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<chapter id="Introduction_to_Xlib"><title>Introduction to Xlib</title>

  <para>
The X Window System is a network-transparent window system that was
designed at MIT. X display servers run on computers with either
monochrome or color bitmap display hardware. The server distributes
user input to and accepts output requests from various client programs
located either on the same machine or elsewhere in the network. Xlib
is a C subroutine library that application programs (clients) use to
interface with the window system by means of a stream connection.
Although a client usually runs on the same machine as the X server
it is talking to, this need not be the case.
  </para>
  <para>
<citetitle>Xlib − C Language X Interface</citetitle> is a reference
guide to the low-level C language interface to the X Window System
protocol. It is neither a tutorial nor a user’s guide to programming
the X Window System. Rather, it provides a detailed description of
each function in the library as well as a discussion of the related
background information. <citetitle>Xlib − C Language X Interface</citetitle>
assumes a basic understanding of a graphics window system and of the C
programming language. Other higher-level abstractions (for example,
those provided by the toolkits for X) are built on top of the Xlib
library. For further information about these higher-level libraries,
see the appropriate toolkit documentation.
The <citetitle>X Window System Protocol</citetitle> provides the
deﬁnitive word on the behavior of X.
Although additional information appears here, the protocol document is
the ruling document.
  </para>
  <para>
To provide an introduction to X programming, this chapter discusses:

    <itemizedlist>
      <listitem><para><link linkend="Overview_of_the_X_Window_System">Overview of the X Window System</link></para></listitem>
      <listitem><para><link linkend="Errors">Errors</link></para></listitem>
      <listitem><para><link linkend="Standard_Header_Files">Standard header files</link></para></listitem>
      <listitem><para><link linkend="Generic_Values_and_Types">Generic values and types</link></para></listitem>
      <listitem><para><link linkend="Naming_and_Argument_Conventions_within_Xlib">Naming and argument conventions within Xlib</link></para></listitem>
      <listitem><para><link linkend="Programming_Considerations">Programming considerations</link></para></listitem>
      <listitem><para><link linkend="Character_Sets_and_Encodings">Character sets and encodings</link></para></listitem>
      <listitem><para><link linkend="Formatting_Conventions">Formatting conventions</link></para></listitem>
    </itemizedlist>
  </para>

  <sect1 id="Overview_of_the_X_Window_System">
    <title>Overview of the X Window System</title>
    <para>
Some of the terms used in this book are unique to X,
and other terms that are common to other window systems
have different meanings in X. You may find it helpful to refer to
<link linkend="glossary">the glossary</link>,
which is located at the end of the book.
    </para>
    <para>
The X Window System supports one or more screens containing
overlapping windows or subwindows.
<indexterm><primary>Screen</primary></indexterm>
A <firstterm>screen</firstterm> is a physical monitor and hardware
that can be color, grayscale, or monochrome.
There can be multiple screens for each display or workstation.
A single X server can provide display services for any number of screens.
A set of screens for a single user with one keyboard and one pointer
(usually a mouse) is called a <firstterm>display</firstterm>.
    </para>
    <para>
All the windows in an X server are arranged in strict hierarchies.
At the top of each hierarchy is a <firstterm>root window</firstterm>,
which covers each of the display screens.
Each root window is partially or completely covered by child windows.
All windows, except for root windows, have parents.
There is usually at least one window for each application program.
<indexterm><primary>Child window</primary></indexterm>
<indexterm><primary>Parent Window</primary></indexterm>
Child windows may in turn have their own children.
In this way,
an application program can create an arbitrarily deep tree
on each screen.
X provides graphics, text, and raster operations for windows.
    </para>
    <para>
A child window can be larger than its parent.
That is, part or all of
the child window can extend beyond the boundaries of the parent,
but all output to a window is clipped by its parent.
<indexterm><primary>Stacking order</primary></indexterm>
If several children of a window have overlapping locations,
one of the children is considered to be on top of or raised over the
others, thus obscuring them.
Output to areas covered by other windows is suppressed by the window
system unless the window has backing store.
If a window is obscured by a second window,
the second window obscures only those ancestors of the second window
that are also ancestors of the first window.
    </para>
    <para>
<indexterm significance="preferred"><primary>Window</primary></indexterm>
A window has a border zero or more pixels in width, which can
be any pattern (pixmap) or solid color you like.
A window usually but not always has a background pattern,
which will be repainted by the window system when uncovered.
Child windows obscure their parents,
and graphic operations in the parent window usually
are clipped by the children.
    </para>
    <para>
Each window and pixmap has its own coordinate system.
The coordinate system has the X axis horizontal and the Y axis vertical
with the origin [0, 0] at the upper-left corner.
Coordinates are integral,
in terms of pixels,
and coincide with pixel centers.
For a window,
the origin is inside the border at the inside, upper-left corner.
    </para>
    <para>
X does not guarantee to preserve the contents of windows.
When part or all of a window is hidden and then brought back onto the screen,
its contents may be lost.
The server then sends the client program an
<systemitem class="event">Expose</systemitem>
event to notify it that part or all of the window needs to be repainted.
Programs must be prepared to regenerate the contents of windows on demand.
    </para>
    <para>
<indexterm><primary>Pixmap</primary></indexterm>
<indexterm><primary>Drawable</primary></indexterm>
<indexterm><primary>Tile</primary></indexterm>
<indexterm><primary>Bitmap</primary></indexterm>
X also provides off-screen storage of graphics objects,
called <firstterm linkend="glossary:Pixmap">pixmaps</firstterm>.
Single plane (depth 1) pixmaps are sometimes referred to as
<firstterm>bitmaps</firstterm>.
Pixmaps can be used in most graphics functions interchangeably with
windows and are used in various graphics operations to define patterns or tiles.
Windows and pixmaps together are referred to as drawables.
    </para>
    <para>
Most of the functions in Xlib just add requests to an output buffer.
These requests later execute asynchronously on the X server.
Functions that return values of information stored in
the server do not return (that is, they block)
until an explicit reply is received or an error occurs.
You can provide an error handler,
which will be called when the error is reported.
    </para>
    <para>
<indexterm><primary>XSync</primary></indexterm>
If a client does not want a request to execute asynchronously,
it can follow the request with a call to
<function>XSync</function>,
which blocks until all previously buffered
asynchronous events have been sent and acted on.
As an important side effect,
the output buffer in Xlib is always flushed by a call to any function
that returns a value from the server or waits for input.
    </para>
    <para>
<indexterm><primary>Resource IDs</primary></indexterm>
<indexterm><primary>Resource IDs</primary><secondary>Window</secondary></indexterm>
<indexterm><primary>Resource IDs</primary><secondary>Font</secondary></indexterm>
<indexterm><primary>Resource IDs</primary><secondary>Pixmap</secondary></indexterm>
<indexterm><primary>Resource IDs</primary><secondary>Colormap</secondary></indexterm>
<indexterm><primary>Resource IDs</primary><secondary>Cursor</secondary></indexterm>
<indexterm><primary>Resource IDs</primary><secondary>GContext</secondary></indexterm>
Many Xlib functions will return an integer resource ID,
which allows you to refer to objects stored on the X server.
These can be of type
<type>Window</type>,
<type>Font</type>,
<type>Pixmap</type>,
<type>Colormap</type>,
<type>Cursor</type>,
and
<type>GContext</type>,
as defined in the file
<filename class="headerfile">&lt;X11/X.h&gt;</filename>.
<indexterm type="file"><primary><filename class="headerfile">X11/X.h</filename></primary></indexterm>
<indexterm><primary>Files</primary><secondary><filename class="headerfile">&lt;X11/X.h&gt;</filename></secondary></indexterm>
<indexterm><primary>Headers</primary><secondary><filename class="headerfile">&lt;X11/X.h&gt;</filename></secondary></indexterm>
These resources are created by requests and are destroyed
(or freed) by requests or when connections are closed.
Most of these resources are potentially sharable between
applications, and in fact, windows are manipulated explicitly by
window manager programs.
Fonts and cursors are shared automatically across multiple screens.
Fonts are loaded and unloaded as needed and are shared by multiple clients.
Fonts are often cached in the server.
Xlib provides no support for sharing graphics contexts between applications.
    </para>
    <para>
<indexterm><primary>Event</primary></indexterm>
Client programs are informed of events.
Events may either be side effects of a request (for example, restacking windows
generates
<systemitem class="event">Expose</systemitem>
events) or completely asynchronous (for example, from the keyboard).
A client program asks to be informed of events.
Because other applications can send events to your application,
programs must be prepared to handle (or ignore) events of all types.
    </para>
    <para>
Input events (for example, a key pressed or the pointer moved)
arrive asynchronously from the server and are queued until they are
requested by an explicit call (for example,
<function>XNextEvent</function>
or
<function>XWindowEvent</function>).
In addition, some library
functions (for example,
<function>XRaiseWindow</function>)
generate
<symbol>Expose</symbol>
and
<symbol>ConfigureRequest</symbol>
events.
These events also arrive asynchronously, but the client may
<indexterm><primary>XSync</primary></indexterm>
wish to explicitly wait for them by calling
<function>XSync</function>
after calling a function that can cause the server to generate events.
    </para>
  </sect1>

  <sect1 id="Errors">
    <title>Errors</title>

    <para>
Some functions return
<type>Status</type>,
an integer error indication.
If the function fails, it returns a zero.
If the function returns a status of zero,
it has not updated the return arguments.
<indexterm><primary>Status</primary></indexterm>
Because C does not provide multiple return values,
many functions must return their results by writing into client-passed storage.
<indexterm><primary>Error</primary><secondary>handling</secondary></indexterm>
By default, errors are handled either by a standard library function
or by one that you provide.
Functions that return pointers to strings return NULL pointers if
the string does not exist.
    </para>
    <para>
The X server reports protocol errors at the time that it detects them.
If more than one error could be generated for a given request,
the server can report any of them.
    </para>
    <para>
Because Xlib usually does not transmit requests to the server immediately
(that is, it buffers them), errors can be reported much later than they
actually occur.
For debugging purposes, however,
Xlib provides a mechanism for forcing synchronous behavior
(see section 11.8.1). <!-- xref -->
When synchronization is enabled,
errors are reported as they are generated.
    </para>
    <para>
When Xlib detects an error,
it calls an error handler,
which your program can provide.
If you do not provide an error handler,
the error is printed, and your program terminates.
    </para>
  </sect1>

  <sect1 id="Standard_Header_Files">
    <title>Standard Header Files</title>

    <para>
The following include files are part of the Xlib standard:
<indexterm><primary>Headers</primary></indexterm>

<variablelist>
  <varlistentry id="Standard_Header_Files:Xlib.h">
    <term><filename class="headerfile">&lt;X11/Xlib.h&gt;</filename></term>
    <listitem>
      <indexterm type="file"><primary><filename class="headerfile">X11/Xlib.h</filename></primary></indexterm>
      <indexterm><primary>Files</primary><secondary>&lt;X11/Xlib.h&gt;</secondary></indexterm>
      <indexterm><primary>Headers</primary><secondary>&lt;X11/Xlib.h&gt;</secondary></indexterm>
      <para>
This is the main header file for Xlib.
The majority of all Xlib symbols are declared by including this file.
This file also contains the preprocessor symbol
<symbol>XlibSpecificationRelease</symbol>.
<indexterm significance="preferred"><primary>XlibSpecificationRelease</primary></indexterm>
This symbol is defined to have the 6 in this release of the standard.
(Release 5 of Xlib was the first release to have this symbol.)
      </para>
    </listitem>
  </varlistentry>
  <varlistentry id="Standard_Header_Files:X.h">
    <term><filename class="headerfile">&lt;X11/X.h&gt;</filename></term>
    <listitem>
      <indexterm type="file"><primary><filename class="headerfile">X11/X.h</filename></primary></indexterm>
      <indexterm><primary>Files</primary><secondary>&lt;X11/X.h&gt;</secondary></indexterm>
      <indexterm><primary>Headers</primary><secondary>&lt;X11/X.h&gt;</secondary></indexterm>
      <para>
This file declares types and constants for the X protocol that are
to be used by applications.  It is included automatically from
<filename class="headerfile">&lt;X11/Xlib.h&gt;</filename>
so application code should never need to
reference this file directly.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry id="Standard_Header_Files:Xcms.h">
    <term><filename class="headerfile">&lt;X11/Xcms.h&gt;</filename></term>
    <listitem>
      <indexterm type="file"><primary><filename class="headerfile">X11/Xcms.h</filename></primary></indexterm>
      <indexterm><primary>Files</primary><secondary>&lt;X11/Xcms.h&gt;</secondary></indexterm>
      <indexterm><primary>Headers</primary><secondary>&lt;X11/Xcms.h&gt;</secondary></indexterm>
      <para>
This file contains symbols for much of the color management facilities
described in chapter 6.  All functions, types, and symbols with the
prefix "Xcms", <!-- xref -->
plus the Color Conversion Contexts macros, are declared in this file.
<filename class="headerfile">&lt;X11/Xlib.h&gt;</filename>
must be included before including this file.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry id="Standard_Header_Files:Xutil.h">
    <term><filename class="headerfile">&lt;X11/Xutil.h&gt;</filename></term>
    <listitem>
      <indexterm type="file"><primary><filename class="headerfile">X11/Xutil.h</filename></primary></indexterm>
      <indexterm><primary>Files</primary><secondary>&lt;X11/Xutil.h&gt;</secondary></indexterm>
      <indexterm><primary>Headers</primary><secondary>&lt;X11/Xutil.h&gt;</secondary></indexterm>
      <para>
This file declares various functions, types, and symbols used for
inter-client communication and application utility functions,
which are described in chapters 14 and 16. <!-- xref -->
<filename class="headerfile">&lt;X11/Xlib.h&gt;</filename> must be included before including this file.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry id="Standard_Header_Files:Xresource.h">
    <term><filename class="headerfile">&lt;X11/Xresource.h&gt;</filename></term>
    <listitem>
      <indexterm type="file"><primary><filename class="headerfile">X11/Xresource.h</filename></primary></indexterm>
      <indexterm><primary>Files</primary><secondary>&lt;X11/Xresource.h&gt;</secondary></indexterm>
      <indexterm><primary>Headers</primary><secondary>&lt;X11/Xresource.h&gt;</secondary></indexterm>
      <para>
This file declares all functions, types, and symbols for the
resource manager facilities, which are described in chapter 15.
<filename class="headerfile">&lt;X11/Xlib.h&gt;</filename> <!-- xref -->
must be included before including this file.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry id="Standard_Header_Files:Xatom.h">
    <term><filename class="headerfile">&lt;X11/Xatom.h&gt;</filename></term>
    <listitem>
      <indexterm type="file"><primary><filename class="headerfile">X11/Xatom.h</filename></primary></indexterm>
      <indexterm><primary>Files</primary><secondary>&lt;X11/Xatom.h&gt;</secondary></indexterm>
      <indexterm><primary>Headers</primary><secondary>&lt;X11/Xatom.h&gt;</secondary></indexterm>
      <para>
This file declares all predefined atoms,
which are symbols with the prefix "XA_".
      </para>
    </listitem>
  </varlistentry>
  <varlistentry id="Standard_Header_Files:cursorfont.h">
    <term><filename class="headerfile">&lt;X11/cursorfont.h&gt;</filename></term>
    <listitem>
      <indexterm type="file"><primary><filename class="headerfile">X11/cursorfont.h</filename></primary></indexterm>
      <indexterm><primary>Files</primary><secondary>&lt;X11/cursorfont.h&gt;</secondary></indexterm>
      <indexterm><primary>Headers</primary><secondary>&lt;X11/cursorfont.h&gt;</secondary></indexterm>
      <para>
This file declares the cursor symbols for the standard cursor font,
which are listed in <link linkend="x_font_cursors">Appendix B</link>.
All cursor symbols have the prefix "XC_".
      </para>
    </listitem>
  </varlistentry>
  <varlistentry id="Standard_Header_Files:keysymdef.h">
    <term><filename class="headerfile">&lt;X11/keysymdef.h&gt;</filename></term>
    <listitem>
      <indexterm type="file"><primary><filename class="headerfile">X11/keysymdef.h</filename></primary></indexterm>
      <indexterm><primary>Files</primary><secondary>&lt;X11/keysymdef.h&gt;</secondary></indexterm>
      <indexterm><primary>Headers</primary><secondary>&lt;X11/keysymdef.h&gt;</secondary></indexterm>
      <para>
This file declares all standard KeySym values,
which are symbols with the prefix "XK_".
The KeySyms are arranged in groups, and a preprocessor symbol controls
inclusion of each group.  The preprocessor symbol must be defined
prior to inclusion of the file to obtain the associated values.
The preprocessor symbols are
<symbol>XK_MISCELLANY</symbol>,
<symbol>XK_XKB_KEYS</symbol>,
<symbol>XK_3270</symbol>,
<symbol>XK_LATIN1</symbol>,
<symbol>XK_LATIN2</symbol>,
<symbol>XK_LATIN3</symbol>,
<symbol>XK_LATIN4</symbol>,
<symbol>XK_KATAKANA</symbol>,
<symbol>XK_ARABIC</symbol>,
<symbol>XK_CYRILLIC</symbol>,
<symbol>XK_GREEK</symbol>,
<symbol>XK_TECHNICAL</symbol>,
<symbol>XK_SPECIAL</symbol>,
<symbol>XK_PUBLISHING</symbol>,
<symbol>XK_APL</symbol>,
<symbol>XK_HEBREW</symbol>,
<symbol>XK_THAI</symbol>, and
<symbol>XK_KOREAN</symbol>.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry id="Standard_Header_Files:keysym.h">
    <term><filename class="headerfile">&lt;X11/keysym.h&gt;</filename></term>
    <listitem>
      <indexterm type="file"><primary><filename class="headerfile">X11/keysym.h</filename></primary></indexterm>
      <indexterm><primary>Files</primary><secondary>&lt;X11/keysym.h&gt;</secondary></indexterm>
      <indexterm><primary>Headers</primary><secondary>&lt;X11/keysym.h&gt;</secondary></indexterm>
      <para>
This file defines the preprocessor symbols
<symbol>XK_MISCELLANY</symbol>,
<symbol>XK_XKB_KEYS</symbol>,
<symbol>XK_LATIN1</symbol>,
<symbol>XK_LATIN2</symbol>,
<symbol>XK_LATIN3</symbol>,
<symbol>XK_LATIN4</symbol>, and
<symbol>XK_GREEK</symbol>
and then includes <filename class="headerfile">&lt;X11/keysymdef.h&gt;</filename>.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry id="Standard_Header_Files:Xlibint.h">
    <term><filename class="headerfile">&lt;X11/Xlibint.h&gt;</filename></term>
    <listitem>
      <indexterm type="file"><primary><filename class="headerfile">X11/Xlibint.h</filename></primary></indexterm>
      <indexterm><primary>Files</primary><secondary>&lt;X11/Xlibint.h&gt;</secondary></indexterm>
      <indexterm><primary>Headers</primary><secondary>&lt;X11/Xlibint.h&gt;</secondary></indexterm>
      <para>
This file declares all the functions, types, and symbols used for
extensions, which are described in <link linkend="extensions">Appendix C</link>.
This file automatically includes
<filename class="headerfile">&lt;X11/Xlib.h&gt;</filename>.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry id="Standard_Header_Files:Xproto.h">
    <term><filename class="headerfile">&lt;X11/Xproto.h&gt;</filename></term>
    <listitem>
      <indexterm type="file"><primary><filename class="headerfile">X11/Xproto.h</filename></primary></indexterm>
      <indexterm><primary>Files</primary><secondary>&lt;X11/Xproto.h&gt;</secondary></indexterm>
      <indexterm><primary>Headers</primary><secondary>&lt;X11/Xproto.h&gt;</secondary></indexterm>
      <para>
This file declares types and symbols for the basic X protocol,
for use in implementing extensions.
It is included automatically from
<filename class="headerfile">&lt;X11/Xlibint.h&gt;</filename>,
so application and extension code should never need to
reference this file directly.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry id="Standard_Header_Files:Xprotostr.h">
    <term><filename class="headerfile">&lt;X11/Xprotostr.h&gt;</filename></term>
    <listitem>
      <indexterm type="file"><primary><filename class="headerfile">X11/Xprotostr.h</filename></primary></indexterm>
      <indexterm><primary>Files</primary><secondary>&lt;X11/Xprotostr.h&gt;</secondary></indexterm>
      <indexterm><primary>Headers</primary><secondary>&lt;X11/Xprotostr.h&gt;</secondary></indexterm>
      <para>
This file declares types and symbols for the basic X protocol,
for use in implementing extensions.
It is included automatically from
<filename class="headerfile">&lt;X11/Xproto.h&gt;</filename>,
so application and extension code should never need to
reference this file directly.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry id="Standard_Header_Files:X10.h">
    <term><filename class="headerfile">&lt;X11/X10.h&gt;</filename></term>
    <listitem>
      <indexterm type="file"><primary><filename class="headerfile">X11/X10.h</filename></primary></indexterm>
      <indexterm><primary>Files</primary><secondary>&lt;X11/X10.h&gt;</secondary></indexterm>
      <indexterm><primary>Headers</primary><secondary>&lt;X11/X10.h&gt;</secondary></indexterm>
      <para>
This file declares all the functions, types, and symbols used for the
X10 compatibility functions, which are described in
<link linkend="X_Version_10_Compatibility_Functions">Appendix D</link>.
      </para>
    </listitem>
  </varlistentry>
</variablelist>
    </para>
  </sect1>

  <sect1 id="Generic_Values_and_Types">
    <title>Generic Values and Types</title>

    <para>
The following symbols are defined by Xlib and used throughout the manual:
<itemizedlist>
  <listitem>
    <para>
<indexterm significance="preferred"><primary>Bool</primary></indexterm>
<indexterm significance="preferred"><primary>True</primary></indexterm>
<indexterm significance="preferred"><primary>False</primary></indexterm>
Xlib defines the type
<type>Bool</type>
and the Boolean values
<symbol>True</symbol>
and
<symbol>False</symbol>.
    </para>
  </listitem>
  <listitem>
    <para>
<indexterm significance="preferred"><primary>None</primary></indexterm>
<symbol>None</symbol>
is the universal null resource ID or atom.
    </para>
  </listitem>
  <listitem>
    <para>
<indexterm significance="preferred"><primary>XID</primary></indexterm>
The type
<type>XID</type>
is used for generic resource IDs.
    </para>
  </listitem>
  <listitem>
    <para>
<indexterm significance="preferred"><primary>XPointer</primary></indexterm>
The type <type>XPointer</type> is defined to be <type>char&hairsp;*</type>
and is used as a generic opaque pointer to data.
    </para>
  </listitem>
</itemizedlist>
    </para>
  </sect1>

  <sect1 id="Naming_and_Argument_Conventions_within_Xlib">
    <title>Naming and Argument Conventions within Xlib</title>

    <para>
Xlib follows a number of conventions for the naming and syntax of the functions.
Given that you remember what information the function requires,
these conventions are intended to make the syntax of the functions more
predictable.
    </para>
    <para>
The major naming conventions are:
<itemizedlist>
  <listitem>
    <para>
To differentiate the X symbols from the other symbols,
the library uses mixed case for external symbols.
It leaves lowercase for variables and all uppercase for user macros,
as per existing convention.
    </para>
  </listitem>
  <listitem>
    <para>
All Xlib functions begin with a capital X.
    </para>
  </listitem>
  <listitem>
    <para>
The beginnings of all function names and symbols are capitalized.
    </para>
  </listitem>
  <listitem>
    <para>
All user-visible data structures begin with a capital X.
More generally,
anything that a user might dereference begins with a capital X.
    </para>
  </listitem>
  <listitem>
    <para>
Macros and other symbols do not begin with a capital X.
To distinguish them from all user symbols,
each word in the macro is capitalized.
    </para>
  </listitem>
  <listitem>
    <para>
All elements  of or variables in a data structure are in lowercase.
Compound words, where needed, are constructed with underscores (_).
    </para>
  </listitem>
  <listitem>
    <para>
The display argument, where used, is always first in the argument list.
    </para>
  </listitem>
  <listitem>
    <para>
All resource objects, where used, occur at the beginning of the argument list
immediately after the display argument.
    </para>
  </listitem>
  <listitem>
    <para>
When a  graphics context is present together with
another type of resource (most commonly, a drawable), the
graphics context occurs in the argument list after the other
resource.
Drawables outrank all other resources.
    </para>
  </listitem>
  <listitem>
    <para>
Source arguments always precede the destination arguments in the argument list.
    </para>
  </listitem>
  <listitem>
    <para>
The x argument always precedes the y argument in the argument list.
    </para>
  </listitem>
  <listitem>
    <para>
The width argument always precedes the height argument in the argument list.
    </para>
  </listitem>
  <listitem>
    <para>
Where the x, y, width, and height arguments are used together,
the x and y arguments always precede the width and height arguments.
    </para>
  </listitem>
  <listitem>
    <para>
Where a mask is accompanied with a structure,
the mask always precedes the pointer to the structure in the argument list.
    </para>
  </listitem>
</itemizedlist>
    </para>
  </sect1>

  <sect1 id="Programming_Considerations">
    <title>Programming Considerations</title>

    <para>
The major programming considerations are:
<itemizedlist>
  <listitem>
    <para>
Coordinates and sizes in X are actually 16-bit quantities.
This decision was made to minimize the bandwidth required for a
given level of performance.
Coordinates usually are declared as an
<type>int</type>
in the interface.
Values larger than 16 bits are truncated silently.
Sizes (width and height) are declared as unsigned quantities.
    </para>
  </listitem>
  <listitem>
    <para>
Keyboards are the greatest variable between different
manufacturers' workstations.
If you want your program to be portable,
you should be particularly conservative here.
    </para>
  </listitem>
  <listitem>
    <para>
Many display systems have limited amounts of off-screen memory.
If you can, you should minimize use of pixmaps and backing
store.
    </para>
  </listitem>
  <listitem>
    <para>
The user should have control of his screen real estate.
Therefore, you should write your applications to react to window management
rather than presume control of the entire screen.
What you do inside of your top-level window, however,
is up to your application.
For further information,
see chapter 14 <!-- xref -->
and the <citetitle>Inter-Client Communication Conventions Manual</citetitle>.
    </para>
  </listitem>
</itemizedlist>
    </para>
  </sect1>

  <sect1 id="Character_Sets_and_Encodings">
    <title>Character Sets and Encodings</title>

    <para>
Some of the Xlib functions make reference to specific character sets
and character encodings.
The following are the most common:

<variablelist>
  <varlistentry>
    <term><firstterm>X Portable Character Set</firstterm></term>
    <listitem><para>
A basic set of 97 characters,
which are assumed to exist in all locales supported by Xlib.
This set contains the following characters:
<literallayout>
a..z A..Z 0..9 !"#$%&amp;'()*+,-./:;&lt;=&gt;?@[\\]^_`{|}~ &lt;space&gt;, &lt;tab&gt;, and &lt;newline&gt;
</literallayout>
      </para>
      <para>
This set is the left/lower half
of the graphic character set of ISO8859-1 plus space, tab, and newline.
It is also the set of graphic characters in 7-bit ASCII plus the same
three control characters.
The actual encoding of these characters on the host is system dependent.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><firstterm>Host Portable Character Encoding</firstterm></term>
    <listitem>
      <para>
The encoding of the X Portable Character Set on the host.
The encoding itself is not defined by this standard,
but the encoding must be the same in all locales supported by Xlib on the host.
If a string is said to be in the Host Portable Character Encoding,
then it only contains characters from the X Portable Character Set,
in the host encoding.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><firstterm>Latin-1</firstterm></term>
    <listitem>
      <para>
The coded character set defined by the ISO8859-1 standard.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><firstterm>Latin Portable Character Encoding</firstterm></term>
    <listitem>
      <para>
The encoding of the X Portable Character Set using the Latin-1 codepoints
plus ASCII control characters.
If a string is said to be in the Latin Portable Character Encoding,
then it only contains characters from the X Portable Character Set,
not all of Latin-1.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><firstterm>STRING Encoding</firstterm></term>
    <listitem>
      <para>
Latin-1, plus tab and newline.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><firstterm><acronym>POSIX</acronym> Portable Filename Character Set</firstterm></term>
    <listitem>
      <para>
The set of 65 characters,
which can be used in naming files on a <acronym>POSIX</acronym>-compliant host,
that are correctly processed in all locales.
The set is:
<literallayout>
a..z A..Z 0..9 ._-
</literallayout>
      </para>
    </listitem>
  </varlistentry>
</variablelist>
    </para>
  </sect1>

  <sect1 id="Formatting_Conventions">
    <title>Formatting Conventions</title>

    <para>
      <citetitle>Xlib − C Language X Interface</citetitle> uses the
      following conventions:

      <itemizedlist>
	<listitem>
	  <para>
Global symbols are printed in
<function>this</function>
<function>special</function>
<function>font</function>.
These can be either function names,
symbols defined in include files, or structure names.
When declared and defined,
function arguments are printed in <emphasis remap='I'>italics</emphasis>.
In the explanatory text that follows,
they usually are printed in regular type.
	  </para>
	</listitem>
	<listitem>
	  <para>
Each function is introduced by a general discussion that
distinguishes it from other functions.
The function declaration itself follows,
and each argument is specifically explained.
Although ANSI C function prototype syntax is not used,
Xlib header files normally declare functions using function prototypes
in ANSI C environments.
General discussion of the function, if any is required,
follows the arguments.
Where applicable,
the last paragraph of the explanation lists the possible
Xlib error codes that the function can generate.
For a complete discussion of the Xlib error codes,
see section 11.8.2. <!-- xref -->
	  </para>
	</listitem>
	<listitem>
	  <para>
To eliminate any ambiguity between those arguments that you pass and those that
a function returns to you,
the explanations for all arguments that you pass start with the word
<emphasis remap='I'>specifies</emphasis> or, in the case of multiple arguments, the word <emphasis remap='I'>specify</emphasis>.
The explanations for all arguments that are returned to you start with the
word <emphasis remap='I'>returns</emphasis> or, in the case of multiple arguments, the word <emphasis remap='I'>return</emphasis>.
The explanations for all arguments that you can pass and are returned start
with the words <emphasis remap='I'>specifies and returns</emphasis>.
	  </para>
	</listitem>
	<listitem>
	  <para>
Any pointer to a structure that is used to return a value is designated as
such by the <emphasis remap='I'>_return</emphasis> suffix as part of its name.
All other pointers passed to these functions are
used for reading only.
A few arguments use pointers to structures that are used for
both input and output and are indicated by using the <emphasis remap='I'>_in_out</emphasis> suffix.
	  </para>
	</listitem>
      </itemizedlist>
    </para>
  </sect1>
</chapter>
