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 <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="prev">Search Path</a>, Up: <a href="Header-Files.html#Header-Files" accesskey="u" rel="up">Header Files</a> &nbsp; [<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 &lsquo;<samp>#ifndef</samp>&rsquo;.  If a subsequent &lsquo;<samp>#include</samp>&rsquo; specifies that
 header, and the macro in the &lsquo;<samp>#ifndef</samp>&rsquo; 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 &lsquo;<samp>_</samp>&rsquo;.  In a system header file, it should begin with
 &lsquo;<samp>__</samp>&rsquo; 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="prev">Search Path</a>, Up: <a href="Header-Files.html#Header-Files" accesskey="u" rel="up">Header Files</a> &nbsp; [<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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	<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="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>
	</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="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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