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| 22 | <title>Once-Only Headers (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Once-Only Headers (GNU C Language Manual)">
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Header-Files.html" rel="up" title="Header Files">
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| 33 | <link href="Computed-Includes.html" rel="next" title="Computed Includes">
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| 54 |
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Once_002dOnly-Headers"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Computed-Includes.html" accesskey="n" rel="next">Computed Includes</a>, Previous: <a href="Search-Path.html" accesskey="p" rel="prev">Search Path</a>, Up: <a href="Header-Files.html" accesskey="u" rel="up">Header Files</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Once_002dOnly-Headers-1"></span><h4 class="subsection">26.4.4 Once-Only Headers</h4>
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| 64 | <span id="index-repeated-inclusion"></span>
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| 65 | <span id="index-including-just-once"></span>
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| 66 | <span id="index-wrapper-_0023ifndef"></span>
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| 67 |
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| 68 | <p>If a header file happens to be included twice, the compiler will process
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| 69 | its contents twice. This is very likely to cause an error, e.g. when the
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| 70 | compiler sees the same structure definition twice.
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| 71 | </p>
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| 72 | <p>The standard way to prevent this is to enclose the entire real contents
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| 73 | of the file in a conditional, like this:
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| 74 | </p>
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| 75 | <div class="example">
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| 76 | <pre class="example">/* File foo. */
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| 77 | #ifndef FILE_FOO_SEEN
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| 78 | #define FILE_FOO_SEEN
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| 79 |
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| 80 | <var>the entire file</var>
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| 81 |
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| 82 | #endif /* !FILE_FOO_SEEN */
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| 83 | </pre></div>
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| 84 |
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| 85 | <p>This construct is commonly known as a <em>wrapper #ifndef</em>. When the
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| 86 | header is included again, the conditional will be false, because
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| 87 | <code>FILE_FOO_SEEN</code> is defined. Preprocessing skips over the entire
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| 88 | contents of the file, so that compilation will never “see” the file
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| 89 | contents twice in one module.
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| 90 | </p>
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| 91 | <p>GCC optimizes this case even further. It remembers when a header file
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| 92 | has a wrapper <code>#ifndef</code>. If a subsequent <code>#include</code>
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| 93 | specifies that header, and the macro in the <code>#ifndef</code> is still
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| 94 | defined, it does not bother to rescan the file at all.
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| 95 | </p>
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| 96 | <p>You can put comments in the header file outside the wrapper. They
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| 97 | do not interfere with this optimization.
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| 98 | </p>
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| 99 | <span id="index-controlling-macro"></span>
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| 100 | <span id="index-guard-macro"></span>
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| 101 | <p>The macro <code>FILE_FOO_SEEN</code> is called the <em>controlling macro</em>
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| 102 | or <em>guard macro</em>. In a user header file, the macro name should
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| 103 | not begin with ‘<samp>_</samp>’. In a system header file, it should begin
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| 104 | with ‘<samp>__</samp>’ (or ‘<samp>_</samp>’ followed by an upper-case letter) to avoid
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| 105 | conflicts with user programs. In any kind of header file, the macro
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| 106 | name should contain the name of the file and some additional text, to
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| 107 | avoid conflicts with other header files.
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| 108 | </p>
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| 109 | <hr>
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| 110 | <div class="header">
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| 111 | <p>
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| 112 | Next: <a href="Computed-Includes.html" accesskey="n" rel="next">Computed Includes</a>, Previous: <a href="Search-Path.html" accesskey="p" rel="prev">Search Path</a>, Up: <a href="Header-Files.html" accesskey="u" rel="up">Header Files</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 113 | </div>
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| 114 |
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| 115 |
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| 116 |
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