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diff --git a/share/doc/cpp/Wrapper-Headers.html b/share/doc/cpp/Wrapper-Headers.html new file mode 100644 index 0000000..8f54504 --- /dev/null +++ b/share/doc/cpp/Wrapper-Headers.html @@ -0,0 +1,137 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1987-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation. A copy of +the license is included in the +section entitled "GNU Free Documentation License". + +This manual contains no Invariant Sections. The Front-Cover Texts are +(a) (see below), and the Back-Cover Texts are (b) (see below). + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>The C Preprocessor: Wrapper Headers</title> + +<meta name="description" content="The C Preprocessor: Wrapper Headers"> +<meta name="keywords" content="The C Preprocessor: Wrapper Headers"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Header-Files.html#Header-Files" rel="up" title="Header Files"> +<link href="System-Headers.html#System-Headers" rel="next" title="System Headers"> +<link href="Computed-Includes.html#Computed-Includes" rel="previous" title="Computed Includes"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Wrapper-Headers"></a> +<div class="header"> +<p> +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="previous">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> +</div> +<hr> +<a name="Wrapper-Headers-1"></a> +<h3 class="section">2.7 Wrapper Headers</h3> +<a name="index-wrapper-headers"></a> +<a name="index-overriding-a-header-file"></a> +<a name="index-_0023include_005fnext"></a> + +<p>Sometimes it is necessary to adjust the contents of a system-provided +header file without editing it directly. GCC’s <code>fixincludes</code> +operation does this, for example. One way to do that would be to create +a new header file with the same name and insert it in the search path +before the original header. That works fine as long as you’re willing +to replace the old header entirely. But what if you want to refer to +the old header from the new one? +</p> +<p>You cannot simply include the old header with ‘<samp>#include</samp>’. That +will start from the beginning, and find your new header again. If your +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. +</p> +<p>You could include the old header with an absolute pathname: +</p><div class="smallexample"> +<pre class="smallexample">#include "/usr/include/old-header.h" +</pre></div> +<p>This works, but is not clean; should the system headers ever move, you +would have to edit the new headers to match. +</p> +<p>There is no way to solve this problem within the C standard, but you can +use the GNU extension ‘<samp>#include_next</samp>’. It means, “Include the +<em>next</em> file with this name”. This directive works like +‘<samp>#include</samp>’ except in searching for the specified file: it starts +searching the list of header file directories <em>after</em> the directory +in which the current file was found. +</p> +<p>Suppose you specify <samp>-I /usr/local/include</samp>, and the list of +directories to search also includes <samp>/usr/include</samp>; and suppose +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 +file contains <code><span class="nolinebreak">#include_next</span> <signal.h><!-- /@w --></code>, it starts searching +after that directory, and finds the file in <samp>/usr/include</samp>. +</p> +<p>‘<samp>#include_next</samp>’ does not distinguish between <code><<var>file</var>></code> +and <code>"<var>file</var>"</code> inclusion, nor does it check that the file you +specify has the same name as the current file. It simply looks for the +file named, starting with the directory in the search path after the one +where the current file was found. +</p> +<p>The use of ‘<samp>#include_next</samp>’ can lead to great confusion. We +recommend it be used only when there is no other alternative. In +particular, it should not be used in the headers belonging to a specific +program; it should be used only to make global corrections along the +lines of <code>fixincludes</code>. +</p> +<hr> +<div class="header"> +<p> +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="previous">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> +</div> + + + +</body> +</html> |