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<h4 class="subsection">4.2.3 Defined</h4>

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<p>The special operator <code>defined</code> is used in &lsquo;<samp>#if</samp>&rsquo; and
&lsquo;<samp>#elif</samp>&rsquo; expressions to test whether a certain name is defined as a
macro.  <code>defined <var>name</var></code> and <code>defined (<var>name</var>)</code> are
both expressions whose value is 1 if <var>name</var> is defined as a macro at
the current point in the program, and 0 otherwise.  Thus,  <code>#if&nbsp;defined&nbsp;MACRO<!-- /@w --></code> is precisely equivalent to <code>#ifdef&nbsp;MACRO<!-- /@w --></code>.
</p>
<p><code>defined</code> is useful when you wish to test more than one macro for
existence at once.  For example,
</p>
<div class="smallexample">
<pre class="smallexample">#if defined (__vax__) || defined (__ns16000__)
</pre></div>

<p>would succeed if either of the names <code>__vax__</code> or
<code>__ns16000__</code> is defined as a macro.
</p>
<p>Conditionals written like this:
</p>
<div class="smallexample">
<pre class="smallexample">#if defined BUFSIZE &amp;&amp; BUFSIZE &gt;= 1024
</pre></div>

<p>can generally be simplified to just <code>#if&nbsp;BUFSIZE&nbsp;&gt;=&nbsp;1024<!-- /@w --></code>,
since if <code>BUFSIZE</code> is not defined, it will be interpreted as having
the value zero.
</p>
<p>If the <code>defined</code> operator appears as a result of a macro expansion,
the C standard says the behavior is undefined.  GNU cpp treats it as a
genuine <code>defined</code> operator and evaluates it normally.  It will warn
wherever your code uses this feature if you use the command-line option
<samp>-Wpedantic</samp>, since other compilers may handle it differently.  The
warning is also enabled by <samp>-Wextra</samp>, and can also be enabled
individually with <samp>-Wexpansion-to-defined</samp>.
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