| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1987-2015 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 5.2, 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="prev" 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="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> |
| </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="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> |
| </div> |
| |
| |
| |
| </body> |
| </html> |
