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<title>The C Preprocessor: Defined</title>
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Next: <a href="Else.html#Else" accesskey="n" rel="next">Else</a>, Previous: <a href="If.html#If" accesskey="p" rel="prev">If</a>, Up: <a href="Conditional-Syntax.html#Conditional-Syntax" accesskey="u" rel="up">Conditional Syntax</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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<a name="Defined-1"></a>
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<h4 class="subsection">4.2.3 Defined</h4>
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<a name="index-defined"></a>
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<p>The special operator <code>defined</code> is used in ‘<samp>#if</samp>’ and
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‘<samp>#elif</samp>’ expressions to test whether a certain name is defined as a
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macro. <code>defined <var>name</var></code> and <code>defined (<var>name</var>)</code> are
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both expressions whose value is 1 if <var>name</var> is defined as a macro at
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the current point in the program, and 0 otherwise. Thus, <code>#if defined MACRO<!-- /@w --></code> is precisely equivalent to <code>#ifdef MACRO<!-- /@w --></code>.
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</p>
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<p><code>defined</code> is useful when you wish to test more than one macro for
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existence at once. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">#if defined (__vax__) || defined (__ns16000__)
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</pre></div>
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<p>would succeed if either of the names <code>__vax__</code> or
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<code>__ns16000__</code> is defined as a macro.
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</p>
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<p>Conditionals written like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#if defined BUFSIZE && BUFSIZE >= 1024
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</pre></div>
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<p>can generally be simplified to just <code>#if BUFSIZE >= 1024<!-- /@w --></code>,
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since if <code>BUFSIZE</code> is not defined, it will be interpreted as having
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the value zero.
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</p>
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<p>If the <code>defined</code> operator appears as a result of a macro expansion,
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the C standard says the behavior is undefined. GNU cpp treats it as a
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genuine <code>defined</code> operator and evaluates it normally. It will warn
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wherever your code uses this feature if you use the command-line option
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<samp>-pedantic</samp>, since other compilers may handle it differently.
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