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diff --git a/share/doc/cppinternals/Files.html b/share/doc/cppinternals/Files.html new file mode 100644 index 0000000..68bfa42 --- /dev/null +++ b/share/doc/cppinternals/Files.html @@ -0,0 +1,135 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>The GNU C Preprocessor Internals: Files</title> + +<meta name="description" content="The GNU C Preprocessor Internals: Files"> +<meta name="keywords" content="The GNU C Preprocessor Internals: Files"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="index.html#Top" rel="up" title="Top"> +<link href="Concept-Index.html#Concept-Index" rel="next" title="Concept Index"> +<link href="Guard-Macros.html#Guard-Macros" rel="previous" title="Guard Macros"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Files"></a> +<div class="header"> +<p> +Next: <a href="Concept-Index.html#Concept-Index" accesskey="n" rel="next">Concept Index</a>, Previous: <a href="Guard-Macros.html#Guard-Macros" accesskey="p" rel="previous">Guard Macros</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="File-Handling"></a> +<h2 class="unnumbered">File Handling</h2> +<a name="index-files"></a> + +<p>Fairly obviously, the file handling code of cpplib resides in the file +<samp>files.cc</samp>. It takes care of the details of file searching, +opening, reading and caching, for both the main source file and all the +headers it recursively includes. +</p> +<p>The basic strategy is to minimize the number of system calls. On many +systems, the basic <code>open ()</code> and <code>fstat ()</code> system calls can +be quite expensive. For every <code>#include</code>-d file, we need to try +all the directories in the search path until we find a match. Some +projects, such as glibc, pass twenty or thirty include paths on the +command line, so this can rapidly become time consuming. +</p> +<p>For a header file we have not encountered before we have little choice +but to do this. However, it is often the case that the same headers are +repeatedly included, and in these cases we try to avoid repeating the +filesystem queries whilst searching for the correct file. +</p> +<p>For each file we try to open, we store the constructed path in a splay +tree. This path first undergoes simplification by the function +<code>_cpp_simplify_pathname</code>. For example, +<samp>/usr/include/bits/../foo.h</samp> is simplified to +<samp>/usr/include/foo.h</samp> before we enter it in the splay tree and try +to <code>open ()</code> the file. CPP will then find subsequent uses of +<samp>foo.h</samp>, even as <samp>/usr/include/foo.h</samp>, in the splay tree and +save system calls. +</p> +<p>Further, it is likely the file contents have also been cached, saving a +<code>read ()</code> system call. We don’t bother caching the contents of +header files that are re-inclusion protected, and whose re-inclusion +macro is defined when we leave the header file for the first time. If +the host supports it, we try to map suitably large files into memory, +rather than reading them in directly. +</p> +<p>The include paths are internally stored on a null-terminated +singly-linked list, starting with the <code>"header.h"</code> directory search +chain, which then links into the <code><header.h></code> directory chain. +</p> +<p>Files included with the <code><foo.h></code> syntax start the lookup directly +in the second half of this chain. However, files included with the +<code>"foo.h"</code> syntax start at the beginning of the chain, but with one +extra directory prepended. This is the directory of the current file; +the one containing the <code>#include</code> directive. Prepending this +directory on a per-file basis is handled by the function +<code>search_from</code>. +</p> +<p>Note that a header included with a directory component, such as +<code>#include "mydir/foo.h"</code> and opened as +<samp>/usr/local/include/mydir/foo.h</samp>, will have the complete path minus +the basename ‘<samp>foo.h</samp>’ as the current directory. +</p> +<p>Enough information is stored in the splay tree that CPP can immediately +tell whether it can skip the header file because of the multiple include +optimization, whether the file didn’t exist or couldn’t be opened for +some reason, or whether the header was flagged not to be re-used, as it +is with the obsolete <code>#import</code> directive. +</p> +<p>For the benefit of MS-DOS filesystems with an 8.3 filename limitation, +CPP offers the ability to treat various include file names as aliases +for the real header files with shorter names. The map from one to the +other is found in a special file called ‘<samp>header.gcc</samp>’, stored in the +command line (or system) include directories to which the mapping +applies. This may be higher up the directory tree than the full path to +the file minus the base name. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Concept-Index.html#Concept-Index" accesskey="n" rel="next">Concept Index</a>, Previous: <a href="Guard-Macros.html#Guard-Macros" accesskey="p" rel="previous">Guard Macros</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |