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 <a name="System-Headers"></a>
 <div class="header">
 <p>
 Previous: <a href="Wrapper-Headers.html#Wrapper-Headers" accesskey="p" rel="prev">Wrapper Headers</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="System-Headers-1"></a>
 <h3 class="section">2.8 System Headers</h3>
 <a name="index-system-header-files-1"></a>

 <p>The header files declaring interfaces to the operating system and
 runtime libraries often cannot be written in strictly conforming C.
 Therefore, GCC gives code found in <em>system headers</em> special
 treatment.  All warnings, other than those generated by &lsquo;<samp>#warning</samp>&rsquo;
 (see <a href="Diagnostics.html#Diagnostics">Diagnostics</a>), are suppressed while GCC is processing a system
 header.  Macros defined in a system header are immune to a few warnings
 wherever they are expanded.  This immunity is granted on an ad-hoc
 basis, when we find that a warning generates lots of false positives
 because of code in macros defined in system headers.
 </p>
 <p>Normally, only the headers found in specific directories are considered
 system headers.  These directories are determined when GCC is compiled.
 There are, however, two ways to make normal headers into system headers.
 </p>
 <p>The <samp>-isystem</samp> command line option adds its argument to the list of
 directories to search for headers, just like <samp>-I</samp>.  Any headers
 found in that directory will be considered system headers.
 </p>
 <p>All directories named by <samp>-isystem</samp> are searched <em>after</em> all
 directories named by <samp>-I</samp>, no matter what their order was on the
 command line.  If the same directory is named by both <samp>-I</samp> and
 <samp>-isystem</samp>, the <samp>-I</samp> option is ignored.  GCC provides an
 informative message when this occurs if <samp>-v</samp> is used.
 </p>
 <a name="index-_0023pragma-GCC-system_005fheader"></a>
 <p>There is also a directive, <code>#pragma&nbsp;GCC&nbsp;<span class="nolinebreak">system_header</span><!-- /@w --></code>, which
 tells GCC to consider the rest of the current include file a system
 header, no matter where it was found.  Code that comes before the
 &lsquo;<samp>#pragma</samp>&rsquo; in the file will not be affected.  <code>#pragma&nbsp;GCC&nbsp;<span class="nolinebreak">system_header</span><!-- /@w --></code> has no effect in the primary source file.
 </p>
 <p>On very old systems, some of the pre-defined system header directories
 get even more special treatment.  GNU C++ considers code in headers
 found in those directories to be surrounded by an <code>extern&nbsp;&quot;C&quot;<!-- /@w --></code>
 block.  There is no way to request this behavior with a &lsquo;<samp>#pragma</samp>&rsquo;,
 or from the command line.
 </p>
 <hr>
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 <p>
 Previous: <a href="Wrapper-Headers.html#Wrapper-Headers" accesskey="p" rel="prev">Wrapper Headers</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="System-Headers"></a>
	<div class="header">
	<p>
	Previous: <a href="Wrapper-Headers.html#Wrapper-Headers" accesskey="p" rel="prev">Wrapper Headers</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="System-Headers-1"></a>
	<h3 class="section">2.8 System Headers</h3>
	<a name="index-system-header-files-1"></a>

	<p>The header files declaring interfaces to the operating system and
	runtime libraries often cannot be written in strictly conforming C.
	Therefore, GCC gives code found in <em>system headers</em> special
	treatment. All warnings, other than those generated by ‘<samp>#warning</samp>’
	(see <a href="Diagnostics.html#Diagnostics">Diagnostics</a>), are suppressed while GCC is processing a system
	header. Macros defined in a system header are immune to a few warnings
	wherever they are expanded. This immunity is granted on an ad-hoc
	basis, when we find that a warning generates lots of false positives
	because of code in macros defined in system headers.
	</p>
	<p>Normally, only the headers found in specific directories are considered
	system headers. These directories are determined when GCC is compiled.
	There are, however, two ways to make normal headers into system headers.
	</p>
	<p>The <samp>-isystem</samp> command line option adds its argument to the list of
	directories to search for headers, just like <samp>-I</samp>. Any headers
	found in that directory will be considered system headers.
	</p>
	<p>All directories named by <samp>-isystem</samp> are searched <em>after</em> all
	directories named by <samp>-I</samp>, no matter what their order was on the
	command line. If the same directory is named by both <samp>-I</samp> and
	<samp>-isystem</samp>, the <samp>-I</samp> option is ignored. GCC provides an
	informative message when this occurs if <samp>-v</samp> is used.
	</p>
	<a name="index-_0023pragma-GCC-system_005fheader"></a>
	<p>There is also a directive, <code>#pragma GCC <span class="nolinebreak">system_header</span><!-- /@w --></code>, which
	tells GCC to consider the rest of the current include file a system
	header, no matter where it was found. Code that comes before the
	‘<samp>#pragma</samp>’ in the file will not be affected. <code>#pragma GCC <span class="nolinebreak">system_header</span><!-- /@w --></code> has no effect in the primary source file.
	</p>
	<p>On very old systems, some of the pre-defined system header directories
	get even more special treatment. GNU C++ considers code in headers
	found in those directories to be surrounded by an <code>extern "C"<!-- /@w --></code>
	block. There is no way to request this behavior with a ‘<samp>#pragma</samp>’,
	or from the command line.
	</p>
	<hr>
	<div class="header">
	<p>
	Previous: <a href="Wrapper-Headers.html#Wrapper-Headers" accesskey="p" rel="prev">Wrapper Headers</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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