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137 lines
6.8 KiB
HTML
137 lines
6.8 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1987-2018 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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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Wrapper Headers (The C Preprocessor)</title>
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<meta name="description" content="Wrapper Headers (The C Preprocessor)">
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<meta name="keywords" content="Wrapper Headers (The C Preprocessor)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Header-Files.html#Header-Files" rel="up" title="Header Files">
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<link href="System-Headers.html#System-Headers" rel="next" title="System Headers">
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<link href="Computed-Includes.html#Computed-Includes" rel="prev" title="Computed Includes">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Wrapper-Headers"></a>
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<div class="header">
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<p>
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Next: <a href="System-Headers.html#System-Headers" accesskey="n" rel="next">System Headers</a>, Previous: <a href="Computed-Includes.html#Computed-Includes" accesskey="p" rel="prev">Computed Includes</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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</div>
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<hr>
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<a name="Wrapper-Headers-1"></a>
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<h3 class="section">2.7 Wrapper Headers</h3>
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<a name="index-wrapper-headers"></a>
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<a name="index-overriding-a-header-file"></a>
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<a name="index-_0023include_005fnext"></a>
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<p>Sometimes it is necessary to adjust the contents of a system-provided
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header file without editing it directly. GCC’s <code>fixincludes</code>
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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>
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<p>You cannot simply include the old header with ‘<samp>#include</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>
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<p>You could include the old header with an absolute pathname:
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</p><div class="smallexample">
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<pre class="smallexample">#include "/usr/include/old-header.h"
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</pre></div>
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<p>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>
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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>#include_next</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>#include</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>
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<p>Suppose you specify <samp>-I /usr/local/include</samp>, and the list of
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directories to search also includes <samp>/usr/include</samp>; and suppose
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both directories contain <samp>signal.h</samp>. Ordinary <code>#include <signal.h><!-- /@w --></code> finds the file under <samp>/usr/local/include</samp>. If that
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file contains <code><span class="nolinebreak">#include_next</span> <signal.h><!-- /@w --></code>, it starts searching
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after that directory, and finds the file in <samp>/usr/include</samp>.
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</p>
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<p>‘<samp>#include_next</samp>’ does not distinguish between <code><<var>file</var>></code>
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and <code>"<var>file</var>"</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>
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<p>The use of ‘<samp>#include_next</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 <code>fixincludes</code>.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="System-Headers.html#System-Headers" accesskey="n" rel="next">System Headers</a>, Previous: <a href="Computed-Includes.html#Computed-Includes" accesskey="p" rel="prev">Computed Includes</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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</div>
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</body>
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</html>
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