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<h3 class="section">2.1 Include Syntax</h3>

<p><a name="index-g_t_0023include-27"></a>Both user and system header files are included using the preprocessing
directive &lsquo;<samp><span class="samp">#include</span></samp>&rsquo;.  It has two variants:

     <dl>
<dt><code>#include &lt;</code><var>file</var><code>&gt;</code><dd>This variant is used for system header files.  It searches for a file
named <var>file</var> in a standard list of system directories.  You can prepend
directories to this list with the <samp><span class="option">-I</span></samp> option (see <a href="Invocation.html#Invocation">Invocation</a>).

     <br><dt><code>#include "</code><var>file</var><code>"</code><dd>This variant is used for header files of your own program.  It
searches for a file named <var>file</var> first in the directory containing
the current file, then in the quote directories and then the same
directories used for <code>&lt;</code><var>file</var><code>&gt;</code>.  You can prepend directories
to the list of quote directories with the <samp><span class="option">-iquote</span></samp> option. 
</dl>

   <p>The argument of &lsquo;<samp><span class="samp">#include</span></samp>&rsquo;, whether delimited with quote marks or
angle brackets, behaves like a string constant in that comments are not
recognized, and macro names are not expanded.  Thus, <code>#include&nbsp;&lt;x/*y&gt;<!-- /@w --></code> specifies inclusion of a system header file named <samp><span class="file">x/*y</span></samp>.

   <p>However, if backslashes occur within <var>file</var>, they are considered
ordinary text characters, not escape characters.  None of the character
escape sequences appropriate to string constants in C are processed. 
Thus, <code>#include&nbsp;"x\n\\y"<!-- /@w --></code> specifies a filename containing three
backslashes.  (Some systems interpret &lsquo;<samp><span class="samp">\</span></samp>&rsquo; as a pathname separator. 
All of these also interpret &lsquo;<samp><span class="samp">/</span></samp>&rsquo; the same way.  It is most portable
to use only &lsquo;<samp><span class="samp">/</span></samp>&rsquo;.)

   <p>It is an error if there is anything (other than comments) on the line
after the file name.

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