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| <title>The C Preprocessor: Defined</title> |
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| <a name="Defined-1"></a> |
| <h4 class="subsection">4.2.3 Defined</h4> |
| |
| <a name="index-defined"></a> |
| <p>The special operator <code>defined</code> is used in ‘<samp>#if</samp>’ and |
| ‘<samp>#elif</samp>’ 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 defined MACRO<!-- /@w --></code> is precisely equivalent to <code>#ifdef 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 && BUFSIZE >= 1024 |
| </pre></div> |
| |
| <p>can generally be simplified to just <code>#if BUFSIZE >= 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>-pedantic</samp>, since other compilers may handle it differently. |
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