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<h4 class="subsection">Defined</h4>
<p>The special operator <code>defined</code> is used in <code>#if</code> and
<code>#elif</code> expressions to test whether a certain name is defined as a
macro. <code>defined </code><var>name</var><code></code> and <code>defined (</code><var>name</var><code>)</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 defined MACRO</code> is precisely equivalent to <code>#ifdef MACRO</code>.
<p><code>defined</code> is useful when you wish to test more than one macro for
existence at once. For example,
<pre class="example"> #if defined (__vax__) || defined (__ns16000__)
</pre>
<p>would succeed if either of the names <code>__vax__</code> or
<code>__ns16000__</code> is defined as a macro.
<p>Conditionals written like this:
<pre class="example"> #if defined BUFSIZE && BUFSIZE >= 1024
</pre>
<p>can generally be simplified to just <code>#if BUFSIZE >= 1024</code>,
since if <code>BUFSIZE</code> is not defined, it will be interpreted as having
the value zero.
<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
<code>-pedantic</code>, since other compilers may handle it differently.
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