xref: /xsrc/external/mit/xorgproto/dist/specs/xextproto/geproto.xml

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<book>
  <bookinfo>
    <title>X Generic Event Extension</title>
    <author>
      <firstname>Peter</firstname>
      <surname>Hutterer</surname>
      <email>peter.hutterer@who-t.net</email>
    </author>
    <releaseinfo>X Version 11, Release &fullrelvers;</releaseinfo>
    <releaseinfo>Version 1.0</releaseinfo>
    <copyright><year>2007</year><holder>Peter Hutterer</holder></copyright>

    <legalnotice>
    <para>
    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
    to deal in the Software without restriction, including without limitation
    the rights to use, copy, modify, merge, publish, distribute, sublicense,
    and/or sell copies of the Software, and to permit persons to whom the
    Software is furnished to do so, subject to the following conditions:
    </para>

    <para>
    The above copyright notice and this permission notice (including the next
    paragraph) shall be included in all copies or substantial portions of the
    Software.
    </para>

    <para>
    THE SOFTWARE IS PROVIDED &ldquo;AS IS&rdquo;, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
    DEALINGS IN THE SOFTWARE.
    </para>
    </legalnotice>

  </bookinfo>

  <chapter>
    <title>Introduction</title>

    <para>X was designed to provide 64 event opcodes for all extensions. These
    events are limited to 32 bytes.</para>

    <para>The Generic Event Extension provides a template event for extensions
    to re-use a single event opcode. GE only provide headers and the most
    basic functionality, leaving the extensions to interpret the events in
    their specific context.</para>

    <para>GenericEvents may be longer than 32 bytes. If so, the number of 4
    byte units following the initial 32 bytes must be specified in the length
    field of the event.</para>
  </chapter>

  <chapter>
    <title>Extension Initialization</title>

    <para>The name of this extension is "Generic Event Extension"</para>

    <programlisting>GEQueryVersion
    client-major-version: CARD16
    client-minor-version: CARD16
==&gt;
    major-version:        CARD16
    minor-version:        CARD16</programlisting>

    <para>The client sends the highest supported version to the server and the
    server sends the highest version it supports, but no higher than the
    requested version. Major versions changes can introduce incompatibilities
    in existing functionality, minor version changes introduce only backward
    compatible changes. It is the clients responsibility to ensure that the
    server supports a version which is compatible with its
    expectations.</para>

    <para>As of version 1.0, no other requests are provided by this extension.
    </para>
  </chapter>

  <chapter>
    <title>Events</title>

    <para>GE defines a single event, to be used by all extensions. The event's
    structure is similar to a reply. This is a core protocol event, ID 35, and
    is not itself an extension event.</para>

    <programlisting>GenericEvent
    type:           BYTE       always GenericEvent (35)
    extension:      CARD8      extension offset
    sequenceNumber: CARD16     low 16 bits of request seq. number
    length:         CARD32     length
    evtype:         CARD16     event type</programlisting>

    <para>The field 'extension' is to be set to the major opcode of the
    extension. The 'evtype' field is the actual opcode of the event. The
    length field specifies the number of 4-byte blocks after the initial 32
    bytes. If length is 0, the event is 32 bytes long.</para>
  </chapter>

  <chapter>
    <title>Notes</title>

    <para>Although the wire event is of arbitrary length, the actual size of
    an XEvent is restricted to sizeof(XEvent) [96 bytes, see Xlib.h]. If an
    extension converts a wire event to an XEvent &gt; 96 bytes, it will
    overwrite the space allocated for the event. See struct _XSQEvent in
    Xlibint.h for details.</para>

    <para>Extensions need to malloc additional data and fill the XEvent
    structure with pointers to the malloc'd data. The client then needs to
    free the data, only the XEvent structure will be released by Xlib.</para>

    <para>The server must not send GenericEvents longer than 32 bytes until it
    has verified that the client is able to interpret these events. If a long
    event is sent to a client unable to process GenericEvents, future
    interpretation of replies and events by this client will fail.</para>
  </chapter>
</book>