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content="David Turner">
<title>FreeType 2 Internals - I/O Frames</title>
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<h1 align=center>
FreeType 2.0 I/O Frames
</h1>
<h3 align=center>
© 2000 David Turner
(<a href="mailto:[email protected]">[email protected]</a>)<br>
© 2000 The FreeType Development Team
(<a href="http://www.freetype.org">www.freetype.org</a>)
</h3>
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<h2>
Introduction
</h2>
<p>This document explains the concept of I/O <b>frames</b> as used in the
FreeType 2 source code. It also enumerates the various functions and
macros that can be used to read them.</p>
<p>It is targeted to FreeType hackers, or more simply to developers who
would like a better understanding of the library's source code.</p>
<hr>
<h2>
I. What frames are
</h2>
<p>Simply speaking, a frame is an array of bytes in a font file that is
"preloaded" into memory in order to be rapidly parsed. Frames are useful
to ensure that every "load" is checked against end-of-file overruns, and
provides nice functions to extract data in a variety of distinct
formats.</p>
<p>But an example is certainly more meaningful than anything else. The
following code</p>
<font color="blue">
<pre>
error = read_short( stream, &str.value1 );
if ( error ) goto ...
error = read_ulong( stream, &str.value2 );
if ( error ) goto ...
error = read_ulong( stream, &str.value3 );
if ( error ) goto ...</pre>
</font>
<p>can easily be replaced with</p>
<font color="blue">
<pre>
error = FT_Access_Frame( stream, 2 + 4 + 4 );
if ( error ) goto ...
str.value1 = FT_Get_Short( stream );
str.value2 = FT_Get_ULong( stream );
str.value3 = FT_Get_ULong( stream );
FT_Forget_Frame( stream );</pre>
</font>
<p>Here, the call to <tt>FT_Access_Frame()</tt> will</p>
<ul>
<li>
<p>Ensure that there are at least 2+4+4=10 bytes left in the
stream.</p>
</li>
<li>
<p>"Preload" (for disk-based streams) 10 bytes from the current
stream position.</p>
</li>
<li>
<p>Set the frame "cursor" to the first byte in the frame.</p>
</li>
</ul>
<p>Each <tt>FT_Get_Short()</tt> or <tt>FT_Get_ULong()</tt> call will read
a big-endian integer from the stream (2 bytes for
<tt>FT_Get_Short()</tt>, 4 bytes for <tt>FT_Get_ULong</tt>) and
advance the frame cursor accordingly.</p>
<p><tt>FT_Forget_Frame()</tt> "releases" the frame from memory.</p>
<p>There are several advantages to using frames:</p>
<ul>
<li>
<p>Single-check when loading tables.</p>
</li>
<li>
<p><em>Making code clearer</em> by providing simple parsing functions
<em>while keeping code safe</em> from file over-runs and invalid
offsets.</p>
</li>
</ul>
<hr>
<h2>
II. Accessing and reading a frame with macros
</h2>
<p>By convention in the FreeType source code, macros are able to use two
implicit variables named <tt>error</tt> and <tt>stream</tt>. This is
useful because these two variables are extremely often used in the
library, and doing this only reduces our typing requirements and make the
source code much clearer.</p>
<p>Note that <tt>error</tt> must be a local variable of type
<tt>FT_Error</tt>, while <tt>stream</tt> must be a local variable or
argument of type <tt>FT_Stream</tt>.</p>
<p>The macro used to access a frame is <font
color="purple"><tt><b>ACCESS_Frame(_size_)</b></tt></font>, it will
translate to</p>
<font color="blue">
<pre>
( error = FT_Access_Frame( stream, _size_ ) )
!= FT_Err_Ok</tt></pre>
</font>
<p>Similarly, the macro <font
color="purple"><b><tt>FORGET_Frame()</tt></b></font> translates to</p>
<font color="blue">
<pre>
<tt>FT_Forget_Frame( stream )</tt></pre>
</font>
<p>Extracting integers can be performed with the <tt>GET_xxx()</tt>
macros, like</p>
<table align=center>
<tr valign="top">
<td>
<b>Macro name</b>
</td>
<td>
Translation
</td>
<td>
Description
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><tt><b>GET_Byte()</b></tt></font>
</td>
<td>
<font color="blue"><tt>FT_Get_Byte(stream)</tt></font>
</td>
<td>
<p>Reads an 8-bit unsigned byte.</p>
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><tt><b>GET_Char()</b></tt></font>
</td>
<td>
<font color="blue"><tt>(FT_Char)<br>
FT_Get_Byte(stream)</tt></font>
</td>
<td>
<p>Reads an 8-bit <em>signed</em> byte.</p>
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><tt><b>GET_Short()</b></tt></font>
</td>
<td>
<font color="blue"><tt>FT_Get_Short(stream)</tt></font>
</td>
<td>
Reads a 16-bit signed big-endian integer.
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><tt><b>GET_UShort()</b></tt></font>
</td>
<td>
<font color="blue"><tt>(FT_UShort)<br>
FT_Get_Short(stream)</tt></font>
</td>
<td>
Reads a 16-bit unsigned big-endian integer.
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><tt><b>GET_Offset()</b></tt></font>
</td>
<td>
<font color="blue"><tt>FT_Get_Offset(stream)</tt></font>
</td>
<td>
Reads a 24-bit signed big-endian integer.
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><tt><b>GET_UOffset()</b></tt></font>
</td>
<td>
<font color="blue"><tt>(FT_UOffset)<br>
FT_Get_Offset(stream)</tt></font>
</td>
<td>
Reads a 24-bit unsigned big-endian integer.
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><tt><b>GET_Long()</b></tt></font>
</td>
<td>
<font color="blue"><tt>FT_Get_Long(stream)</tt></font>
</td>
<td>
Reads a 32-bit signed big-endian integer.
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><tt><b>GET_ULong()</b></tt></font>
</td>
<td>
<font color="blue"><tt>(FT_ULong)<br>
FT_Get_Long(stream)</tt></font>
</td>
<td>
Reads a 32-bit unsigned big-endian integer.
</td>
</tr>
</table>
<p>(Note that an <b>Offset</b> is an integer stored with 3 bytes on
the file.)</p>
<p>All this means that the following code</p>
<font color="blue">
<pre>
error = FT_Access_Frame( stream, 2 + 4 + 4 );
if ( error ) goto ...
str.value1 = FT_Get_Short( stream );
str.value2 = FT_Get_ULong( stream );
str.value3 = FT_Get_ULong( stream );
FT_Forget_Frame( stream );</pre>
</font>
<p>can be simplified with macros:</p>
<font color="blue">
<pre>
if ( ACCESS_Frame( 2 +4 + 4 ) ) goto ...
str.value1 = GET_Short();
str.value2 = GET_ULong();
str.value3 = GET_ULong();
FORGET_Frame();</pre>
</font>
<p>Which is clearer. Notice that <tt>error</tt> and <tt>stream</tt>
must be defined locally though for this code to work!</p>
<hr>
<h2>
III. Alternatives
</h2>
<p>It is sometimes useful to read small integers from a font file without
using a frame. Some functions have been introduced in FreeType 2 to
do just that, and they are of the form <font
color="blue"><tt>FT_Read_xxxx</tt></font>.</p>
<p>For example, <font color="blue"><tt>FT_Read_Short(stream,
&error)</tt></font> reads and returns a 2-byte big-endian integer from a
<tt>stream</tt>, and places an error code in the <tt>error</tt>
variable.</p>
<p>Thus, reading a single big-endian integer is shorter than using a frame
for it.</p>
<p>Note that there are also macros <font
color="purple"><tt>READ_xxx()</tt></font> which translate to</p>
<font color="blue">
<pre>
FT_Read_xxx( stream, &error ), error != FT_Err_Ok</pre>
</font>
<p>and can be used as in</p>
<font color="blue">
<pre>
if ( READ_UShort( variable1 ) ||
READ_ULong ( variable2 ) )
goto Fail;</pre>
</font>
<p>if <tt>error</tt> and <tt>stream</tt> are already defined locally.</p>
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