<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE appendix PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
	  "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd">
<appendix id="extensions">
<title>Extensions</title>
<para>
<!-- .XE -->
Because X can evolve by extensions to the core protocol, 
it is important that extensions not be perceived as second-class citizens.
At some point, 
your favorite extensions may be adopted as additional parts of the 
X Standard.
</para>
<para>
<!-- .LP -->
Therefore, there should be little to distinguish the use of an extension from 
that of the core protocol.
To avoid having to initialize extensions explicitly in application programs, 
it is also important that extensions perform lazy evaluations,
automatically initializing themselves when called for the first time.  
</para>
<para>
<!-- .LP -->
This appendix describes techniques for writing extensions to Xlib that will
run at essentially the same performance as the core protocol requests.
</para>
<!-- .NT -->
<note><para>
It is expected that a given extension to X consists of multiple
requests.
Defining 10 new features as 10 separate extensions is a bad practice.
Rather, they should be packaged into a single extension
and should use minor opcodes to distinguish the requests.
</para></note>
<!-- .NE -->
<para>
<!-- .LP -->
The symbols and macros used for writing stubs to Xlib are listed in
<filename class="headerfile">&lt;X11/Xlibint.h&gt;</filename>.
</para>
<sect1 id="Basic_Protocol_Support_Routines">
<title>Basic Protocol Support Routines</title>
<para>
The basic protocol requests for extensions are 
<xref linkend='XQueryExtension' xrefstyle='select: title'/>
and
<xref linkend='XListExtensions' xrefstyle='select: title'/>.
</para>
<indexterm significance="preferred"><primary>XQueryExtension</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XQueryExtension'>
<funcprototype>
  <funcdef>Bool <function>XQueryExtension</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>char *<parameter>name</parameter></paramdef>
  <paramdef>int *<parameter>major_opcode_return</parameter></paramdef>
  <paramdef>int *<parameter>first_event_return</parameter></paramdef>
  <paramdef>int *<parameter>first_error_return</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>display</term>
    <listitem>
      <para>Specifies the connection to the X server.</para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>name</term>
    <listitem>
      <para>Specifies the extension name.</para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>major_opcode_return</term>
    <listitem>
      <para>Returns the major opcode.</para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>first_event_return</term>
    <listitem>
      <para>Returns the first event code, if any.</para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>first_error_return</term>
    <listitem>
      <para>Returns the first error code, if any.</para>
    </listitem>
  </varlistentry>
</variablelist>

<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XQueryExtension' xrefstyle='select: title'/>
function determines if the named extension is present.  
If the extension is not present,
<xref linkend='XQueryExtension' xrefstyle='select: title'/>
returns
<symbol>False</symbol>;
otherwise, it returns
<symbol>True</symbol>.
If the extension is present,
<xref linkend='XQueryExtension' xrefstyle='select: title'/>
returns the major opcode for the extension to major_opcode_return;
otherwise,
it returns zero.
Any minor opcode and the request formats are specific to the
extension.  
If the extension involves additional event types, 
<xref linkend='XQueryExtension' xrefstyle='select: title'/>
returns the base event type code to first_event_return;
otherwise, 
it returns zero.  
The format of the events is specific to the extension.  
If the extension involves additional error codes, 
<xref linkend='XQueryExtension' xrefstyle='select: title'/>
returns the base error code to first_error_return;
otherwise, 
it returns zero.  
The format of additional data in the errors is specific to the extension.
</para>
<para>
<!-- .LP -->
If the extension name is not in the Host Portable Character Encoding
the result is implementation-dependent.
Uppercase and lowercase matter;
the strings ``thing'', ``Thing'', and ``thinG'' 
are all considered different names.
</para>
<indexterm significance="preferred"><primary>XListExtensions</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XListExtensions'>
<funcprototype>
  <funcdef>char **<function>XListExtensions</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int *<parameter>nextensions_return</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>nextensions_return</emphasis>
    </term>
    <listitem>
      <para>
Returns the number of extensions listed.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XListExtensions' xrefstyle='select: title'/>
function returns a list of all extensions supported by the server.
If the data returned by the server is in the Latin Portable Character Encoding,
then the returned strings are in the Host Portable Character Encoding.
Otherwise, the result is implementation-dependent.
</para>
<indexterm significance="preferred"><primary>XFreeExtensionList</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XFreeExtensionList'>
<funcprototype>
  <funcdef><function>XFreeExtensionList</function></funcdef>
  <paramdef>char **<parameter>list</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>list</emphasis>
    </term>
    <listitem>
      <para>
Specifies the list of extension names.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XFreeExtensionList' xrefstyle='select: title'/>
function frees the memory allocated by 
<xref linkend='XListExtensions' xrefstyle='select: title'/>.
</para>
</sect1>
<sect1 id="Hooking_into_Xlib">
<title>Hooking into Xlib</title>
<para>
<!-- .LP -->
These functions allow you to hook into the library.  
They are not normally used by application programmers but are used 
by people who need to extend the core X protocol and
the X library interface.
The functions, which generate protocol requests for X, are typically
called stubs.
</para>
<para>
<!-- .LP -->
In extensions, stubs first should check to see if they have initialized 
themselves on a connection.
If they have not, they then should call 
<xref linkend='XInitExtension' xrefstyle='select: title'/>
to attempt to initialize themselves on the connection.
</para>
<para>
<!-- .LP -->
If the extension needs to be informed of GC/font allocation or
deallocation or if the extension defines new event types, 
the functions described here allow the extension to be 
called when these events occur.
</para>
<para>
<!-- .LP -->
The
<structname>XExtCodes</structname>
structure returns the information from 
<xref linkend='XInitExtension' xrefstyle='select: title'/>
and is defined in
<filename class="headerfile">&lt;X11/Xlib.h&gt;</filename>:
</para>
<para>
<!-- .LP -->
<indexterm significance="preferred"><primary>XExtCodes</primary></indexterm>
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<synopsis>
typedef struct _XExtCodes {	/* public to extension, cannot be changed */
	int extension;		/* extension number */
	int major_opcode;	/* major op-code assigned by server */
	int first_event;	/* first event number for the extension */
	int first_error;	/* first error number for the extension */
} XExtCodes;
</synopsis>
</para>
<!-- .LP -->
<!-- .eM -->
<indexterm significance="preferred"><primary>XInitExtension</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XInitExtension'>
<funcprototype>
  <funcdef>XExtCodes *<function>XInitExtension</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>char *<parameter>name</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>name</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension name.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XInitExtension' xrefstyle='select: title'/>
function determines if the named extension exists. 
Then, it allocates storage for maintaining the 
information about the extension on the connection, 
chains this onto the extension list for the connection,
and returns the information the stub implementor will need to access
the extension.
If the extension does not exist,
<xref linkend='XInitExtension' xrefstyle='select: title'/>
returns NULL.
</para>
<para>
<!-- .LP -->
If the extension name is not in the Host Portable Character Encoding,
the result is implementation-dependent.
Uppercase and lowercase matter;
the strings ``thing'', ``Thing'', and ``thinG'' 
are all considered different names.
</para>
<para>
<!-- .LP -->
The extension number in the 
<structname>XExtCodes</structname>
structure is
needed in the other calls that follow.  
This extension number is unique only to a single connection.
</para>
<!-- .LP -->
<indexterm significance="preferred"><primary>XAddExtension</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XAddExtension'>
<funcprototype>
  <funcdef>XExtCodes *<function>XAddExtension</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
For local Xlib extensions, the
<xref linkend='XAddExtension' xrefstyle='select: title'/>
function allocates the
<structname>XExtCodes</structname>
structure, bumps the extension number count,
and chains the extension onto the extension list.
(This permits extensions to Xlib without requiring server extensions.)
</para>
<sect2 id="Hooks_into_the_Library">
<title>Hooks into the Library</title>
<para>
<!-- .LP -->
These functions allow you to define procedures that are to be
called when various circumstances occur.
The procedures include the creation of a new GC for a connection,
the copying of a GC, the freeing of a GC, the creating and freeing of fonts,
the conversion of events defined by extensions to and from wire
format, and the handling of errors.
</para>
<para>
<!-- .LP -->
All of these functions return the previous procedure defined for this
extension.
</para>
<indexterm significance="preferred"><primary>XESetCloseDisplay</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XESetCloseDisplay'>
<funcprototype>
  <funcdef>int <function>XESetCloseDisplay</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>extension</parameter></paramdef>
  <paramdef>int <parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>extension</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension number.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call when the display is closed.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XESetCloseDisplay' xrefstyle='select: title'/>
function defines a procedure to be called whenever
<function>XCloseDisplay</function>
is called.  
It returns any previously defined procedure, usually NULL.
</para>
<para>
<!-- .LP -->
When 
<function>XCloseDisplay</function>
is called, 
your procedure is called 
with these arguments:
<funcsynopsis>
<funcprototype>
  <funcdef>int <function>(*<replaceable>proc</replaceable>)</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>XExtCodes *<parameter>codes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</para>
<indexterm significance="preferred"><primary>XESetCreateGC</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XESetCreateGC'>
<funcprototype>
  <funcdef>int *<function>XESetCreateGC</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>extension</parameter></paramdef>
  <paramdef>int <parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>extension</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension number.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call when a GC is closed.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XESetCreateGC' xrefstyle='select: title'/>
function defines a procedure to be called whenever
a new GC is created.  
It returns any previously defined procedure, usually NULL.
</para>
<para>
<!-- .LP -->
When a GC is created, 
your procedure is called with these arguments:
<!-- .LP -->
<!-- .sM -->
<funcsynopsis>
<funcprototype>
  <funcdef>int <function>(*<replaceable>proc</replaceable>)</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>GC <parameter>gc</parameter></paramdef>
  <paramdef>XExtCodes *<parameter>codes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</para>
<indexterm significance="preferred"><primary>XESetCopyGC</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XESetCopyGC'>
<funcprototype>
  <funcdef>int *<function>XESetCopyGC</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>extension</parameter></paramdef>
  <paramdef>int <parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>extension</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension number.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call when GC components are copied.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XESetCopyGC' xrefstyle='select: title'/>
function defines a procedure to be called whenever
a GC is copied.  
It returns any previously defined procedure, usually NULL.
</para>
<para>
<!-- .LP -->
When a GC is copied, 
your procedure is called with these arguments:
<funcsynopsis>
<funcprototype>
  <funcdef>int <function>(*<replaceable>proc</replaceable>)</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>GC <parameter>gc</parameter></paramdef>
  <paramdef>XExtCodes *<parameter>codes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</para>
<funcsynopsis id='XESetFreeGC'>
<funcprototype>
  <funcdef>int *<function>XESetFreeGC</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>extension</parameter></paramdef>
  <paramdef>int <parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>extension</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension number.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call when a GC is freed.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The
<xref linkend='XESetFreeGC' xrefstyle='select: title'/>
function defines a procedure to be called whenever
a GC is freed.  
It returns any previously defined procedure, usually NULL.
</para>
<para>
<!-- .LP -->
When a GC is freed, 
your procedure is called with these arguments:
<!-- .LP -->
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<!-- .R -->
<funcsynopsis>
<funcprototype>
  <funcdef>int <function>(*<replaceable>proc</replaceable>)</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>GC <parameter>gc</parameter></paramdef>
  <paramdef>XExtCodes *<parameter>codes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</para>
<!-- .LP -->
<!-- .eM -->
<indexterm significance="preferred"><primary>XESetCreateFont</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XESetCreateFont'>
<funcprototype>
  <funcdef>int *<function>XESetCreateFont</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>extension</parameter></paramdef>
  <paramdef>int <parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>extension</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension number.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call when a font is created.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XESetCreateFont' xrefstyle='select: title'/>
function defines a procedure to be called whenever
<xref linkend='XLoadQueryFont' xrefstyle='select: title'/>
and
<xref linkend='XQueryFont' xrefstyle='select: title'/>
are called.  
It returns any previously defined procedure, usually NULL.
</para>
<para>
<!-- .LP -->
When 
<xref linkend='XLoadQueryFont' xrefstyle='select: title'/>
or
<xref linkend='XQueryFont' xrefstyle='select: title'/>
is called, 
your procedure is called with these arguments:
<!-- .LP -->
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<!-- .R -->
<funcsynopsis>
<funcprototype>
  <funcdef>int <function>(*<replaceable>proc</replaceable>)</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>XFontStruct *<parameter>fs</parameter></paramdef>
  <paramdef>XExtCodes *<parameter>codes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</para>
<!-- .LP -->
<!-- .eM -->
<indexterm significance="preferred"><primary>XESetFreeFont</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XESetFreeFont'>
<funcprototype>
  <funcdef>int *<function>XESetFreeFont</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>extension</parameter></paramdef>
  <paramdef>int <parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>extension</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension number.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call when a font is freed.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XESetFreeFont' xrefstyle='select: title'/>
function defines a procedure to be called whenever
<xref linkend='XFreeFont' xrefstyle='select: title'/>
is called.  
It returns any previously defined procedure, usually NULL.
</para>
<para>
<!-- .LP -->
When 
<xref linkend='XFreeFont' xrefstyle='select: title'/>
is called, your procedure is called with these arguments:
<!-- .LP -->
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<!-- .R -->
<funcsynopsis>
<funcprototype>
  <funcdef>int <function>(*<replaceable>proc</replaceable>)</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>XFontStruct *<parameter>fs</parameter></paramdef>
  <paramdef>XExtCodes *<parameter>codes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</para>
<para>
<!-- .LP -->
<!-- .eM -->
The 
<xref linkend='XESetWireToEvent' xrefstyle='select: title'/>
and
<xref linkend='XESetEventToWire' xrefstyle='select: title'/>
functions allow you to define new events to the library.
An 
<structname>XEvent</structname>
structure always has a type code (type
<type>int</type>)
as the first component.
This uniquely identifies what kind of event it is.
The second component is always the serial number (type
<type>unsigned</type>
<type>long</type>)
of the last request processed by the server.
The third component is always a Boolean (type
<type>Bool</type>)
indicating whether the event came from a
<systemitem>SendEvent</systemitem>
protocol request.
The fourth component is always a pointer to the display
the event was read from.
The fifth component is always a resource ID of one kind or another,
usually a window, carefully selected to be useful to toolkit dispatchers.
The fifth component should always exist, even if
the event does not have a natural destination;
if there is no value
from the protocol to put in this component, initialize it to zero.
<!-- .NT -->
There is an implementation limit such that your host event
structure size cannot be bigger than the size of the
<structname>XEvent</structname>
union of structures.
There also is no way to guarantee that more than 24 elements or 96 characters
in the structure will be fully portable between machines.
</para>
<!-- .NE -->
<indexterm significance="preferred"><primary>XESetWireToEvent</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XESetWireToEvent'>
<funcprototype>
  <funcdef>int *<function>XESetWireToEvent</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>event_number</parameter></paramdef>
  <paramdef>Status <parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>event_number</emphasis>
    </term>
    <listitem>
      <para>
Specifies the event code.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call when converting an event.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XESetWireToEvent' xrefstyle='select: title'/>
function defines a procedure to be called when an event
needs to be converted from wire format 
(<structname>xEvent</structname>)
to host format 
(<structname>XEvent</structname>).
The event number defines which protocol event number to install a
conversion procedure for.
<xref linkend='XESetWireToEvent' xrefstyle='select: title'/>
returns any previously defined procedure.
<!-- .NT -->
You can replace a core event conversion function with one
of your own, although this is not encouraged.
It would, however, allow you to intercept a core event 
and modify it before being placed in the queue or otherwise examined.
<!-- .NE -->
When Xlib needs to convert an event from wire format to host
format, your procedure is called with these arguments:
<!-- .LP -->
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<!-- .R -->
<funcsynopsis>
<funcprototype>
  <funcdef>int <function>(*<replaceable>proc</replaceable>)</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>XEvent *<parameter>re</parameter></paramdef>
  <paramdef>xEvent *<parameter>event</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .LP -->
<!-- .eM -->
Your procedure must return status to indicate if the conversion succeeded.
The re argument is a pointer to where the host format event should be stored,
and the event argument is the 32-byte wire event structure.
In the 
<structname>XEvent</structname>
structure you are creating, 
you must fill in the five required members of the event structure.
You should fill in the type member with the type specified for the 
<structname>xEvent</structname>
structure.
You should copy all other members from the 
<structname>xEvent</structname>
structure (wire format) to the
<structname>XEvent</structname>
structure (host format).
Your conversion procedure should return 
<symbol>True</symbol>
if the event should be placed in the queue or
<symbol>False</symbol>
if it should not be placed in the queue.
</para>
<para>
<!-- .LP -->
To initialize the serial number component of the event, call
<xref linkend='_XSetLastRequestRead' xrefstyle='select: title'/>
with the event and use the return value.
</para>
<!-- .LP -->
<indexterm significance="preferred"><primary>_XSetLastRequestRead</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='_XSetLastRequestRead'>
<funcprototype>
  <funcdef>unsigned long<function>_XSetLastRequestRead</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>xGenericReply *<parameter>rep</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>rep</emphasis>
    </term>
    <listitem>
      <para>
Specifies the wire event structure.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='_XSetLastRequestRead' xrefstyle='select: title'/>
function computes and returns a complete serial number from the partial
serial number in the event.
<!-- .sp -->
</para>
<!-- .LP -->
<indexterm significance="preferred"><primary>XESetEventToWire</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XESetEventToWire'>
<funcprototype>
  <funcdef>Status *<function>XESetEventToWire</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>event_number</parameter></paramdef>
  <paramdef>int <parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>event_number</emphasis>
    </term>
    <listitem>
      <para>
Specifies the event code.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call when converting an event.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XESetEventToWire' xrefstyle='select: title'/>
function defines a procedure to be called when an event
needs to be converted from host format
(<structname>XEvent</structname>)
to wire format
(<structname>xEvent</structname>)
form.  
The event number defines which protocol event number to install a
conversion procedure for.
<xref linkend='XESetEventToWire' xrefstyle='select: title'/>
returns any previously defined procedure.
It returns zero if the conversion fails or nonzero otherwise.
<!-- .NT -->
You can replace a core event conversion function with one
of your own, although this is not encouraged.  
It would, however, allow you to intercept a core event 
and modify it before being sent to another client.
<!-- .NE -->
When Xlib needs to convert an event from host format to wire format, 
your procedure is called with these arguments:
<!-- .LP -->
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<!-- .R -->
<funcsynopsis>
<funcprototype>
  <funcdef>int <function>(*<replaceable>proc</replaceable>)</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>XEvent *<parameter>re</parameter></paramdef>
  <paramdef>xEvent *<parameter>event</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .LP -->
<!-- .eM -->
The re argument is a pointer to the host format event,
and the event argument is a pointer to where the 32-byte wire event 
structure should be stored.
You should fill in the type with the type from the 
<structname>XEvent</structname>
structure.
All other members then should be copied from the host format to the 
<structname>xEvent</structname>
structure.
</para>
<indexterm significance="preferred"><primary>XESetWireToError</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XESetWireToError'>
<funcprototype>
  <funcdef>Bool *<function>XESetWireToError</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>error_number</parameter></paramdef>
  <paramdef>Bool <parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>error_number</emphasis>
    </term>
    <listitem>
      <para>
Specifies the error code.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call when an error is received.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XESetWireToError' xrefstyle='select: title'/>
function defines a procedure to be called when an extension
error needs to be converted from wire format to host format.
The error number defines which protocol error code to install
the conversion procedure for.
<xref linkend='XESetWireToError' xrefstyle='select: title'/>
returns any previously defined procedure.
</para>
<para>
<!-- .LP -->
Use this function for extension errors that contain additional error values
beyond those in a core X error, when multiple wire errors must be combined
into a single Xlib error, or when it is necessary to intercept an
X error before it is otherwise examined.
</para>
<para>
<!-- .LP -->
When Xlib needs to convert an error from wire format to host format,
the procedure is called with these arguments:
<!-- .LP -->
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<!-- .R -->
<funcsynopsis>
<funcprototype>
  <funcdef>int <function>(*<replaceable>proc</replaceable>)</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>XErrorEvent *<parameter>he</parameter></paramdef>
  <paramdef>xError *<parameter>we</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .LP -->
<!-- .eM -->
The he argument is a pointer to where the host format error should be stored.
The structure pointed at by he is guaranteed to be as large as an
<structname>XEvent</structname>
structure and so can be cast to a type larger than an
<structname>XErrorEvent</structname>
to store additional values.
If the error is to be completely ignored by Xlib
(for example, several protocol error structures will be combined into
one Xlib error),
then the function should return
<symbol>False</symbol>;
otherwise, it should return
<symbol>True</symbol>.
</para>
<indexterm significance="preferred"><primary>XESetError</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XESetError'>
<funcprototype>
  <funcdef>int *<function>XESetError</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>extension</parameter></paramdef>
  <paramdef>int <parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>extension</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension number.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call when an error is received.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
Inside Xlib, there are times that you may want to suppress the
calling of the external error handling when an error occurs.
This allows status to be returned on a call at the cost of the call
being synchronous (though most such functions are query operations, in any
case, and are typically programmed to be synchronous).
</para>
<para>
<!-- .LP -->
When Xlib detects a protocol error in 
<xref linkend='_XReply' xrefstyle='select: title'/>,
it calls your procedure with these arguments:
<!-- .LP -->
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<!-- .R -->
<funcsynopsis>
<funcprototype>
  <funcdef>int <function>(*<replaceable>proc</replaceable>)</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>xError *<parameter>err</parameter></paramdef>
  <paramdef>XExtCodes *<parameter>codes</parameter></paramdef>
  <paramdef>int *<parameter>ret_code</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .LP -->
<!-- .eM -->
The err argument is a pointer to the 32-byte wire format error.
The codes argument is a pointer to the extension codes structure.
The ret_code argument is the return code you may want 
<xref linkend='_XReply' xrefstyle='select: title'/>
returned to.
</para>
<para>
<!-- .LP -->
If your procedure returns a zero value, 
the error is not suppressed, and 
the client's error handler is called.
(For further information,
see <link linkend="Using_the_Default_Error_Handlers">section 11.8.2</link>.)
If your procedure returns nonzero, 
the error is suppressed, and 
<xref linkend='_XReply' xrefstyle='select: title'/>
returns the value of ret_code.
</para>
<indexterm significance="preferred"><primary>XESetErrorString</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XESetErrorString'>
<funcprototype>
  <funcdef>char *<function>XESetErrorString</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>extension</parameter></paramdef>
  <paramdef>char *<parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>extension</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension number.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call to obtain an error string.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The 
<xref linkend='XGetErrorText' xrefstyle='select: title'/>
function returns a string to the user for an error.
<xref linkend='XESetErrorString' xrefstyle='select: title'/>
allows you to define a procedure to be called that
should return a pointer to the error message.
The following is an example.
</para>
<para>
<!-- .LP -->
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<!-- .R -->
<funcsynopsis>
<funcprototype>
  <funcdef>int <function>(*<replaceable>proc</replaceable>)</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>code</parameter></paramdef>
  <paramdef>XExtCodes *<parameter>codes</parameter></paramdef>
  <paramdef>char *<parameter>buffer</parameter></paramdef>
  <paramdef>int <parameter>nbytes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .LP -->
<!-- .eM -->
Your procedure is called with the error code for every error detected.
You should copy nbytes of a null-terminated string containing the
error message into buffer.
</para>
<indexterm significance="preferred"><primary>XESetPrintErrorValues</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XESetPrintErrorValues'>
<funcprototype>
  <funcdef>void *<function>XESetPrintErrorValues</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>extension</parameter></paramdef>
  <paramdef>void <parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>extension</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension number.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call when an error is printed.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XESetPrintErrorValues' xrefstyle='select: title'/>
function defines a procedure to be called when an extension
error is printed, to print the error values.
Use this function for extension errors that contain additional error values
beyond those in a core X error.
It returns any previously defined procedure.
</para>
<para>
<!-- .LP -->
When Xlib needs to print an error,
the procedure is called with these arguments:
<!-- .LP -->
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<!-- .R -->
<funcsynopsis>
<funcprototype>
  <funcdef>void <function>(*<replaceable>proc</replaceable>)</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>XErrorEvent *<parameter>ev</parameter></paramdef>
  <paramdef>void *<parameter>fp</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .LP -->
<!-- .eM -->
The structure pointed at by ev is guaranteed to be as large as an
<structname>XEvent</structname>
structure and so can be cast to a type larger than an
<structname>XErrorEvent</structname>
to obtain additional values set by using
<xref linkend='XESetWireToError' xrefstyle='select: title'/>.
The underlying type of the fp argument is system dependent;
on a <acronym>POSIX</acronym>-compliant system, fp should be cast to type FILE*.
</para>
<indexterm significance="preferred"><primary>XESetFlushGC</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XESetFlushGC'>
<funcprototype>
  <funcdef>int *<function>XESetFlushGC</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>extension</parameter></paramdef>
  <paramdef>int *<parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>extension</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension number.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call when a GC is flushed.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The procedure set by the
<xref linkend='XESetFlushGC' xrefstyle='select: title'/>
function has the same interface as the procedure set by the
<xref linkend='XESetCopyGC' xrefstyle='select: title'/>
function, but is called when a GC cache needs to be updated in the server.
</para>
<indexterm significance="preferred"><primary>XESetBeforeFlush</primary></indexterm>
<!-- .sM -->
<funcsynopsis>
<funcprototype>
  <funcdef>int *<function>XESetCopyGC</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>int <parameter>extension</parameter></paramdef>
  <paramdef>int *<parameter>(*proc)()</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>extension</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension number.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the procedure to call when a buffer is flushed.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<function>XESetBeforeFlush</function>
function defines a procedure to be called when data is about to be
sent to the server.  When data is about to be sent, your procedure is  
called one or more times with these arguments:
<!-- .LP -->
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<!-- .R -->
<funcsynopsis>
<funcprototype>
  <funcdef>void <function>(*<replaceable>proc</replaceable>)</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>XExtCodes *<parameter>codes</parameter></paramdef>
  <paramdef>char *<parameter>data</parameter></paramdef>
  <paramdef>long <parameter>len</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .LP -->
<!-- .eM -->
The data argument specifies a portion of the outgoing data buffer,
and its length in bytes is specified by the len argument.
Your procedure must not alter the contents of the data and must not
do additional protocol requests to the same display.
</para>
</sect2>
<sect2 id="Hooks_onto_Xlib_Data_Structures">
<title>Hooks onto Xlib Data Structures</title>
<para>
<!-- .LP -->
Various Xlib data structures have provisions for extension procedures
to chain extension supplied data onto a list.
These structures are
<structname>GC</structname>,
<structname>Visual</structname>,
<type>Screen</type>,
<structname>ScreenFormat</structname>,
<type>Display</type>,
and 
<structname>XFontStruct</structname>.
Because the list pointer is always the first member in the structure, 
a single set of procedures can be used to manipulate the data
on these lists.
</para>
<para>
<!-- .LP -->
The following structure is used in the functions in this section
and is defined in 
<filename class="headerfile">&lt;X11/Xlib.h&gt;</filename>
</para>
<para>
<!-- .LP -->
<indexterm significance="preferred"><primary>XExtData</primary></indexterm>
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<synopsis>
typedef struct _XExtData {
	int number;	/* number returned by XInitExtension */
	struct _XExtData *next;	/* next item on list of data for structure */
	int (*free_private)();	/* if defined,  called to free private */
	XPointer private_data;	/* data private to this extension. */
} XExtData;
</synopsis>
</para>
<para>
<!-- .LP -->
<!-- .eM -->
When any of the data structures listed above are freed, 
the list is walked, and the structure's free procedure (if any) is called. 
If free is NULL, 
then the library frees both the data pointed to by the private_data member
and the structure itself. 
</para>
<para>
<!-- .LP -->
<!-- .sM -->
<!-- .TA .5i -->
<!-- .ta .5i -->
<synopsis>
union {	Display *display;
	GC gc;
	Visual *visual;
	Screen *screen;
	ScreenFormat *pixmap_format;
	XFontStruct *font } XEDataObject;
</synopsis>
</para>
<!-- .LP -->
<!-- .eM -->
<indexterm significance="preferred"><primary>XEHeadOfExtensionList</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XEHeadOfExtensionList'>
<funcprototype>
  <funcdef>XExtData **<function>XEHeadOfExtensionList</function></funcdef>
  <paramdef>XEDataObject <parameter>object</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>object</emphasis>
    </term>
    <listitem>
      <para>
Specifies the object.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XEHeadOfExtensionList' xrefstyle='select: title'/>
function returns a pointer to the list of extension structures attached
to the specified object.
In concert with 
<xref linkend='XAddToExtensionList' xrefstyle='select: title'/>,
<xref linkend='XEHeadOfExtensionList' xrefstyle='select: title'/>
allows an extension to attach arbitrary data to any of the structures
of types contained in
<structname>XEDataObject</structname>.
</para>
<!-- .LP -->
<indexterm significance="preferred"><primary>XAddToExtensionList</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XAddToExtensionList'>
<funcprototype>
  <funcdef><function>XAddToExtensionList</function></funcdef>
  <paramdef>XExtData **<parameter>structure</parameter></paramdef>
  <paramdef>XExtData *<parameter>ext_data</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>structure</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension list.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>ext_data</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension data structure to add.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The structure argument is a pointer to one of the data structures
enumerated above.
You must initialize ext_data-&gt;number with the extension number
before calling this function.
</para>
<indexterm significance="preferred"><primary>XFindOnExtensionList</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XFindOnExtensionList'>
<funcprototype>
  <funcdef>XExtData *<function>XFindOnExtensionList</function></funcdef>
  <paramdef>struct_XExtData **<parameter>structure</parameter></paramdef>
  <paramdef>int <parameter>number</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>structure</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension list.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>number</emphasis>
    </term>
    <listitem>
      <para>
Specifies the extension number from
<xref linkend='XInitExtension' xrefstyle='select: title'/>.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='XFindOnExtensionList' xrefstyle='select: title'/>
function returns the first extension data structure
for the extension numbered number.
It is expected that an extension will add at most one extension
data structure to any single data structure's extension data list.
There is no way to find additional structures.
</para>
<para>
<!-- .LP -->
The 
<xref linkend='XAllocID' xrefstyle='select: title'/>
macro, which allocates and returns a resource ID, is defined in 
<filename class="headerfile">&lt;X11/Xlib.h&gt;</filename>.
</para>
<indexterm significance="preferred"><primary>XAllocID</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XAllocID'>
<funcprototype>
  <funcdef><function>XAllocID</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
This macro is a call through the 
<type>Display</type>
structure to an internal resource ID allocator.
It returns a resource ID that you can use when creating new resources.
</para>
<para>
<!-- .LP -->
The 
<xref linkend='XAllocIDs' xrefstyle='select: title'/>
macro allocates and returns an array of resource ID.
</para>
<indexterm significance="preferred"><primary>XAllocIDs</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='XAllocIDs'>
<funcprototype>
  <funcdef><function>XAllocIDs</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>XID *<parameter>ids_return</parameter></paramdef>
  <paramdef>int <parameter>count</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>ids_return</emphasis>
    </term>
    <listitem>
      <para>
Returns the resource IDs.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>rep</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of resource IDs requested.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
This macro is a call through the 
<type>Display</type>
structure to an internal resource ID allocator.
It returns resource IDs to the array supplied by the caller.
To correctly handle automatic reuse of resource IDs, you must call
<xref linkend='XAllocIDs' xrefstyle='select: title'/>
when requesting multiple resource IDs.  This call might generate
protocol requests.
</para>
</sect2>
</sect1>
<sect1 id="GC_Caching">
<title>GC Caching</title>
<para>
<!-- .LP -->
GCs are cached by the library to allow merging of independent change
requests to the same GC into single protocol requests.
This is typically called a write-back cache.
Any extension procedure whose behavior depends on the contents of a GC
must flush the GC cache to make sure the server has up-to-date contents
in its GC.
</para>
<para>
<!-- .LP -->
The 
<xref linkend='FlushGC' xrefstyle='select: title'/>
macro checks the dirty bits in the library's GC structure and calls
<xref linkend='_XFlushGCCache' xrefstyle='select: title'/>
if any elements have changed.
The
<xref linkend='FlushGC' xrefstyle='select: title'/>
macro is defined as follows:
</para>
<indexterm significance="preferred"><primary>FlushGC</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='FlushGC'>
<funcprototype>
  <funcdef><function>FlushGC</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>GC <parameter>gc</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>gc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the GC.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
Note that if you extend the GC to add additional resource ID components,
you should ensure that the library stub sends the change request immediately.
This is because a client can free a resource immediately after
using it, so if you only stored the value in the cache without
forcing a protocol request, the resource might be destroyed before being
set into the GC.
You can use the
<xref linkend='_XFlushGCCache' xrefstyle='select: title'/>
procedure 
to force the cache to be flushed.
The
<xref linkend='_XFlushGCCache' xrefstyle='select: title'/>
procedure
is defined as follows:
</para>
<indexterm significance="preferred"><primary>_XFlushGCCache</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='_XFlushGCCache'>
<funcprototype>
  <funcdef><function>_XFlushGCCache</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>GC <parameter>gc</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>gc</emphasis>
    </term>
    <listitem>
      <para>
Specifies the GC.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<!-- .LP -->
<!-- .eM -->
</sect1>
<sect1 id="Graphics_Batching">
<title>Graphics Batching</title>
<para>
<!-- .LP -->
If you extend X to add more poly graphics primitives, you may be able to
take advantage of facilities in the library to allow back-to-back 
single calls to be transformed into poly requests.
This may dramatically improve performance of programs that are not
written using poly requests. 
A pointer to an 
<structname>xReq</structname>,
called last_req in the display structure, is the last request being processed.  
By checking that the last request
type, drawable, gc, and other options are the same as the new one
and that there is enough space left in the buffer, you may be able
to just extend the previous graphics request by extending the length
field of the request and appending the data to the buffer.  
This can improve performance by five times or more in naive programs.  
For example, here is the source for the 
<xref linkend='XDrawPoint' xrefstyle='select: title'/>
stub.
(Writing extension stubs is discussed in the next section.)
</para>
<!-- .sM -->
<!-- .nf -->
<programlisting>
#include &lt;X11/Xlibint.h&gt;

/* precompute the maximum size of batching request allowed */

static int size = sizeof(xPolyPointReq) + EPERBATCH * sizeof(xPoint);

XDrawPoint(dpy, d, gc, x, y)
    register Display *dpy;
    Drawable d;
    GC gc;
    int x, y; /* INT16 */
{
    xPoint *point;
    LockDisplay(dpy);
    FlushGC(dpy, gc);
    {
    register xPolyPointReq *req = (xPolyPointReq *) dpy-&gt;last_req;
    /* if same as previous request, with same drawable, batch requests */
    if (
          (req-&gt;reqType == X_PolyPoint)
       &amp;&amp; (req-&gt;drawable == d)
       &amp;&amp; (req-&gt;gc == gc-&gt;gid)
       &amp;&amp; (req-&gt;coordMode == CoordModeOrigin)
       &amp;&amp; ((dpy-&gt;bufptr + sizeof (xPoint)) &lt;= dpy-&gt;bufmax)
       &amp;&amp; (((char *)dpy-&gt;bufptr - (char *)req) &lt; size) ) {
         point = (xPoint *) dpy-&gt;bufptr;
         req-&gt;length += sizeof (xPoint) &gt;&gt; 2;
         dpy-&gt;bufptr += sizeof (xPoint);
         }

    else {
        GetReqExtra(PolyPoint, 4, req); /* 1 point = 4 bytes */
        req-&gt;drawable = d;
        req-&gt;gc = gc-&gt;gid;
        req-&gt;coordMode = CoordModeOrigin;
        point = (xPoint *) (req + 1);
        }
    point-&gt;x = x;
    point-&gt;y = y;
    }
    UnlockDisplay(dpy);
    SyncHandle();
}
</programlisting>
<!-- .fi -->
<para>
<!-- .LP -->
<!-- .eM -->
To keep clients from generating very long requests that may monopolize the 
server,
there is a symbol defined in
<filename class="headerfile">&lt;X11/Xlibint.h&gt;</filename>
of EPERBATCH on the number of requests batched.
Most of the performance benefit occurs in the first few merged requests.
Note that 
<xref linkend='FlushGC' xrefstyle='select: title'/>
is called <emphasis remap='I'>before</emphasis> picking up the value of last_req,
because it may modify this field.
</para>
</sect1>
<sect1 id="Writing_Extension_Stubs">
<title>Writing Extension Stubs</title>
<para>
<!-- .LP -->
All X requests always contain the length of the request,
expressed as a 16-bit quantity of 32 bits.
This means that a single request can be no more than 256K bytes in
length.
Some servers may not support single requests of such a length.
The value of dpy-&gt;max_request_size contains the maximum length as
defined by the server implementation.
For further information,
see <olink targetdoc='x11protocol' targetptr='Maximum-request-length'
><citetitle>X Window System Protocol</citetitle></olink>.
</para>
<sect2 id="Requests_Replies_and_Xproto.h">
<title>Requests, Replies, and Xproto.h</title>
<para>
<!-- .LP -->
The 
<filename class="headerfile">&lt;X11/Xproto.h&gt;</filename>
file contains three sets of definitions that
are of interest to the stub implementor:  
request names, request structures, and reply structures.
</para>
<para>
<!-- .LP -->
You need to generate a file equivalent to 
<filename class="headerfile">&lt;X11/Xproto.h&gt;</filename>
for your extension and need to include it in your stub procedure.
Each stub procedure also must include 
<filename class="headerfile">&lt;X11/Xlibint.h&gt;</filename>.
</para>
<para>
<!-- .LP -->
The identifiers are deliberately chosen in such a way that, if the
request is called X_DoSomething, then its request structure is
xDoSomethingReq, and its reply is xDoSomethingReply.  
The GetReq family of macros, defined in 
<filename class="headerfile">&lt;X11/Xlibint.h&gt;</filename>,
takes advantage of this naming scheme.
</para>
<para>
<!-- .LP -->
For each X request, 
there is a definition in 
<filename class="headerfile">&lt;X11/Xproto.h&gt;</filename>
that looks similar to this:
</para>
<para>
<!-- .LP -->
<!-- .R -->
<programlisting>
#define X_DoSomething   42
</programlisting>
In your extension header file, 
this will be a minor opcode, 
instead of a major opcode.
</para>
</sect2>
<sect2 id="Request_Format">
<title>Request Format</title>
<para>
<!-- .LP -->
Every request contains an 8-bit major opcode and a 16-bit length field
expressed in units of 4 bytes.  
Every request consists of 4 bytes of header
(containing the major opcode, the length field, and a data byte) followed by
zero or more additional bytes of data. 
The length field defines the total length of the request, including the header.
The length field in a request must equal the minimum length required to contain 
the request. 
If the specified length is smaller or larger than the required length, 
the server should generate a 
<errorname>BadLength</errorname>
error.
Unused bytes in a request are not required to be zero.  
Extensions should be designed in such a way that long protocol requests
can be split up into smaller requests,
if it is possible to exceed the maximum request size of the server.
The protocol guarantees the maximum request size to be no smaller than
4096 units (16384 bytes).
</para>
<para>
<!-- .LP -->
Major opcodes 128 through 255 are reserved for extensions.
Extensions are intended to contain multiple requests, 
so extension requests typically have an additional minor opcode encoded 
in the second data byte in the request header, 
but the placement and interpretation of this minor opcode as well as all
other fields in extension requests are not defined by the core protocol.
Every request is implicitly assigned a sequence number (starting with one)
used in replies, errors, and events.
</para>
<para>
<!-- .LP -->
To help but not cure portability problems to certain machines, the
<symbol>B16</symbol>
and
<symbol>B32</symbol>
macros have been defined so that they can become bitfield specifications 
on some machines.
For example, on a Cray,
these should be used for all 16-bit and 32-bit quantities, as discussed below.
</para>
<para>
<!-- .LP -->
Most protocol requests have a corresponding structure typedef in
<filename class="headerfile">&lt;X11/Xproto.h&gt;</filename>,
which looks like:
</para>
<para>
<!-- .LP -->
<indexterm significance="preferred"><primary>xDoSomethingReq</primary></indexterm>
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<synopsis>
typedef struct _DoSomethingReq {
	CARD8 reqType;		/* X_DoSomething */
	CARD8 someDatum;	/* used differently in different requests */
	CARD16 length B16;	/* total # of bytes in request, divided by 4 */
	...
	/* request-specific data */
	...
} xDoSomethingReq;
</synopsis>
</para>
<para>
<!-- .LP -->
<!-- .eM -->
If a core protocol request has a single 32-bit argument, 
you need not declare a request structure in your extension header file.
Instead, such requests use the
<structname>xResourceReq</structname>
structure in
<filename class="headerfile">&lt;X11/Xproto.h&gt;</filename>.
This structure is used for any request whose single argument is a 
<type>Window</type>,
<type>Pixmap</type>,
<type>Drawable</type>,
<type>GContext</type>,
<type>Font</type>,
<type>Cursor</type>,
<type>Colormap</type>,
<type>Atom</type>,
or
<type>VisualID</type>.
</para>
<para>
<!-- .LP -->
<indexterm significance="preferred"><primary>xResourceReq</primary></indexterm>
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<synopsis>
typedef struct _ResourceReq {
	CARD8 reqType;	/* the request type, e.g. X_DoSomething */
	BYTE pad;	/* not used */
	CARD16 length B16;	/* 2 (= total # of bytes in request, divided by 4) */
	CARD32 id B32;	/* the Window, Drawable, Font, GContext, etc. */
} xResourceReq;
</synopsis>
</para>
<para>
<!-- .LP -->
<!-- .eM -->
If convenient,
you can do something similar in your extension header file. 
</para>
<para>
<!-- .LP -->
In both of these structures, 
the reqType field identifies the type of the request (for example, 
X_MapWindow or X_CreatePixmap).  
The length field tells how long the request is
in units of 4-byte longwords. 
This length includes both the request structure itself and any
variable-length data, such as strings or lists, that follow the
request structure.  
Request structures come in different sizes, 
but all requests are padded to be multiples of four bytes long.
</para>
<para>
<!-- .LP -->
A few protocol requests take no arguments at all. 
Instead, they use the
<structname>xReq</structname>
structure in
<filename class="headerfile">&lt;X11/Xproto.h&gt;</filename>,
which contains only a reqType and a length (and a pad byte).
</para>
<para>
<!-- .LP  -->
If the protocol request requires a reply, 
then
<filename class="headerfile">&lt;X11/Xproto.h&gt;</filename>
also contains a reply structure typedef:
</para>
<para>
<!-- .LP -->
<indexterm significance="preferred"><primary>xDoSomethingReply</primary></indexterm>
<!-- .sM -->
<!-- .TA .5i 3i -->
<!-- .ta .5i 3i -->
<synopsis>
typedef struct _DoSomethingReply {
	BYTE type;	/* always X_Reply */
	BYTE someDatum;	/* used differently in different requests */
	CARD16 sequenceNumber B16;	/* # of requests sent so far */
	CARD32 length B32;	/* # of additional bytes, divided by 4 */
	...
	/* request-specific data */
	...
} xDoSomethingReply;
</synopsis>
</para>
<para>
<!-- .LP -->
<!-- .eM -->
Most of these reply structures are 32 bytes long. 
If there are not that many reply values, 
then they contain a sufficient number of pad fields
to bring them up to 32 bytes.  
The length field is the total number of bytes in the request minus 32, 
divided by 4.  
This length will be nonzero only if:
</para>
<itemizedlist>
  <listitem>
    <para>
The reply structure is followed by variable-length data,
such as a list or string.
    </para>
  </listitem>
  <listitem>
    <para>
The reply structure is longer than 32 bytes.
    </para>
  </listitem>
</itemizedlist>
<para>
<!-- .LP -->
Only 
<systemitem>GetWindowAttributesl</systemitem>,
<systemitem>QueryFont</systemitem>,
<systemitem>QueryKeymap</systemitem>,
and
<systemitem>GetKeyboardControl</systemitem>
have reply structures longer than 32 bytes in the core protocol.
</para>
<para>
<!-- .LP -->
A few protocol requests return replies that contain no data.  
<filename class="headerfile">&lt;X11/Xproto.h&gt;</filename>
does not define reply structures for these.
Instead, they use the 
<structname>xGenericReply</structname>
structure, which contains only a type, length,
and sequence number (and sufficient padding to make it 32 bytes long).
</para>
</sect2>
<sect2 id="Starting_to_Write_a_Stub_Procedure">
<title>Starting to Write a Stub Procedure</title>
<para>
<!-- .LP -->
An Xlib stub procedure should start like this:
</para>
<para>
<!-- .LP -->
<!-- .R -->
<programlisting>
#include "&lt;X11/Xlibint.h&gt;

XDoSomething (arguments, ... )
/* argument declarations */
{

register XDoSomethingReq *req;
...
</programlisting>
If the protocol request has a reply, 
then the variable declarations should include the reply structure for the request.
The following is an example:
</para>
<para>
<!-- .LP -->
<!-- .R -->
<programlisting>
xDoSomethingReply rep;
</programlisting>
</para>
</sect2>
<sect2 id="Locking_Data_Structures">
<title>Locking Data Structures</title>
<para>
<!-- .LP -->
To lock the display structure for systems that
want to support multithreaded access to a single display connection,
each stub will need to lock its critical section.
Generally, this section is the point from just before the appropriate GetReq
call until all arguments to the call have been stored into the buffer.
The precise instructions needed for this locking depend upon the machine
architecture. 
Two calls, which are generally implemented as macros, have been provided.
<indexterm significance="preferred"><primary>LockDisplay</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='LockDisplay'>
<funcprototype>
  <funcdef><function>LockDisplay</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
</para>
<!-- .LP -->
<indexterm significance="preferred"><primary>UnlockDisplay</primary></indexterm>
<funcsynopsis id='UnlockDisplay'>
<funcprototype>
  <funcdef><function>UnlockDisplay</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<!-- .LP -->
<!-- .eM -->
</sect2>
<sect2 id="Sending_the_Protocol_Request_and_Arguments">
<title>Sending the Protocol Request and Arguments</title>
<para>
<!-- .LP -->
After the variable declarations, 
a stub procedure should call one of four macros defined in 
<filename class="headerfile">&lt;X11/Xlibint.h&gt;</filename>:
<function>GetReq</function>,
<function>GetReqExtra</function>,
<function>GetResReq</function>,
or 
<function>GetEmptyReq</function>.
All of these macros take, as their first argument,
the name of the protocol request as declared in 
<filename class="headerfile">&lt;X11/Xproto.h&gt;</filename>
except with X_ removed.   
Each one declares a 
<type>Display</type>
structure pointer,
called dpy, and a pointer to a request structure, called req,
which is of the appropriate type.
The macro then appends the request structure to the output buffer, 
fills in its type and length field, and sets req to point to it.
</para>
<para>
<!-- .LP -->
If the protocol request has no arguments (for instance, X_GrabServer),
then use 
<function>GetEmptyReq</function>.
</para>
<para>
<!-- .LP -->
<!-- .R  -->
<programlisting>
GetEmptyReq (DoSomething, req);
</programlisting>
If the protocol request has a single 32-bit argument (such as a
<type>Pixmap</type>,
<type>Window</type>,
<type>Drawable</type>,
<type>Atom</type>,
and so on),
then use 
<function>GetResReq</function>.
The second argument to the macro is the 32-bit object.  
<symbol>X_MapWindow</symbol>
is a good example.
</para>
<para>
<!-- .LP -->
<!-- .R -->
<programlisting>
GetResReq (DoSomething, rid, req);
</programlisting>
The rid argument is the 
<type>Pixmap</type>,
<type>Window</type>,
or other resource ID.
</para>
<para>
<!-- .LP -->
If the protocol request takes any other argument list, 
then call 
<function>GetReq</function>.
After the 
<function>GetReq</function>,
you need to set all the other fields in the request structure,
usually from arguments to the stub procedure.
</para>
<para>
<!-- .LP -->
<!-- .R -->
<programlisting>
GetReq (DoSomething, req);
/* fill in arguments here */
req-&gt;arg1 = arg1;
req-&gt;arg2 = arg2;
...
</programlisting>
A few stub procedures (such as 
<xref linkend='XCreateGC' xrefstyle='select: title'/>
and 
<xref linkend='XCreatePixmap' xrefstyle='select: title'/>)
return a resource ID to the caller but pass a resource ID as an argument
to the protocol request.   
Such procedures use the macro 
<xref linkend='XAllocID' xrefstyle='select: title'/>
to allocate a resource ID from the range of IDs 
that were assigned to this client when it opened the connection.
</para>
<para>
<!-- .LP -->
<!-- .R -->
<programlisting>
rid = req-&gt;rid = XAllocID();
...
return (rid);
</programlisting>
Finally, some stub procedures transmit a fixed amount of variable-length
data after the request.  
Typically, these procedures (such as
<xref linkend='XMoveWindow' xrefstyle='select: title'/>
and 
<xref linkend='XSetBackground' xrefstyle='select: title'/>)
are special cases of more general functions like 
<xref linkend='XMoveResizeWindow' xrefstyle='select: title'/>
and 
<xref linkend='XChangeGC' xrefstyle='select: title'/>.
These procedures use 
<function>GetReqExtra</function>,
which is the same as 
<function>GetReq</function>
except that it takes an additional argument (the number of
extra bytes to allocate in the output buffer after the request structure).  
This number should always be a multiple of four. Note that it is possible
for req to be set to NULL as a defensive measure if the requested length
exceeds the Xlib's buffer size (normally 16K).
</para>
</sect2>
<sect2 id="Variable_Length_Arguments">
<title>Variable Length Arguments</title>
<para>
<!-- .LP -->
Some protocol requests take additional variable-length data that
follow the 
<type>xDoSomethingReq</type>
structure.    
The format of this data varies from request to request. 
Some requests require a sequence of 8-bit bytes, 
others a sequence of 16-bit or 32-bit entities, 
and still others a sequence of structures.
</para>
<para>
<!-- .LP -->
It is necessary to add the length of any variable-length data to the
length field of the request structure.  
That length field is in units of 32-bit longwords.  
If the data is a string or other sequence of 8-bit bytes, 
then you must round the length up and shift it before adding:
</para>
<para>
<!-- .LP -->
<!-- .R -->
<programlisting>
req-&gt;length += (nbytes+3)&gt;&gt;2;
</programlisting>
To transmit variable-length data, use the 
<xref linkend='Data' xrefstyle='select: title'/>
macros.
If the data fits into the output buffer, 
then this macro copies it to the buffer.  
If it does not fit, however,
the 
<xref linkend='Data' xrefstyle='select: title'/>
macro calls 
<function>_XSend</function>,
which transmits first the contents of the buffer and then your data.
The 
<xref linkend='Data' xrefstyle='select: title'/>
macros take three arguments:  
the display, a pointer to the beginning of the data, 
and the number of bytes to be sent.
<!-- .sM -->
<funcsynopsis id='Data'>
<funcprototype role='macro'>
  <funcdef><function>Data</function></funcdef>
  <paramdef><parameter>display</parameter></paramdef>
  <paramdef>(char *) <parameter>data</parameter></paramdef>
  <paramdef><parameter>nbytes</parameter></paramdef>
</funcprototype>
<funcprototype role='macro'>
  <funcdef><function>Data16</function></funcdef>
  <paramdef><parameter>display</parameter></paramdef>
  <paramdef>(short *) <parameter>data</parameter></paramdef>
  <paramdef><parameter>nbytes</parameter></paramdef>
</funcprototype>
<funcprototype role='macro'>
  <funcdef><function>Data32</function></funcdef>
  <paramdef><parameter>display</parameter></paramdef>
  <paramdef>(long *) <parameter>data</parameter></paramdef>
  <paramdef><parameter>nbytes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
</para>
<para>
<!-- .LP -->
<!-- .eM -->
<function>Data</function>,
<function>Data16</function>,
and
<function>Data32</function>
are macros that may use their last argument
more than once, so that argument should be a variable rather than
an expression such as ``nitems*sizeof(item)''.  
You should do that kind of computation in a separate statement before calling 
them.
Use the appropriate macro when sending byte, short, or long data.
</para>
<para>
<!-- .LP -->
If the protocol request requires a reply, 
then call the procedure 
<function>_XSend</function>
instead of the 
<xref linkend='Data' xrefstyle='select: title'/>
macro.  
<function>_XSend</function>
takes the same arguments, but because it sends your data immediately instead of 
copying it into the output buffer (which would later be flushed
anyway by the following call on 
<xref linkend='_XReply' xrefstyle='select: title'/>),
it is faster.
</para>
</sect2>
<sect2 id="Replies">
<title>Replies</title>
<para>
<!-- .LP -->
If the protocol request has a reply, 
then call 
<xref linkend='_XReply' xrefstyle='select: title'/>
after you have finished dealing with 
all the fixed-length and variable-length arguments.  
<xref linkend='_XReply' xrefstyle='select: title'/>
flushes the output buffer and waits for an 
<structname>xReply</structname>
packet to arrive.  
If any events arrive in the meantime,
<xref linkend='_XReply' xrefstyle='select: title'/>
places them in the queue for later use.
</para>
<indexterm significance="preferred"><primary>_XReply</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='_XReply'>
<funcprototype>
  <funcdef>Status <function>_XReply</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>xReply *<parameter>rep</parameter></paramdef>
  <paramdef>int <parameter>extra</parameter></paramdef>
  <paramdef>Bool <parameter>discard</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>rep</emphasis>
    </term>
    <listitem>
      <para>
Specifies the reply structure.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>extra</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of 32-bit words expected after the replay.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>discard</emphasis>
    </term>
    <listitem>
      <para>
Specifies if any data beyond that specified in the extra argument
should be discarded.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='_XReply' xrefstyle='select: title'/>
function waits for a reply packet and copies its contents into the
specified rep.  
<xref linkend='_XReply' xrefstyle='select: title'/>
handles error and event packets that occur before the reply is received.
<xref linkend='_XReply' xrefstyle='select: title'/>
takes four arguments:
</para>
<itemizedlist>
  <listitem>
    <para>
A 
<type>Display</type>
* structure
    </para>
  </listitem>
  <listitem>
    <para>
A pointer to a reply structure (which must be cast to an 
<structname>xReply</structname>
*)
    </para>
  </listitem>
  <listitem>
    <para>
The number of additional 32-bit words (beyond 
<function>sizeof( xReply</function>)
= 32 bytes)
in the reply structure
    </para>
  </listitem>
  <listitem>
    <para>
A Boolean that indicates whether
<xref linkend='_XReply' xrefstyle='select: title'/>
is to discard any additional bytes
beyond those it was told to read
    </para>
  </listitem>
</itemizedlist>
<para>
<!-- .LP -->
Because most reply structures are 32 bytes long, 
the third argument is usually 0.  
The only core protocol exceptions are the replies to 
<systemitem>GetWindowAttributesl</systemitem>,
<systemitem>QueryFont</systemitem>,
<systemitem>QueryKeymap</systemitem>,
and 
<systemitem>GetKeyboardControl</systemitem>,
which have longer replies.
</para>
<para>
<!-- .LP -->
The last argument should be 
<symbol>False</symbol>
if the reply structure is followed
by additional variable-length data (such as a list or string).  
It should be 
<symbol>True</symbol>
if there is not any variable-length data.
<!-- .NT -->
This last argument is provided for upward-compatibility reasons
to allow a client to communicate properly with a hypothetical later
version of the server that sends more data than the client expected.
For example, some later version of 
<systemitem>GetWindowAttributesl</systemitem>
might use a
larger, but compatible, 
<structname>xGetWindowAttributesReply</structname>
that contains additional attribute data at the end.
<!-- .NE -->
<xref linkend='_XReply' xrefstyle='select: title'/>
returns 
<symbol>True</symbol>
if it received a reply successfully or 
<symbol>False</symbol>
if it received any sort of error. 
</para>
<para>
<!-- .LP -->
For a request with a reply that is not followed by variable-length
data, you write something like:
</para>
<para>
<!-- .LP -->
<!-- .R -->
<programlisting>
_XReply(display, (xReply *)&amp;rep, 0, True);
*ret1 = rep.ret1;
*ret2 = rep.ret2;
*ret3 = rep.ret3;
...
UnlockDisplay(dpy);
SyncHandle();
return (rep.ret4);
}
</programlisting>
If there is variable-length data after the reply, 
change the 
<symbol>True</symbol>
to 
<symbol>False</symbol>,
and use the appropriate
<xref linkend='_XRead' xrefstyle='select: title'/>
function to read the variable-length data.
</para>
<!-- .LP -->
<!-- .sM -->
<funcsynopsis id='_XRead'>
<funcprototype>
  <funcdef><function>_XRead</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>char *<parameter>data_return</parameter></paramdef>
  <paramdef>long <parameter>nbytes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>data_return</emphasis>
    </term>
    <listitem>
      <para>
Specifies the buffer.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>nbytes</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of bytes required.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='_XRead' xrefstyle='select: title'/>
function reads the specified number of bytes into data_return.
</para>
<!-- .LP -->
<!-- .sM -->
<funcsynopsis id='_XRead16'>
<funcprototype>
  <funcdef><function>_XRead16</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>short *<parameter>data_return</parameter></paramdef>
  <paramdef>long <parameter>nbytes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>data_return</emphasis>
    </term>
    <listitem>
      <para>
Specifies the buffer.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>nbytes</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of bytes required.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='_XRead16' xrefstyle='select: title'/>
function reads the specified number of bytes,
unpacking them as 16-bit quantities,
into the specified array as shorts.
</para>
<!-- .LP -->
<!-- .sM -->
<funcsynopsis id='_XRead32'>
<funcprototype>
  <funcdef><function>_XRead32</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>long *<parameter>data_return</parameter></paramdef>
  <paramdef>long <parameter>nbytes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>data_return</emphasis>
    </term>
    <listitem>
      <para>
Specifies the buffer.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>nbytes</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of bytes required.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='_XRead32' xrefstyle='select: title'/>
function reads the specified number of bytes,
unpacking them as 32-bit quantities,
into the specified array as longs.
</para>
<!-- .LP -->
<!-- .sM -->
<funcsynopsis id='_XRead16Pad'>
<funcprototype>
  <funcdef><function>_XRead16Pad</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>short *<parameter>data_return</parameter></paramdef>
  <paramdef>long <parameter>nbytes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>data_return</emphasis>
    </term>
    <listitem>
      <para>
Specifies the buffer.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>nbytes</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of bytes required.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='_XRead16Pad' xrefstyle='select: title'/>
function reads the specified number of bytes,
unpacking them as 16-bit quantities,
into the specified array as shorts.
If the number of bytes is not a multiple of four,
<xref linkend='_XRead16Pad' xrefstyle='select: title'/>
reads and discards up to two additional pad bytes.
</para>
<!-- .LP -->
<!-- .sM -->
<funcsynopsis id='_XReadPad'>
<funcprototype>
  <funcdef><function>_XReadPad</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>char *<parameter>data_return</parameter></paramdef>
  <paramdef>long <parameter>nbytes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>data_return</emphasis>
    </term>
    <listitem>
      <para>
Specifies the buffer.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>nbytes</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of bytes required.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
The
<xref linkend='_XReadPad' xrefstyle='select: title'/>
function reads the specified number of bytes into data_return.
If the number of bytes is not a multiple of four,
<xref linkend='_XReadPad' xrefstyle='select: title'/>
reads and discards up to three additional pad bytes.
</para>
<para>
<!-- .LP -->
Each protocol request is a little different. 
For further information,
see the Xlib sources for examples.
</para>
</sect2>
<sect2 id="Synchronous_Calling">
<title>Synchronous Calling</title>
<para>
<!-- .LP -->
Each procedure should have a call, just before returning to the user, 
to a macro called
<systemitem>SyncHandle</systemitem>.
If synchronous mode is enabled (see 
<function>XSynchronize</function>),
the request is sent immediately.
The library, however, waits until any error the procedure could generate
at the server has been handled.
</para>
</sect2>
<sect2 id="Allocating_and_Deallocating_Memory">
<title>Allocating and Deallocating Memory</title>
<para>
<!-- .LP -->
To support the possible reentry of these procedures, 
you must observe several conventions when allocating and deallocating memory,
most often done when returning data to the user from the window
system of a size the caller could not know in advance
(for example, a list of fonts or a list of extensions).
The standard C library functions on many systems
are not protected against signals or other multithreaded uses.
The following analogies to standard I/O library functions
have been defined:
</para>
<para>
<!-- .LP -->
These should be used in place of any calls you would make to the normal
C library functions.
</para>
<para>
<!-- .LP -->
If you need a single scratch buffer inside a critical section 
(for example, to pack and unpack data to and from the wire protocol),
the general memory allocators may be too expensive to use
(particularly in output functions, which are performance critical).  
The following function returns a scratch buffer for use within a
critical section:
</para>
<indexterm significance="preferred"><primary>_XAllocScratch</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='_XAllocScratch'>
<funcprototype>
  <funcdef>char *<function>_XAllocScratch</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>unsigned long <parameter>nbytes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>nbytes</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of bytes required.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
This storage must only be used inside of a critical section of your
stub.  The returned pointer cannot be assumed valid after any call
that might permit another thread to execute inside Xlib.  For example,
the pointer cannot be assumed valid after any use of the
<function>GetReq</function>
or
<xref linkend='Data' xrefstyle='select: title'/>
families of macros,
after any use of
<xref linkend='_XReply' xrefstyle='select: title'/>,
or after any use of the
<function>_XSend</function>
or
<xref linkend='_XRead' xrefstyle='select: title'/>
families of functions.
</para>
<para>
<!-- .LP -->
<!-- .sp -->
The following function returns a scratch buffer for use across
critical sections:
</para>
<indexterm significance="preferred"><primary>_XAllocTemp</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='_XAllocTemp'>
<funcprototype>
  <funcdef>char *<function>_XAllocTemp</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>unsigned long <parameter>nbytes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>nbytes</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of bytes required.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
This storage can be used across calls that might permit another thread to
execute inside Xlib.  The storage must be explicitly returned to Xlib.
The following function returns the storage:
</para>
<indexterm significance="preferred"><primary>_XFreeTemp</primary></indexterm>
<!-- .sM -->
<funcsynopsis id='_XFreeTemp'>
<funcprototype>
  <funcdef>void <function>_XFreeTemp</function></funcdef>
  <paramdef>Display *<parameter>display</parameter></paramdef>
  <paramdef>char *<parameter>buf</parameter></paramdef>
  <paramdef>unsigned long <parameter>nbytes</parameter></paramdef>
</funcprototype>
</funcsynopsis>
<!-- .FN -->
<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>display</emphasis>
    </term>
    <listitem>
      <para>
Specifies the connection to the X server.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>buf</emphasis>
    </term>
    <listitem>
      <para>
Specifies the buffer to return.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>nbytes</emphasis>
    </term>
    <listitem>
      <para>
Specifies the size of the buffer.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
<para>
<!-- .LP -->
<!-- .eM -->
You must pass back the same pointer and size that were returned by
<xref linkend='_XAllocTemp' xrefstyle='select: title'/>.
</para>
</sect2>
<sect2 id="Portability_Considerations">
<title>Portability Considerations</title>
<para>
<!-- .LP -->
Many machine architectures
do not correctly or efficiently access data at unaligned locations;
their compilers pad out structures to preserve this characteristic.
Many other machines capable of unaligned references pad inside of structures
as well to preserve alignment, because accessing aligned data is
usually much faster.
Because the library and the server use structures to access data at
arbitrary points in a byte stream,
all data in request and reply packets <emphasis remap='I'>must</emphasis> be naturally aligned;
that is, 16-bit data starts on 16-bit boundaries in the request
and 32-bit data on 32-bit boundaries.
All requests <emphasis remap='I'>must</emphasis> be a multiple of 32 bits in length to preserve
the natural alignment in the data stream.
You must pad structures out to 32-bit boundaries.
Pad information does not have to be zeroed unless you want to
preserve such fields for future use in your protocol requests,
but it is recommended to zero it to avoid inadvertant data leakage
and improve compressability.
Floating point varies radically between machines and should be
avoided completely if at all possible.
</para>
<para>
<!-- .LP -->
This code may run on machines with 16-bit ints.  
So, if any integer argument, variable, or return value either can take 
only nonnegative values or is declared as a
<type>CARD16</type>
in the protocol, be sure to declare it as
<type>unsigned</type>
<type>int</type>
and not as
<type>int</type>.
(This, of course, does not apply to Booleans or enumerations.)
</para>
<para>
<!-- .LP -->
Similarly, 
if any integer argument or return value is declared
<type>CARD32</type>
in the protocol, 
declare it as an
<type>unsigned</type>
<type>long</type>
and not as
<type>int</type>
or
<type>long</type>.
This also goes for any internal variables that may
take on values larger than the maximum 16-bit
<type>unsigned</type>
<type>int</type>.
</para>
<para>
<!-- .LP -->
The library has always assumed that a
<type>char</type>
is 8 bits, a
<type>short</type>
is 16 bits, an
<type>int</type>
is 16 or 32 bits, and a
<type>long</type>
is 32 bits.
Unfortunately, this assumption remains on machines where a <type>long</type>
can hold 64-bits, and many functions and structures require unnecessarily
large fields to avoid breaking compatibility with existing code.  Special
care must be taken with arrays of values that are transmitted in the
protocol as CARD32 or INT32 but have to be converted to arrays of 64-bit
<type>long</type> when passed to or from client applications.
</para>
<para>
The 
<function>PackData</function>
macro is a half-hearted attempt to deal with the possibility of 32 bit shorts. 
However, much more work is needed to make this work properly.
</para>
</sect2>
<sect2 id="Deriving_the_Correct_Extension_Opcode">
<title>Deriving the Correct Extension Opcode</title>
<para>
<!-- .LP -->
The remaining problem a writer of an extension stub procedure faces that
the core protocol does not face is to map from the call to the proper
major and minor opcodes.  
While there are a number of strategies, 
the simplest and fastest is outlined below.
</para>
<itemizedlist>
  <listitem>
    <para>
Declare an array of pointers, _NFILE long (this is normally found
in 
<filename class="headerfile">&lt;stdio.h&gt;</filename>
and is the number of file descriptors supported on the system)
of type 
<structname>XExtCodes</structname>.
Make sure these are all initialized to NULL.
    </para>
  </listitem>
  <listitem>
    <para>
When your stub is entered, your initialization test is just to use
the display pointer passed in to access the file descriptor and an index
into the array.  
If the entry is NULL, then this is the first time you
are entering the procedure for this display.  
Call your initialization procedure and pass to it the display pointer.
    </para>
  </listitem>
  <listitem>
    <para>
Once in your initialization procedure, call 
<xref linkend='XInitExtension' xrefstyle='select: title'/>;
if it succeeds, store the pointer returned into this array.  
Make sure to establish a close display handler to allow you to zero the entry.
Do whatever other initialization your extension requires.
(For example, install event handlers and so on.)
Your initialization procedure would normally return a pointer to the
<structname>XExtCodes</structname>
structure for this extension, which is what would normally
be found in your array of pointers.
    </para>
  </listitem>
  <listitem>
    <para>
After returning from your initialization procedure, 
the stub can now continue normally, because it has its major opcode safely 
in its hand in the 
<structname>XExtCodes</structname>
structure.
<!-- .bp -->
    </para>
  </listitem>
</itemizedlist>
</sect2>
</sect1>
</appendix>
