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| 3 | <!-- Copyright (C) 2022 Richard Stallman and Free Software Foundation, Inc.
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| 5 | (The work of Trevis Rothwell and Nelson Beebe has been assigned or
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| 22 | <title>defined (GNU C Language Manual)</title>
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| 23 |
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Conditional-Syntax.html" rel="up" title="Conditional Syntax">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="defined"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="else.html" accesskey="n" rel="next">else</a>, Previous: <a href="if.html" accesskey="p" rel="prev">if</a>, Up: <a href="Conditional-Syntax.html" accesskey="u" rel="up">Conditional Syntax</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="The-defined-test"></span><h4 class="subsubsection">26.6.2.3 The <code>defined</code> test</h4>
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| 64 |
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| 65 | <span id="index-defined"></span>
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| 66 | <p>The special operator <code>defined</code> is used in <code>#if</code> and
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| 67 | <code>#elif</code> expressions to test whether a certain name is defined as a
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| 68 | macro. <code>defined <var>name</var></code> and <code>defined (<var>name</var>)</code> are
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| 69 | both expressions whose value is 1 if <var>name</var> is defined as a macro at
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| 70 | 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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| 71 | </p>
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| 72 | <p><code>defined</code> is useful when you wish to test more than one macro for
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| 73 | existence at once. For example,
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| 74 | </p>
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| 75 | <div class="example">
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| 76 | <pre class="example">#if defined (__arm__) || defined (__PPC__)
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| 77 | </pre></div>
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| 78 |
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| 79 | <p>would succeed if either of the names <code>__arm__</code> or
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| 80 | <code>__PPC__</code> is defined as a macro—in other words,
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| 81 | when compiling for ARM processors or PowerPC processors.
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| 82 | </p>
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| 83 | <p>Conditionals written like this:
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| 84 | </p>
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| 85 | <div class="example">
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| 86 | <pre class="example">#if defined BUFSIZE && BUFSIZE >= 1024
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| 87 | </pre></div>
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| 88 |
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| 89 | <p>can generally be simplified to just <code>#if BUFSIZE >= 1024<!-- /@w --></code>,
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| 90 | since if <code>BUFSIZE</code> is not defined, it will be interpreted as having
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| 91 | the value zero.
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| 92 | </p>
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| 93 | <p>In GCC, you can include <code>defined</code> as part of another macro definition,
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| 94 | like this:
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| 95 | </p>
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| 96 | <div class="example">
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| 97 | <pre class="example">#define MACRO_DEFINED(X) defined X
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| 98 |
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| 99 | #if MACRO_DEFINED(BUFSIZE)
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| 100 | </pre></div>
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| 101 |
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| 102 | <p>which would expand the <code>#if</code> expression to:
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| 103 | </p>
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| 104 | <div class="example">
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| 105 | <pre class="example">#if defined BUFSIZE
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| 106 | </pre></div>
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| 107 |
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| 108 | <p>Generating <code>defined</code> in this way is a GNU C extension.
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| 109 | </p>
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| 110 |
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| 111 |
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| 112 |
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| 114 | </html>
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