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103 lines
5.3 KiB
HTML
103 lines
5.3 KiB
HTML
4 years ago
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<title>Files - The GNU C Preprocessor Internals</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="The GNU C Preprocessor Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="prev" href="Guard-Macros.html#Guard-Macros" title="Guard Macros">
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<link rel="next" href="Concept-Index.html#Concept-Index" title="Concept Index">
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</head>
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<body>
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<div class="node">
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<a name="Files"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Concept-Index.html#Concept-Index">Concept Index</a>,
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Previous: <a rel="previous" accesskey="p" href="Guard-Macros.html#Guard-Macros">Guard Macros</a>,
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Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
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<hr>
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</div>
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<h2 class="unnumbered">File Handling</h2>
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<p><a name="index-files-21"></a>
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Fairly obviously, the file handling code of cpplib resides in the file
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<samp><span class="file">files.c</span></samp>. It takes care of the details of file searching,
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opening, reading and caching, for both the main source file and all the
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headers it recursively includes.
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<p>The basic strategy is to minimize the number of system calls. On many
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systems, the basic <code>open ()</code> and <code>fstat ()</code> system calls can
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be quite expensive. For every <code>#include</code>-d file, we need to try
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all the directories in the search path until we find a match. Some
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projects, such as glibc, pass twenty or thirty include paths on the
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command line, so this can rapidly become time consuming.
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<p>For a header file we have not encountered before we have little choice
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but to do this. However, it is often the case that the same headers are
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repeatedly included, and in these cases we try to avoid repeating the
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filesystem queries whilst searching for the correct file.
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<p>For each file we try to open, we store the constructed path in a splay
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tree. This path first undergoes simplification by the function
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<code>_cpp_simplify_pathname</code>. For example,
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<samp><span class="file">/usr/include/bits/../foo.h</span></samp> is simplified to
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<samp><span class="file">/usr/include/foo.h</span></samp> before we enter it in the splay tree and try
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to <code>open ()</code> the file. CPP will then find subsequent uses of
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<samp><span class="file">foo.h</span></samp>, even as <samp><span class="file">/usr/include/foo.h</span></samp>, in the splay tree and
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save system calls.
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<p>Further, it is likely the file contents have also been cached, saving a
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<code>read ()</code> system call. We don't bother caching the contents of
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header files that are re-inclusion protected, and whose re-inclusion
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macro is defined when we leave the header file for the first time. If
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the host supports it, we try to map suitably large files into memory,
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rather than reading them in directly.
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<p>The include paths are internally stored on a null-terminated
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singly-linked list, starting with the <code>"header.h"</code> directory search
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chain, which then links into the <code><header.h></code> directory chain.
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<p>Files included with the <code><foo.h></code> syntax start the lookup directly
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in the second half of this chain. However, files included with the
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<code>"foo.h"</code> syntax start at the beginning of the chain, but with one
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extra directory prepended. This is the directory of the current file;
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the one containing the <code>#include</code> directive. Prepending this
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directory on a per-file basis is handled by the function
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<code>search_from</code>.
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<p>Note that a header included with a directory component, such as
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<code>#include "mydir/foo.h"</code> and opened as
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<samp><span class="file">/usr/local/include/mydir/foo.h</span></samp>, will have the complete path minus
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the basename ‘<samp><span class="samp">foo.h</span></samp>’ as the current directory.
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<p>Enough information is stored in the splay tree that CPP can immediately
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tell whether it can skip the header file because of the multiple include
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optimization, whether the file didn't exist or couldn't be opened for
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some reason, or whether the header was flagged not to be re-used, as it
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is with the obsolete <code>#import</code> directive.
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<p>For the benefit of MS-DOS filesystems with an 8.3 filename limitation,
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CPP offers the ability to treat various include file names as aliases
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for the real header files with shorter names. The map from one to the
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other is found in a special file called ‘<samp><span class="samp">header.gcc</span></samp>’, stored in the
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command line (or system) include directories to which the mapping
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applies. This may be higher up the directory tree than the full path to
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the file minus the base name.
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</body></html>
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