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105 lines
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HTML
105 lines
5.5 KiB
HTML
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<title>Wrapper Headers - The C Preprocessor</title>
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<meta name="description" content="The C Preprocessor">
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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="Computed-Includes.html#Computed-Includes" title="Computed Includes">
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<link rel="next" href="System-Headers.html#System-Headers" title="System Headers">
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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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<a name="Wrapper-Headers"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="System-Headers.html#System-Headers">System Headers</a>,
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Previous: <a rel="previous" accesskey="p" href="Computed-Includes.html#Computed-Includes">Computed Includes</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">2.7 Wrapper Headers</h3>
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<p><a name="index-wrapper-headers-36"></a><a name="index-overriding-a-header-file-37"></a><a name="index-g_t_0023include_005fnext-38"></a>
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Sometimes it is necessary to adjust the contents of a system-provided
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header file without editing it directly. GCC's <samp><span class="command">fixincludes</span></samp>
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operation does this, for example. One way to do that would be to create
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a new header file with the same name and insert it in the search path
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before the original header. That works fine as long as you're willing
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to replace the old header entirely. But what if you want to refer to
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the old header from the new one?
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<p>You cannot simply include the old header with ‘<samp><span class="samp">#include</span></samp>’. That
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will start from the beginning, and find your new header again. If your
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header is not protected from multiple inclusion (see <a href="Once_002dOnly-Headers.html#Once_002dOnly-Headers">Once-Only Headers</a>), it will recurse infinitely and cause a fatal error.
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<p>You could include the old header with an absolute pathname:
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<pre class="smallexample"> #include "/usr/include/old-header.h"
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</pre>
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<p class="noindent">This works, but is not clean; should the system headers ever move, you
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would have to edit the new headers to match.
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<p>There is no way to solve this problem within the C standard, but you can
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use the GNU extension ‘<samp><span class="samp">#include_next</span></samp>’. It means, “Include the
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<em>next</em> file with this name”. This directive works like
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‘<samp><span class="samp">#include</span></samp>’ except in searching for the specified file: it starts
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searching the list of header file directories <em>after</em> the directory
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in which the current file was found.
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<p>Suppose you specify <samp><span class="option">-I /usr/local/include</span></samp>, and the list of
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directories to search also includes <samp><span class="file">/usr/include</span></samp>; and suppose
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both directories contain <samp><span class="file">signal.h</span></samp>. Ordinary <code>#include <signal.h><!-- /@w --></code> finds the file under <samp><span class="file">/usr/local/include</span></samp>. If that
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file contains <code>#include_next <signal.h><!-- /@w --></code>, it starts searching
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after that directory, and finds the file in <samp><span class="file">/usr/include</span></samp>.
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<p>‘<samp><span class="samp">#include_next</span></samp>’ does not distinguish between <code><</code><var>file</var><code>></code>
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and <code>"</code><var>file</var><code>"</code> inclusion, nor does it check that the file you
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specify has the same name as the current file. It simply looks for the
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file named, starting with the directory in the search path after the one
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where the current file was found.
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<p>The use of ‘<samp><span class="samp">#include_next</span></samp>’ can lead to great confusion. We
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recommend it be used only when there is no other alternative. In
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particular, it should not be used in the headers belonging to a specific
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program; it should be used only to make global corrections along the
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lines of <samp><span class="command">fixincludes</span></samp>.
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</body></html>
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