ref: 321cd27d0f89f8646c95218b16dca4a3c02d7f17
dir: /docs/design/io-frames.html/
<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
<html>
<head>
<meta http-equiv="Content-Type"
content="text/html; charset=iso-8859-1">
<meta name="Author"
content="David Turner">
<title>FreeType 2 Internals - I/O Frames</title>
</head>
<body text="#000000"
bgcolor="#FFFFFF"
link="#0000EF"
vlink="#51188E"
alink="#FF0000">
<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>
<center>
<table width="70%">
<tr><td>
<hr>
<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 <code>FT_Access_Frame()</code> 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 <code>FT_Get_Short()</code> or <code>FT_Get_ULong()</code> call
will read a big-endian integer from the stream (2 bytes for
<code>FT_Get_Short()</code>, 4 bytes for <code>FT_Get_ULong</code>)
and advance the frame cursor accordingly.</p>
<p><code>FT_Forget_Frame()</code> `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 <var>error</var> and <var>stream</var>. 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 <var>error</var> must be a local variable of type
<code>FT_Error</code>, while <var>stream</var> must be a local variable or
argument of type <code>FT_Stream</code>.</p>
<p>The macro used to access a frame is <font
color="purple"><code><b>ACCESS_Frame(_size_)</b></code></font>, it will
translate to</p>
<font color="blue">
<pre>
( error = FT_Access_Frame( stream, _size_ ) )
!= FT_Err_Ok</pre>
</font>
<p>Similarly, the macro <font
color="purple"><b><code>FORGET_Frame()</code></b></font> translates to</p>
<font color="blue">
<pre>
FT_Forget_Frame( stream )</pre>
</font>
<p>Extracting integers can be performed with the <code>GET_xxx()</code>
macros, like</p>
<table align=center
cellpadding=5>
<tr valign="top">
<th>
Macro name
</th>
<th>
Translation
</th>
<th>
Description
</th>
</tr>
<tr valign="top">
<td>
<font color="purple"><code><b>GET_Byte()</b></code></font>
</td>
<td>
<font color="blue"><code>FT_Get_Byte(stream)</code></font>
</td>
<td>
<p>Reads an 8-bit unsigned byte.</p>
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><code><b>GET_Char()</b></code></font>
</td>
<td>
<font color="blue"><code>(FT_Char)<br>
FT_Get_Byte(stream)</code></font>
</td>
<td>
<p>Reads an 8-bit <em>signed</em> byte.</p>
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><code><b>GET_Short()</b></code></font>
</td>
<td>
<font color="blue"><code>FT_Get_Short(stream)</code></font>
</td>
<td>
Reads a 16-bit signed big-endian integer.
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><code><b>GET_UShort()</b></code></font>
</td>
<td>
<font color="blue"><code>(FT_UShort)<br>
FT_Get_Short(stream)</code></font>
</td>
<td>
Reads a 16-bit unsigned big-endian integer.
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><code><b>GET_Offset()</b></code></font>
</td>
<td>
<font color="blue"><code>FT_Get_Offset(stream)</code></font>
</td>
<td>
Reads a 24-bit signed big-endian integer.
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><code><b>GET_UOffset()</b></code></font>
</td>
<td>
<font color="blue"><code>(FT_UOffset)<br>
FT_Get_Offset(stream)</code></font>
</td>
<td>
Reads a 24-bit unsigned big-endian integer.
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><code><b>GET_Long()</b></code></font>
</td>
<td>
<font color="blue"><code>FT_Get_Long(stream)</code></font>
</td>
<td>
Reads a 32-bit signed big-endian integer.
</td>
</tr>
<tr valign="top">
<td>
<font color="purple"><code><b>GET_ULong()</b></code></font>
</td>
<td>
<font color="blue"><code>(FT_ULong)<br>
FT_Get_Long(stream)</code></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 <var>error</var> and <var>stream</var>
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"><code>FT_Read_xxxx</code></font>.</p>
<p>For example, <font color="blue"><code>FT_Read_Short(stream,
&error)</code></font> reads and returns a 2-byte big-endian integer from a
<var>stream</var>, and places an error code in the <var>error</var>
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"><code>READ_xxx()</code></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 <var>error</var> and <var>stream</var> are already defined
locally.</p>
</td></tr>
</table>
</center>
</body>
</html>