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95 lines
4.7 KiB
HTML
95 lines
4.7 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>System 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="Wrapper-Headers.html#Wrapper-Headers" title="Wrapper Headers">
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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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<style type="text/css"><!--
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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="System-Headers"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Wrapper-Headers.html#Wrapper-Headers">Wrapper Headers</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">2.8 System Headers</h3>
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<p><a name="index-system-header-files-39"></a>
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The header files declaring interfaces to the operating system and
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runtime libraries often cannot be written in strictly conforming C.
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Therefore, GCC gives code found in <dfn>system headers</dfn> special
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treatment. All warnings, other than those generated by ‘<samp><span class="samp">#warning</span></samp>’
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(see <a href="Diagnostics.html#Diagnostics">Diagnostics</a>), are suppressed while GCC is processing a system
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header. Macros defined in a system header are immune to a few warnings
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wherever they are expanded. This immunity is granted on an ad-hoc
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basis, when we find that a warning generates lots of false positives
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because of code in macros defined in system headers.
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<p>Normally, only the headers found in specific directories are considered
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system headers. These directories are determined when GCC is compiled.
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There are, however, two ways to make normal headers into system headers.
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<p>The <samp><span class="option">-isystem</span></samp> command-line option adds its argument to the list of
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directories to search for headers, just like <samp><span class="option">-I</span></samp>. Any headers
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found in that directory will be considered system headers.
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<p>All directories named by <samp><span class="option">-isystem</span></samp> are searched <em>after</em> all
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directories named by <samp><span class="option">-I</span></samp>, no matter what their order was on the
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command line. If the same directory is named by both <samp><span class="option">-I</span></samp> and
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<samp><span class="option">-isystem</span></samp>, the <samp><span class="option">-I</span></samp> option is ignored. GCC provides an
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informative message when this occurs if <samp><span class="option">-v</span></samp> is used.
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<p><a name="index-g_t_0023pragma-GCC-system_005fheader-40"></a>There is also a directive, <code>#pragma GCC system_header<!-- /@w --></code>, which
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tells GCC to consider the rest of the current include file a system
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header, no matter where it was found. Code that comes before the
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‘<samp><span class="samp">#pragma</span></samp>’ in the file will not be affected. <code>#pragma GCC system_header<!-- /@w --></code> has no effect in the primary source file.
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<p>On very old systems, some of the pre-defined system header directories
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get even more special treatment. GNU C++ considers code in headers
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found in those directories to be surrounded by an <code>extern "C"<!-- /@w --></code>
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block. There is no way to request this behavior with a ‘<samp><span class="samp">#pragma</span></samp>’,
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or from the command line.
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</body></html>
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