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89 lines
3.9 KiB
HTML
89 lines
3.9 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Defined - The C Preprocessor</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="The C Preprocessor">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Conditional-Syntax.html#Conditional-Syntax" title="Conditional Syntax">
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<link rel="prev" href="If.html#If" title="If">
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<link rel="next" href="Else.html#Else" title="Else">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1987-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation. A copy of
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the license is included in the
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section entitled ``GNU Free Documentation License''.
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This manual contains no Invariant Sections. The Front-Cover Texts are
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(a) (see below), and the Back-Cover Texts are (b) (see below).
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development.
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<style type="text/css"><!--
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span.sansserif { font-family:sans-serif; font-weight:normal; }
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Defined"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Else.html#Else">Else</a>,
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Previous: <a rel="previous" accesskey="p" href="If.html#If">If</a>,
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Up: <a rel="up" accesskey="u" href="Conditional-Syntax.html#Conditional-Syntax">Conditional Syntax</a>
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<hr>
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</div>
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<h4 class="subsection">4.2.3 Defined</h4>
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<p><a name="index-g_t_0040code_007bdefined_007d-89"></a>The special operator <code>defined</code> is used in ‘<samp><span class="samp">#if</span></samp>’ and
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‘<samp><span class="samp">#elif</span></samp>’ expressions to test whether a certain name is defined as a
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macro. <code>defined </code><var>name</var> and <code>defined (</code><var>name</var><code>)</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><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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<pre class="smallexample"> #if defined (__vax__) || defined (__ns16000__)
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</pre>
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<p class="noindent">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>Conditionals written like this:
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<pre class="smallexample"> #if defined BUFSIZE && BUFSIZE >= 1024
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</pre>
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<p class="noindent">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>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><span class="option">-pedantic</span></samp>, since other compilers may handle it differently.
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</body></html>
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