id="auth-states">
      <title>Authentication state diagrams</title>
      
      <para>
        WRITEME
      </para>
    </sect2>
    <sect2 id="auth-mechanisms">
      <title>Authentication mechanisms</title>
      <para>
        This section describes some new authentication mechanisms.
        D-BUS also allows any standard SASL mechanism of course.
      </para>
      <sect3 id="auth-mechanisms-sha">
        <title>DBUS_COOKIE_SHA1</title>
        <para>
          The DBUS_COOKIE_SHA1 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.
        </para>
        <para>
          Authentication proceeds as follows:
          <itemizedlist>
            <listitem>
              <para>
                The client sends the username it would like to authenticate 
                as.
              </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
                hex-encoded randomly-generated challenge string.
              </para>
            </listitem>
            <listitem>
              <para>
                The client locates the cookie, and generates its own hex-encoded
                randomly-generated challenge string.  The client then
                concatentates the server's hex-encoded challenge, a ":"
                character, its own hex-encoded challenge, another ":" character,
                and the hex-encoded cookie.  It computes the SHA-1 hash of this
                composite string.  It sends back to the server the client's
                hex-encoded challenge string, a space character, and the SHA-1
                hash.
              </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_global" 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.
              </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). If no recent keys remain, the 
                server may generate a new key.
              </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>
    </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.
      [FIXME how do you escape colon, comma, and semicolon in the values of the key=value pairs?]
    </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>
      [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>
      [FIXME we need to specify in detail each transport and its possible arguments]
      Current transports include: unix domain sockets (including 
      abstract namespace on linux), TCP/IP, and a debug/testing transport using 
      in-process pipes. Future possible transports include one that 
      tunnels over X11 protocol.
    </para>
  </sect1>

  <sect1 id="standard-messages">
    <title>Standard Peer-to-Peer Messages</title>
    <para>
      See <xref linkend="message-protocol-types-notation"> for details on 
       the notation used in this section.
    </para>
    <sect2 id="standard-messages-ping">
      <title><literal>org.freedesktop.Peer.Ping</literal></title>
      <para>        
        <programlisting>
          org.freedesktop.Peer.Ping ()
        </programlisting>
      </para>
      <para>
        On receipt of the METHOD_CALL
        message <literal>org.freedesktop.Peer.Ping</literal>, an application
        should do nothing other than reply with a METHOD_RETURN as usual.
      </para>
    </sect2>

    <sect2 id="standard-messages-get-props">
      <title><literal>org.freedesktop.Props.Get</literal></title>
      <para>
        [FIXME this is just a bogus made-up method that isn't implemented 
        or thought through, to save an example of table formatting for the 
        argument descriptions]
        <programlisting>
          org.freedesktop.Props.Get (in STRING property_name,
                                     out ANY_OR_NIL property_value)
        </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>in STRING</entry>
                <entry>Name of the property to get</entry>
              </row>
              <row>
                <entry>1</entry>
                <entry>out ANY_OR_NIL</entry>
                <entry>The value of the property. The type depends on the property.</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
    </sect2>
  </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 send and receive messages from 
        the message bus, as in the peer-to-peer case.
      </para>
      <para>
        The message bus keeps track of a set of
        <firstterm>services</firstterm>. A service is simply a name, such as
        <literal>com.yoyodyne.Screensaver</literal>, which can be
        <firstterm>owned</firstterm> by one or more of the connected
        applications.  The message bus itself always owns the special service
        <literal>org.freedesktop.DBus</literal>.
      </para>
      <para>
        Services may have <firstterm>secondary owners</firstterm>. Secondary owners
        of a service are kept in a queue; if the primary owner of a service 
        disconnects, or releases the service, the next secondary owner becomes
        the new owner of the service.
      </para>
      <para>
        Messages may have a <literal>SERVICE</literal> field (see <xref
                                                                  linkend="message-protocol-header-fields">).  When the message bus
          receives a message, if the <literal>SERVICE</literal> field is absent, the
          message is taken to be a standard peer-to-peer message and interpreted
          by the message bus itself. For example, sending
          an <literal>org.freedesktop.Peer.Ping</literal> message with no 
          <literal>SERVICE</literal> will cause the message bus itself to reply 
          to the ping immediately; the message bus would never make 
          this message visible to other applications.
      </para>
      <para>
        If the <literal>SERVICE</literal> field is present, then it indicates a
        request for the message bus to route the message. In the usual case,
        messages are routed to the owner of the named service.
        Messages may also be <firstterm>broadcast</firstterm>
        by sending them to the special service 
        <literal>org.freedesktop.DBus.Broadcast</literal>. Broadcast messages are
        sent to all applications with <firstterm>message matching
          rules</firstterm> that match the message.
      </para>
      <para>
        Continuing the <literal>org.freedesktop.Peer.Ping</literal> example, if
        the ping message were sent with a <literal>SERVICE</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. If
        <literal>org.freedesktop.Peer.Ping</literal> were sent to
        <literal>org.freedesktop.DBus.Broadcast</literal>, then multiple applications
        might receive the ping, and all would normally reply to it.
      </para>
    </sect2>

    <sect2 id="message-bus-services">
      <title>Message Bus Services</title>
      <para>
        A service is a name that identifies a certain application. Each
        application connected to the message bus has at least one service name
        assigned at connection time and returned in response to the
        <literal>org.freedesktop.DBus.Hello</literal> message.
        This automatically-assigned service name is called 
        the application's <firstterm>base service</firstterm>. 
        Base service names are unique and MUST never be reused for two different 
        applications.
      </para>
      <para>
        Ownership of the base service is a prerequisite for interaction with 
        the message bus. It logically follows that the base service is always 
        the first service that an application comes to own, and the last 
        service that it loses ownership of.
      </para>
      <para>
        Base service names must begin with the character ':' (ASCII colon
        character); service names that are not base service names must not begin
        with this character. (The bus must reject any attempt by an application
        to manually create a service name beginning with ':'.) This restriction
        categorically prevents "spoofing"; messages sent to a base service name
        will always go to a single application instance and that instance only.
      </para>
      <para>
        An application can request additional service names to be associated
        with it using the
        <literal>org.freedesktop.DBus.AcquireService</literal>
        message. [FIXME what service names are allowed; ASCII or unicode; 
        length limit; etc.]
      </para>
      <para>
        [FIXME this needs more detail, and should move the service-related message 
        descriptions up into this section perhaps]
        Service ownership handling can be specified in the flags part
        of the <literal>org.freedesktop.DBus.AcquireService</literal>
        message. If an application specifies the
        DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT flag, then all applications
        trying to acquire the service will be put in a queue. When the
        primary owner disconnects from the bus or removes ownership
        from the service, the next application in the queue will be the
        primary owner. If the DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT
        flag is not specified, then the primary owner will lose
        ownership whenever another application requests ownership of the
        service.
      </para>
      <para>
        When a client disconnects from the bus, all the services that
        the clients own are deleted, or in the case of a service that
        prohibits replacement, ownership is transferred to the next
        client in the queue, if any.
      </para>
    </sect2>
    <sect2 id="message-bus-routing">
      <title>Message Bus Message Routing</title>
      <para>
        When a message is received by the message bus, the message's 
        <literal>sndr</literal> header field MUST be set to the base service of
        the application which sent the message.  If the service already has
        a <literal>sndr</literal> field, the pre-existing field is replaced.
        This rule means that a replies are always sent to the base service name,
        i.e. to the same application that sent the message being replied to.
      </para>
      <para>
        [FIXME go into detail about broadcast, multicast, unicast, etc.]
      </para>
    </sect2>
    <sect2 id="message-bus-activation">
      <title>Message Bus Service Activation</title>
      <para>
        <firstterm>Activation</firstterm> means to locate a service 
        owner for a service that is currently unowned. For now, it 
        means to launch an executable that will take ownership of 
        a particular service.
      </para>
      <para>
        To find an executable corresponding to a particular service, the bus
        daemon looks for <firstterm>service description files</firstterm>.
        Service description files define a mapping from service names to
        executables. Different kinds of message bus will look for these files
        in different places, see <xref linkend="message-bus-types">.
      </para>
      <para>
        [FIXME the file format should be much better specified than
        "similar to .desktop entries" esp. since desktop entries are
        already badly-specified. ;-)] 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://www.freedesktop.org/standards/desktop-entry-spec/desktop-entry-spec.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.

        <figure>
	  <title>Example service description file</title>
	  <programlisting>
            # Sample service description file
            [D-BUS Service]
            Name=org.gnome.ConfigurationDatabase
            Exec=/usr/libexec/gconfd-2
          </programlisting>
	</figure>
      </para>
      <para>
        When an application requests a service to be activated, the
        bus daemon tries to find it in the list of activation
        entries. It then tries to spawn the executable associated with
        it. If this fails, it will report an error. [FIXME what
        happens if two .service files offer the same service; what
        kind of error is reported, should we have a way for the client
        to choose one?]
      </para>
      <para>
        The executable launched will have the environment variable
        <literal>DBUS_ACTIVATION_ADDRESS</literal> set to the address of the
        message bus so it can connect and register the appropriate services.
      </para>
      <para>
        The executable being launched may want to know whether the message bus
        activating 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_ACTIVATION_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 activated executable must still connect to the address given
        in <literal>DBUS_ACTIVATION_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>
    </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>
          [FIXME specify location of .service files, probably using 
          DESKTOP_DIRS etc. from basedir specification, though login session 
          bus is not really desktop-specific]
        </para>
      </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 login session 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>.
          <footnote>
            <para>
              The D-BUS reference implementation actually honors the 
              <literal>$(localstatedir)</literal> configure option 
              for this address, on both client and server side.
            </para>
          </footnote>
        </para>
        <para>
          [FIXME specify location of system bus .service files]
        </para>
      </sect3>
    </sect2>

    <sect2 id="message-bus-messages">
      <title>Message Bus Messages</title>
      <para>
        The special message bus service <literal>org.freedesktop.DBus</literal>
        responds to a number of messages, allowing applications to 
        interact with the message bus.
      </para>

      <sect3 id="bus-messages-hello">
        <title><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>Name of the service assigned to the application</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 service. If an application tries to send a
          message to another application, or a message to the message
          bus service that isn't the
          <literal>org.freedesktop.DBus.Hello</literal> message, it
          will be disconnected from the bus. If a client wishes to
          disconnect from the bus, it just has to disconnect from the
          transport used. No de-registration message is necessary.
        </para>
        <para>
          The reply message contains the name of the application's base service.
        </para>
      </sect3>
      <sect3 id="bus-messages-list-services">
        <title><literal>org.freedesktop.DBus.ListServices</literal></title>
        <para>
          As a method:
          <programlisting>
            STRING_ARRAY ListServices ()
          </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_ARRAY</entry>
                  <entry>Array of strings where each string is the name of a service</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Returns a list of all existing services registered with the message bus. 
        </para>
      </sect3>
      <sect3 id="bus-messages-service-exists">
        <title><literal>org.freedesktop.DBus.ServiceExists</literal></title>
        <para>
          As a method:
          <programlisting>
            BOOLEAN ServiceExists (in STRING service_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 of the service</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 service exists</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Checks if a service with a specified name exists.
        </para>
      </sect3>

      <sect3 id="bus-messages-acquire-service">
        <title><literal>org.freedesktop.DBus.AcquireService</literal></title>
        <para>
          As a method:
          <programlisting>
            UINT32 AcquireService (in STRING service_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 of the service</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>
          Tries to become owner of a specific service. The flags
          specified can be the following values logically ORed together:

          <informaltable>
            <tgroup cols=3>
              <thead>
                <row>
                  <entry>Identifier</entry>
                  <entry>Value</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
	        <row>
		  <entry>DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT</entry>
		  <entry>0x1</entry>
		  <entry>
                    If the application succeeds in being the owner of the specified service,
                    then ownership of the service can't be transferred until the service
                    disconnects. If this flag is not set, then any application trying to become
                    the owner of the service will succeed and the previous owner will be
                    sent a <literal>org.freedesktop.DBus.ServiceLost</literal> message.
                  </entry>
	        </row>
	        <row>
		  <entry>DBUS_SERVICE_FLAGS_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 service if
                  there is no current owner.</entry>
	        </row>
	      </tbody>
	    </tgroup>
	  </informaltable>

          [FIXME if it's one of the following values, why are the values
          done as flags instead of just 0, 1, 2, 3, 4]
          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_SERVICE_REPLY_PRIMARY_OWNER</entry>
		  <entry>0x1</entry>
		  <entry>The application is now the primary owner of the service.</entry>
	        </row>
	        <row>
		  <entry>DBUS_SERVICE_REPLY_IN_QUEUE</entry>
		  <entry>0x2</entry>
		  <entry>The service already has an owner which do not want to give up ownership and therefore the application has been put in a queue.</entry>
	        </row>
	        <row>
		  <entry>DBUS_SERVICE_REPLY_SERVICE_EXISTS</entry>
		  <entry>0x4</entry>
		  <entry>The service does already have a primary owner, and DBUS_SERVICE_FLAG_REPLACE_EXISTING was not specified when trying to acquire the service.</entry>
	        </row>
	        <row>
		  <entry>DBUS_SERVICE_REPLY_ALREADY_OWNER</entry>
		  <entry>0x8</entry>
		  <entry>The application trying to request ownership of the service is already the owner of it.</entry>
	        </row>
	      </tbody>
	    </tgroup>
	  </informaltable>
        </para>
      </sect3>
      <sect3 id="bus-messages-service-acquired">
        <title><literal>org.freedesktop.DBus.ServiceAcquired</literal></title>
        <para>
          As a method:
          <programlisting>
            ServiceAcquired (in STRING service_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 of the service</entry>
                </row>
	        <row>
		  <entry>1</entry>
		  <entry>UINT32</entry>
		  <entry>Flags</entry>
	        </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This message is sent to a specific application when it becomes the
          primary owner of a service.
        </para>
      </sect3>
      <sect3 id="bus-messages-service-lost">
        <title><literal>org.freedesktop.DBus.ServiceLost</literal></title>
        <para>
          As a method:
          <programlisting>
            ServiceLost (in STRING service_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 of the service</entry>
                </row>
	        <row>
		  <entry>1</entry>
		  <entry>UINT32</entry>
		  <entry>Flags</entry>
	        </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This message is sent to a specific application when it loses primary
          ownership of a service.

          [FIXME instead of ServiceLost/ServiceCreated going only to 
          a specific app, why not just OwnerChanged that covers both 
          lost and created and changed owner and deleted]
        </para>
      </sect3>

      <sect3 id="bus-messages-service-created">
        <title><literal>org.freedesktop.DBus.ServiceCreated</literal></title>
        <para>
          As a method:
          <programlisting>
            ServiceCreated (in STRING service_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 of the service</entry>
                </row>
	        <row>
		  <entry>1</entry>
		  <entry>UINT32</entry>
		  <entry>Flags</entry>
	        </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This message is broadcast to all applications when a service has been
          successfully registered on the message bus.
        </para>
      </sect3>

      <sect3 id="bus-messages-service-deleted">
        <title><literal>org.freedesktop.DBus.ServiceDeleted</literal></title>
        <para>
          As a method:
          <programlisting>
            ServiceDeleted (in STRING service_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 of the service</entry>
                </row>
	        <row>
		  <entry>1</entry>
		  <entry>UINT32</entry>
		  <entry>Flags</entry>
	        </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This message is broadcast to all applications when a service has been
          deleted from the message bus.
        </para>
      </sect3>

      <sect3 id="bus-messages-activate-service">
        <title><literal>org.freedesktop.DBus.ActivateService</literal></title>
        <para>
          As a method:
          <programlisting>
            UINT32 ActivateService (in STRING service_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 activate</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>Result code; DBUS_ACTIVATION_REPLY_ACTIVATED if
                the service was activated successfully or
                DBUS_ACTIVATION_REPLY_ALREADY_ACTIVE if the service is
                already active.</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
        </para>
        <para>
          Tries to launch the executable associated with a service. For more information, see <xref linkend="message-bus-activation">.

            [FIXME need semantics in much more detail here; for example, 
            if I activate a service then send it a message, is the message 
            queued for the new service or is there a race]
        </para>
      </sect3>

      <sect3 id="bus-messages-out-of-memory">
        <title><literal>org.freedesktop.DBus.Error.NoMemory</literal></title>
        <para>
          As a method:
          <programlisting>
            void NoMemory ()
          </programlisting>
        </para>
        <para>
          Sent by the message bus when it can't process a message due to an out of memory failure.
        </para>
      </sect3>

      <sect3 id="bus-messages-service-does-not-exist">
        <title><literal>org.freedesktop.DBus.Error.ServiceDoesNotExist</literal></title>
        <para>
          As a method:
          <programlisting>
            void ServiceDoesNotExist (in STRING error)
          </programlisting>
        </para>
        <para>
          Sent by the message bus as a reply to a client that tried to send a message to a service that doesn't exist.
        </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-activation"><glossterm>Activation</glossterm>
      <glossdef>
        <para>
          The process of creating an owner for a particular service, 
          typically by launching an executable.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-base-service"><glossterm>Base Service</glossterm>
      <glossdef>
        <para>
          The special service automatically assigned to an application by the 
          message bus. This service may never change owner, and the service 
          name will be unique (never reused during the lifetime of the 
          message bus).
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-broadcast"><glossterm>Broadcast</glossterm>
      <glossdef>
        <para>
          A message sent to the special <literal>org.freedesktop.DBus.Broadcast</literal>
          service; the message bus will forward the broadcast message 
          to all applications that have expressed interest in it.
        </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 broadcasts messages between a group of applications
          connected to the message bus. It also manages 
          <firstterm>services</firstterm>.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="namespace"><glossterm>Namespace</glossterm>
      <glossdef>
	<para>
          Used to prevent collisions when defining message and service
	  names. The convention used is the same as Java uses for
	  defining classes: a reversed domain name.
        </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> or 
          <firstterm>object reference</firstterm>.
        </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="peer-to-peer"><glossterm>Peer-to-peer</glossterm>
      <glossdef>
	<para>
          An application talking directly to another application, without going through a message bus. 
        </para>
      </glossdef>
    </glossentry>
    <glossentry id="term-secondary-owner"><glossterm>Secondary service owner</glossterm>
      <glossdef>
        <para>
          Each service has a primary owner; messages sent to the service name 
          go to the primary owner. However, certain services also maintain 
          a queue of secondary owners "waiting in the wings." If 
          the primary owner releases the service, then the first secondary 
          owner in the queue automatically becomes the primary owner.
        </para>
      </glossdef>
    </glossentry>
    <glossentry id="term-service"><glossterm>Service</glossterm>
      <glossdef>
        <para>
          A service is simply a named list of applications. For example, the
          hypothetical <literal>com.yoyodyne.Screensaver</literal> service might
          accept messages that affect a screensaver from Yoyodyne Corporation.
          An application is said to <firstterm>own</firstterm> a service if the
          message bus has associated the application with the service name.
          Services may also have <firstterm>secondary owners</firstterm> (see
          <xref linkend="term-secondary-owner">).
        </para>
      </glossdef>
    </glossentry>
    <glossentry id="term-service-name"><glossterm>Service name</glossterm>
      <glossdef>
	<para>
          The name used when referring to a service. If the service is
          a base service it has a unique service name, for example
          ":1-20", and otherwise it should be namespaced.
        </para>
      </glossdef>
    </glossentry>
    <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
      <glossdef>
        <para>
          ".service files" tell the bus how to activate a particular service.
          See <xref linkend="term-activation">
        </para>
      </glossdef>
    </glossentry>

  </glossary>
</article>