X Generic Event Extension

Peter Hutterer

   <peter.hutterer@who-t.net>

   X Version 11, Release 7.7

   Version 1.0

   Copyright © 2007 Peter Hutterer

   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:

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

   THE SOFTWARE IS PROVIDED "AS IS", 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.
     __________________________________________________________

   Table of Contents

   1. Introduction
   2. Extension Initialization
   3. Events
   4. Notes

Chapter 1. Introduction

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

   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.

   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.

Chapter 2. Extension Initialization

   The name of this extension is "Generic Event Extension"
GEQueryVersion
    client-major-version: CARD16
    client-minor-version: CARD16
==>
    major-version:        CARD16
    minor-version:        CARD16

   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.

   As of version 1.0, no other requests are provided by this
   extension.

Chapter 3. Events

   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.
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

   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.

Chapter 4. Notes

   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 >
   96 bytes, it will overwrite the space allocated for the event.
   See struct _XSQEvent in Xlibint.h for details.

   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.

   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.
