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| <title>The C Preprocessor: Search Path</title> |
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| <a name="Search-Path-1"></a> |
| <h3 class="section">2.3 Search Path</h3> |
| |
| <p>GCC looks in several different places for headers. On a normal Unix |
| system, if you do not instruct it otherwise, it will look for headers |
| requested with <code>#include <<var>file</var>><!-- /@w --></code> in: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">/usr/local/include |
| <var>libdir</var>/gcc/<var>target</var>/<var>version</var>/include |
| /usr/<var>target</var>/include |
| /usr/include |
| </pre></div> |
| |
| <p>For C++ programs, it will also look in |
| <samp><var>libdir</var>/../include/c++/<var>version</var></samp>, |
| first. In the above, <var>target</var> is the canonical name of the system |
| GCC was configured to compile code for; often but not always the same as |
| the canonical name of the system it runs on. <var>version</var> is the |
| version of GCC in use. |
| </p> |
| <p>You can add to this list with the <samp>-I<var>dir</var></samp> command line |
| option. All the directories named by <samp>-I</samp> are searched, in |
| left-to-right order, <em>before</em> the default directories. The only |
| exception is when <samp>dir</samp> is already searched by default. In |
| this case, the option is ignored and the search order for system |
| directories remains unchanged. |
| </p> |
| <p>Duplicate directories are removed from the quote and bracket search |
| chains before the two chains are merged to make the final search chain. |
| Thus, it is possible for a directory to occur twice in the final search |
| chain if it was specified in both the quote and bracket chains. |
| </p> |
| <p>You can prevent GCC from searching any of the default directories with |
| the <samp>-nostdinc</samp> option. This is useful when you are compiling an |
| operating system kernel or some other program that does not use the |
| standard C library facilities, or the standard C library itself. |
| <samp>-I</samp> options are not ignored as described above when |
| <samp>-nostdinc</samp> is in effect. |
| </p> |
| <p>GCC looks for headers requested with <code>#include "<var>file</var>"<!-- /@w --></code> |
| first in the directory containing the current file, then in the |
| directories as specified by <samp>-iquote</samp> options, then in the same |
| places it would have looked for a header requested with angle |
| brackets. For example, if <samp>/usr/include/sys/stat.h</samp> contains |
| <code>#include "types.h"<!-- /@w --></code>, GCC looks for <samp>types.h</samp> first in |
| <samp>/usr/include/sys</samp>, then in its usual search path. |
| </p> |
| <p>‘<samp>#line</samp>’ (see <a href="Line-Control.html#Line-Control">Line Control</a>) does not change GCC’s idea of the |
| directory containing the current file. |
| </p> |
| <p>You may put <samp>-I-</samp> at any point in your list of <samp>-I</samp> options. |
| This has two effects. First, directories appearing before the |
| <samp>-I-</samp> in the list are searched only for headers requested with |
| quote marks. Directories after <samp>-I-</samp> are searched for all |
| headers. Second, the directory containing the current file is not |
| searched for anything, unless it happens to be one of the directories |
| named by an <samp>-I</samp> switch. <samp>-I-</samp> is deprecated, <samp>-iquote</samp> |
| should be used instead. |
| </p> |
| <p><samp>-I. -I-</samp> is not the same as no <samp>-I</samp> options at all, and does |
| not cause the same behavior for ‘<samp><></samp>’ includes that ‘<samp>""</samp>’ |
| includes get with no special options. <samp>-I.</samp> searches the |
| compiler’s current working directory for header files. That may or may |
| not be the same as the directory containing the current file. |
| </p> |
| <p>If you need to look for headers in a directory named <samp>-</samp>, write |
| <samp>-I./-</samp>. |
| </p> |
| <p>There are several more ways to adjust the header search path. They are |
| generally less useful. See <a href="Invocation.html#Invocation">Invocation</a>. |
| </p> |
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| Next: <a href="Once_002dOnly-Headers.html#Once_002dOnly-Headers" accesskey="n" rel="next">Once-Only Headers</a>, Previous: <a href="Include-Operation.html#Include-Operation" accesskey="p" rel="prev">Include Operation</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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