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