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<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;</code><var>file</var><code>&gt;<!-- /@w --></code> in:

<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>
   <p>For C++ programs, it will also look in <samp><span class="file">/usr/include/g++-v3</span></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>You can add to this list with the <samp><span class="option">-I</span><var>dir</var></samp> command line
option.  All the directories named by <samp><span class="option">-I</span></samp> are searched, in
left-to-right order, <em>before</em> the default directories.  The only
exception is when <samp><span class="file">dir</span></samp> is already searched by default.  In
this case, the option is ignored and the search order for system
directories remains unchanged.

   <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>You can prevent GCC from searching any of the default directories with
the <samp><span class="option">-nostdinc</span></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><span class="option">-I</span></samp> options are not ignored as described above when
<samp><span class="option">-nostdinc</span></samp> is in effect.

   <p>GCC looks for headers requested with <code>#include&nbsp;"</code><var>file</var><code>"<!-- /@w --></code>
first in the directory containing the current file, then in the
directories as specified by <samp><span class="option">-iquote</span></samp> options, then in the same
places it would have looked for a header requested with angle
brackets.  For example, if <samp><span class="file">/usr/include/sys/stat.h</span></samp> contains
<code>#include&nbsp;"types.h"<!-- /@w --></code>, GCC looks for <samp><span class="file">types.h</span></samp> first in
<samp><span class="file">/usr/include/sys</span></samp>, then in its usual search path.

   <p>&lsquo;<samp><span class="samp">#line</span></samp>&rsquo; (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>You may put <samp><span class="option">-I-</span></samp> at any point in your list of <samp><span class="option">-I</span></samp> options. 
This has two effects.  First, directories appearing before the
<samp><span class="option">-I-</span></samp> in the list are searched only for headers requested with
quote marks.  Directories after <samp><span class="option">-I-</span></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><span class="option">-I</span></samp> switch.  <samp><span class="option">-I-</span></samp> is deprecated, <samp><span class="option">-iquote</span></samp>
should be used instead.

   <p><samp><span class="option">-I. -I-</span></samp> is not the same as no <samp><span class="option">-I</span></samp> options at all, and does
not cause the same behavior for &lsquo;<samp><span class="samp">&lt;&gt;</span></samp>&rsquo; includes that &lsquo;<samp><span class="samp">""</span></samp>&rsquo;
includes get with no special options.  <samp><span class="option">-I.</span></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>If you need to look for headers in a directory named <samp><span class="file">-</span></samp>, write
<samp><span class="option">-I./-</span></samp>.

   <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>.

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