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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&nbsp;&lt;<var>file</var>&gt;<!-- /@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&nbsp;&quot;<var>file</var>&quot;<!-- /@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&nbsp;&quot;types.h&quot;<!-- /@w --></code>, GCC looks for <samp>types.h</samp> first in
<samp>/usr/include/sys</samp>, then in its usual search path.
</p>
<p>&lsquo;<samp>#line</samp>&rsquo; (see <a href="Line-Control.html#Line-Control">Line Control</a>) does not change GCC&rsquo;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 &lsquo;<samp>&lt;&gt;</samp>&rsquo; includes that &lsquo;<samp>&quot;&quot;</samp>&rsquo;
includes get with no special options.  <samp>-I.</samp> searches the
compiler&rsquo;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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