135 lines
6.0 KiB
HTML
135 lines
6.0 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1987-2016 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation. A copy of
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the license is included in the
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section entitled "GNU Free Documentation License".
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This manual contains no Invariant Sections. The Front-Cover Texts are
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(a) (see below), and the Back-Cover Texts are (b) (see below).
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>The C Preprocessor: Once-Only Headers</title>
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<meta name="description" content="The C Preprocessor: Once-Only Headers">
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<meta name="keywords" content="The C Preprocessor: Once-Only Headers">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Header-Files.html#Header-Files" rel="up" title="Header Files">
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<link href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef" rel="next" title="Alternatives to Wrapper #ifndef">
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<link href="Search-Path.html#Search-Path" rel="prev" title="Search Path">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.indentedblock {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style:oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nocodebreak {white-space:nowrap}
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span.nolinebreak {white-space:nowrap}
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span.roman {font-family:serif; font-weight:normal}
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span.sansserif {font-family:sans-serif; font-weight:normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Once_002dOnly-Headers"></a>
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<div class="header">
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<p>
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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="prev">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>
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</div>
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<hr>
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<a name="Once_002dOnly-Headers-1"></a>
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<h3 class="section">2.4 Once-Only Headers</h3>
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<a name="index-repeated-inclusion"></a>
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<a name="index-including-just-once"></a>
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<a name="index-wrapper-_0023ifndef"></a>
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<p>If a header file happens to be included twice, the compiler will process
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its contents twice. This is very likely to cause an error, e.g. when the
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compiler sees the same structure definition twice. Even if it does not,
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it will certainly waste time.
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</p>
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<p>The standard way to prevent this is to enclose the entire real contents
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of the file in a conditional, like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">/* File foo. */
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#ifndef FILE_FOO_SEEN
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#define FILE_FOO_SEEN
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<var>the entire file</var>
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#endif /* !FILE_FOO_SEEN */
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</pre></div>
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<p>This construct is commonly known as a <em>wrapper #ifndef</em>.
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When the header is included again, the conditional will be false,
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because <code>FILE_FOO_SEEN</code> is defined. The preprocessor will skip
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over the entire contents of the file, and the compiler will not see it
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twice.
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</p>
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<p>CPP optimizes even further. It remembers when a header file has a
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wrapper ‘<samp>#ifndef</samp>’. If a subsequent ‘<samp>#include</samp>’ specifies that
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header, and the macro in the ‘<samp>#ifndef</samp>’ is still defined, it does
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not bother to rescan the file at all.
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</p>
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<p>You can put comments outside the wrapper. They will not interfere with
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this optimization.
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</p>
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<a name="index-controlling-macro"></a>
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<a name="index-guard-macro"></a>
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<p>The macro <code>FILE_FOO_SEEN</code> is called the <em>controlling macro</em> or
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<em>guard macro</em>. In a user header file, the macro name should not
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begin with ‘<samp>_</samp>’. In a system header file, it should begin with
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‘<samp>__</samp>’ to avoid conflicts with user programs. In any kind of header
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file, the macro name should contain the name of the file and some
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additional text, to avoid conflicts with other header files.
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</p>
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<hr>
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<div class="header">
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<p>
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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="prev">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>
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</div>
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</body>
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</html>
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