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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<book>
<bookinfo>
<title>X Generic Event Extension</title>
<author>
<firstname>Peter</firstname>
<surname>Hutterer</surname>
<affiliation>
<orgname>peter.hutterer@who-t.net</orgname>
</affiliation>
</author>
</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
==>
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 > 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>
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