248 lines
8.2 KiB
HTML
248 lines
8.2 KiB
HTML
<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="Author" content="David Turner">
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<meta name="GENERATOR" content="Mozilla/4.5 [fr] (Win98; I) [Netscape]">
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<title>FreeType 2 Internals - I/O Frames</title>
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</head>
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<body>
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<body text="#000000"
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bgcolor="#FFFFFF"
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link="#0000EF"
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vlink="#51188E"
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alink="#FF0000">
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<center>
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<h1>
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FreeType 2.0 I/O Frames</h1></center>
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<center>
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<h3>
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© 2000 David Turner (<a href="mailto:david@freetype.org">david@freetype.org</a>)<br>
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© 2000 The FreeType Development Team (<a href="http://www.freetype.org">www.freetype.org</a>)</h3></center>
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<p><br>
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<hr WIDTH="100%">
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<br>
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<h2>Introduction:</h2>
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<ul>
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This document explains the concept of i/o <b>frames</b> as used in the
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FreeType 2 source code. It also enumerates the various functions and macros
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that can be used to read them.
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<p>
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It is targetted to FreeType hackers, or more simply to developers who would
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like a better understanding of the library's source code.
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</ul>
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<p><hr><p>
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<h2>I. What frames are:</h2>
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<ul>
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Simply speaking, a frame is an array of bytes in a font file that is
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"preloaded" into memory in order to be rapidly parsed. Frames are useful to
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ensure that every "load" is checked against end-of-file overruns, and
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provides nice functions to extract data in a variety of distinct formats.
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<p>
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But an example is certainly more meaningful than anything else.
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The following code:
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<p>
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<font color="blue"><pre>
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error = read_short(stream, &str.value1);
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if (error) goto ...
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error = read_ulong(stream, &str.value2);
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if (error) goto ...
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error = read_ulong(stream, &str.value3);
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if (error) goto ...
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</pre></font>
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can easily be replaced with:
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<p>
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<font color="blue"><pre>
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error = FT_Access_Frame(stream, 2+4+4);
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if (error) goto ...
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str.value1 = FT_Get_Short(stream);
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str.value2 = FT_Get_ULong(stream);
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str.value3 = FT_Get_ULong(stream);
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FT_Forget_Frame(stream);
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</pre></font>
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<p>
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Here, the call to <tt>FT_Access_Frame</tt> will:<p>
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<ul>
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<li>Ensure that there are at least 2+4+4=10 bytes left in the stream.
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<li>"Preload" (for disk-based streams) 10 bytes from the current
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stream position.
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<li>Set the frame "cursor" to the first byte in the frame;
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</ul>
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<p>
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Each <tt>FT_Get_Short</tt> or <tt>FT_Get_ULong</tt> call will read a
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big-endian integer from the stream (2 bytes for <tt>FT_Get_Short</tt>,
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4 bytes for <tt>FT_Get_ULong</tt>) and advance the frame cursor accordingly.
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<p>
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<tt>FT_Forget_Frame</tt> "releases" the frame from memory
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<p>
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There are several advantages to using frames :<p>
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<ul>
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<li>single-check when loading tables
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<li><em>making code clearer</em> by providing simple parsing functions
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<em>while keeping code safe</em> from file over-runs and invalid
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offsets.
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</ul>
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<p>
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</ul>
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<p><hr><p>
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<h2>II. Accessing and reading a frame with macros:</h2>
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<ul>
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By convention in the FreeType source code, macros are able to use two implicit
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variables named "<tt>error</tt>" and "<tt>stream</tt>". This is useful because
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these two variables are extremely used in the library, and doing this only
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reduces our typing requirements and make the source code much clearer.
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<p>
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Note that <tt>error</tt> must be a local variable of type <tt>FT_Error</tt>,<br>
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while <tt>stream</tt> must be a local variable or argument of type <tt>FT_Stream</tt>;
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<p>
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The macro used to access a frame is
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<font color="purple"><tt><b>ACCESS_Frame(_size_)</b></tt></font>, it will
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translate to:<p>
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<ul><font color="blue">
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<tt>(error=FT_Access_Frame(stream,_size_)) != FT_Err_Ok</tt>.
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</font></ul>
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<p>
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Similarly, the macro
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<font color="purple"><b><tt>FORGET_Frame()</tt></b></font>
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translates to:<p>
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<ul><font color="blue">
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<tt>FT_Forget_Frame(stream)</tt>
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</font></ul>
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<p>
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Extracting integers can be performed with the <tt>GET_xxx</tt> macros, like:<p>
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<ul>
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<table>
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<tr>
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<td><b>Macro name</b> <td>Translation <td>Description
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<tr><td><font color="purple"><tt><b>
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GET_Byte()
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</b></tt></font><td><font color="blue"><tt>
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(FT_Get_Byte(stream))
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</tt></font><td>
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reads an 8-bit unsigned byte
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<tr><td><font color="purple"><tt><b>
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GET_Char()
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</b></tt></font><td><font color="blue"><tt>
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((FT_Char)FT_Get_Byte(stream))
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</tt></font><td>
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reads an 8-bit <em>signed</em> byte
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<tr><td><font color="purple"><tt><b>
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GET_Short()
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</b></tt></font><td><font color="blue"><tt>
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(FT_Get_Short(stream))
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</tt></font><td>
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reads a 16-bit signed big-endian integer
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<tr><td><font color="purple"><tt><b>
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GET_UShort()
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</b></tt></font><td><font color="blue"><tt>
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((FT_UShort)FT_Get_Short(stream))
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</tt></font><td>
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reads a 16-bit unsigned big-endian integer
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<tr><td><font color="purple"><tt><b>
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GET_Offset()
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</b></tt></font><td><font color="blue"><tt>
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(FT_Get_Offset(stream))
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</tt></font><td>
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reads a 24-bit signed big-endian integer
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<tr><td><font color="purple"><tt><b>
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GET_UOffset()
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</b></tt></font><td><font color="blue"><tt>
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((FT_UOffset)FT_Get_Offset(stream))
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</tt></font><td>
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reads a 24-bit unsigned big-endian integer
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<tr><td><font color="purple"><tt><b>
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GET_Long()
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</b></tt></font><td><font color="blue"><tt>
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(FT_Get_Long(stream))
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</tt></font><td>
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reads a 32-bit signed big-endian integer
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<tr><td><font color="purple"><tt><b>
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GET_ULong()
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</b></tt></font><td><font color="blue"><tt>
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((FT_ULong)FT_Get_Long(stream))
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</tt></font><td>
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reads a 32-bit unsigned big-endian integer
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</table>
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</ul>
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<p>
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(Note that an <b>Offset</b> is an integer stored with 3 bytes on the file).
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<p>
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All this means that the following code:<p>
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<font color="blue"><pre>
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error = FT_Access_Frame(stream, 2+4+4);
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if (error) goto ...
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str.value1 = FT_Get_Short(stream);
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str.value2 = FT_Get_ULong(stream);
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str.value3 = FT_Get_ULong(stream);
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FT_Forget_Frame(stream);<br>
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</pre></font>
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<p>
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Can be replaced with macros by:<p>
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<font color="blue"><pre>
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if ( ACCESS_Frame( 2+4+4 ) ) goto ...
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str.value1 = GET_Short();
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str.value2 = GET_ULong();
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str.value3 = GET_ULong();
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FORGET_Frame();
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</pre></font>
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<p>
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Which is clearer. Notice that <b>error</b> and <b>stream</b> must be defined
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locally though for this code to work.. !!
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</ul>
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<p><hr><p>
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<h2>III. Alternatives:</h2>
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<ul>
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It is sometimes useful to read small integers from a font file without using
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a frame. Some functions have been introduced in FreeType 2 to do just that,
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and they are of the form <font color="blue"><tt>FT_Read_xxxx</tt></font>.
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<p>
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For example,
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<font color="blue"><tt>FT_Read_Short( stream, &error )</tt></font> reads and
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returns a 2-byte big-endian integer from a <tt>stream</tt>, and place an
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error code in the <tt>error</tt> variable.
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<p>
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Thus, reading a single big-endian integer is shorter than using a frame
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for it.
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<p>
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Note that there is also the macros
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<font color="purple"><tt>READ_xxx()</tt></font> which translate to:<p>
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<font color="blue"><pre>
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<tt>( FT_Read_xxx(stream,&error), error != FT_Err_Ok )
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</pre></font>
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<p>
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and can be used as in:<p>
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<font color="blue"><pre>
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if ( READ_UShort(variable1) || READ_ULong (variable2) ) goto Fail;
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</pre></font>
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<p>
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when <b>error</b> and <b>stream</b> are already defined locally..
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</ul>
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