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diff --git a/share/doc/cpp/Once_002dOnly-Headers.html b/share/doc/cpp/Once_002dOnly-Headers.html new file mode 100644 index 0000000..612b60d --- /dev/null +++ b/share/doc/cpp/Once_002dOnly-Headers.html @@ -0,0 +1,134 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1987-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation. A copy of +the license is included in the +section entitled "GNU Free Documentation License". + +This manual contains no Invariant Sections. The Front-Cover Texts are +(a) (see below), and the Back-Cover Texts are (b) (see below). + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>The C Preprocessor: Once-Only Headers</title> + +<meta name="description" content="The C Preprocessor: Once-Only Headers"> +<meta name="keywords" content="The C Preprocessor: Once-Only Headers"> +<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="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Header-Files.html#Header-Files" rel="up" title="Header Files"> +<link href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef" rel="next" title="Alternatives to Wrapper #ifndef"> +<link href="Search-Path.html#Search-Path" rel="previous" title="Search Path"> +<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="Once_002dOnly-Headers"></a> +<div class="header"> +<p> +Next: <a href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef" accesskey="n" rel="next">Alternatives to Wrapper #ifndef</a>, Previous: <a href="Search-Path.html#Search-Path" accesskey="p" rel="previous">Search Path</a>, Up: <a href="Header-Files.html#Header-Files" accesskey="u" rel="up">Header Files</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Once_002dOnly-Headers-1"></a> +<h3 class="section">2.4 Once-Only Headers</h3> +<a name="index-repeated-inclusion"></a> +<a name="index-including-just-once"></a> +<a name="index-wrapper-_0023ifndef"></a> + +<p>If a header file happens to be included twice, the compiler will process +its contents twice. This is very likely to cause an error, e.g. when the +compiler sees the same structure definition twice. Even if it does not, +it will certainly waste time. +</p> +<p>The standard way to prevent this is to enclose the entire real contents +of the file in a conditional, like this: +</p> +<div class="smallexample"> +<pre class="smallexample">/* File foo. */ +#ifndef FILE_FOO_SEEN +#define FILE_FOO_SEEN + +<var>the entire file</var> + +#endif /* !FILE_FOO_SEEN */ +</pre></div> + +<p>This construct is commonly known as a <em>wrapper #ifndef</em>. +When the header is included again, the conditional will be false, +because <code>FILE_FOO_SEEN</code> is defined. The preprocessor will skip +over the entire contents of the file, and the compiler will not see it +twice. +</p> +<p>CPP optimizes even further. It remembers when a header file has a +wrapper ‘<samp>#ifndef</samp>’. If a subsequent ‘<samp>#include</samp>’ specifies that +header, and the macro in the ‘<samp>#ifndef</samp>’ is still defined, it does +not bother to rescan the file at all. +</p> +<p>You can put comments outside the wrapper. They will not interfere with +this optimization. +</p> +<a name="index-controlling-macro"></a> +<a name="index-guard-macro"></a> +<p>The macro <code>FILE_FOO_SEEN</code> is called the <em>controlling macro</em> or +<em>guard macro</em>. In a user header file, the macro name should not +begin with ‘<samp>_</samp>’. In a system header file, it should begin with +‘<samp>__</samp>’ to avoid conflicts with user programs. In any kind of header +file, the macro name should contain the name of the file and some +additional text, to avoid conflicts with other header files. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef" accesskey="n" rel="next">Alternatives to Wrapper #ifndef</a>, Previous: <a href="Search-Path.html#Search-Path" accesskey="p" rel="previous">Search Path</a>, Up: <a href="Header-Files.html#Header-Files" accesskey="u" rel="up">Header Files</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |