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100 lines
4.5 KiB
HTML
100 lines
4.5 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Once-Only Headers - The C Preprocessor</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="The C Preprocessor">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Header-Files.html#Header-Files" title="Header Files">
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<link rel="prev" href="Search-Path.html#Search-Path" title="Search Path">
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<link rel="next" href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef" title="Alternatives to Wrapper #ifndef">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1987-2015 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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-->
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<meta http-equiv="Content-Style-Type" content="text/css">
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<style type="text/css"><!--
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pre.display { font-family:inherit }
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pre.format { font-family:inherit }
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pre.smalldisplay { font-family:inherit; font-size:smaller }
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pre.smallformat { font-family:inherit; font-size:smaller }
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pre.smallexample { font-size:smaller }
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pre.smalllisp { font-size:smaller }
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span.sc { font-variant:small-caps }
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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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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Once-Only-Headers"></a>
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<a name="Once_002dOnly-Headers"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef">Alternatives to Wrapper #ifndef</a>,
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Previous: <a rel="previous" accesskey="p" href="Search-Path.html#Search-Path">Search Path</a>,
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Up: <a rel="up" accesskey="u" href="Header-Files.html#Header-Files">Header Files</a>
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<hr>
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</div>
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<h3 class="section">2.4 Once-Only Headers</h3>
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<p><a name="index-repeated-inclusion-28"></a><a name="index-including-just-once-29"></a><a name="index-wrapper-_0040code_007b_0023ifndef_007d-30"></a>
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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>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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<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>
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<p>This construct is commonly known as a <dfn>wrapper #ifndef</dfn>.
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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>CPP optimizes even further. It remembers when a header file has a
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wrapper ‘<samp><span class="samp">#ifndef</span></samp>’. If a subsequent ‘<samp><span class="samp">#include</span></samp>’ specifies that
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header, and the macro in the ‘<samp><span class="samp">#ifndef</span></samp>’ is still defined, it does
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not bother to rescan the file at all.
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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><a name="index-controlling-macro-31"></a><a name="index-guard-macro-32"></a>The macro <code>FILE_FOO_SEEN</code> is called the <dfn>controlling macro</dfn> or
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<dfn>guard macro</dfn>. In a user header file, the macro name should not
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begin with ‘<samp><span class="samp">_</span></samp>’. In a system header file, it should begin with
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‘<samp><span class="samp">__</span></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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</body></html>
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