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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 ‘<samp><span class="samp">#include</span></samp>’. It has two variants:
<dl>
<dt><code>#include <</code><var>file</var><code>></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><</code><var>file</var><code>></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 ‘<samp><span class="samp">#include</span></samp>’, 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 <x/*y><!-- /@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 "x\n\\y"<!-- /@w --></code> specifies a filename containing three
backslashes. (Some systems interpret ‘<samp><span class="samp">\</span></samp>’ as a pathname separator.
All of these also interpret ‘<samp><span class="samp">/</span></samp>’ the same way. It is most portable
to use only ‘<samp><span class="samp">/</span></samp>’.)
<p>It is an error if there is anything (other than comments) on the line
after the file name.
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