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| <h3 class="section">2.7 Wrapper Headers</h3> |
| |
| <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> |
| Sometimes it is necessary to adjust the contents of a system-provided |
| header file without editing it directly. GCC's <samp><span class="command">fixincludes</span></samp> |
| 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>You cannot simply include the old header with ‘<samp><span class="samp">#include</span></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>You could include the old header with an absolute pathname: |
| <pre class="smallexample"> #include "/usr/include/old-header.h" |
| </pre> |
| <p class="noindent">This works, but is not clean; should the system headers ever move, you |
| would have to edit the new headers to match. |
| |
| <p>There is no way to solve this problem within the C standard, but you can |
| use the GNU extension ‘<samp><span class="samp">#include_next</span></samp>’. It means, “Include the |
| <em>next</em> file with this name”. This directive works like |
| ‘<samp><span class="samp">#include</span></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>Suppose you specify <samp><span class="option">-I /usr/local/include</span></samp>, and the list of |
| directories to search also includes <samp><span class="file">/usr/include</span></samp>; and suppose |
| 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 |
| file contains <code>#include_next <signal.h><!-- /@w --></code>, it starts searching |
| after that directory, and finds the file in <samp><span class="file">/usr/include</span></samp>. |
| |
| <p>‘<samp><span class="samp">#include_next</span></samp>’ does not distinguish between <code><</code><var>file</var><code>></code> |
| and <code>"</code><var>file</var><code>"</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>The use of ‘<samp><span class="samp">#include_next</span></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 <samp><span class="command">fixincludes</span></samp>. |
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