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| 22 | <title>Undefining and Redefining Macros (GNU C Language Manual)</title>
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Macros.html" rel="up" title="Macros">
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| 33 | <link href="Directives-Within-Macro-Arguments.html" rel="next" title="Directives Within Macro Arguments">
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| 34 | <link href="Predefined-Macros.html" rel="prev" title="Predefined Macros">
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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="Undefining-and-Redefining-Macros"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Directives-Within-Macro-Arguments.html" accesskey="n" rel="next">Directives Within Macro Arguments</a>, Previous: <a href="Predefined-Macros.html" accesskey="p" rel="prev">Predefined Macros</a>, Up: <a href="Macros.html" accesskey="u" rel="up">Macros</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="Undefining-and-Redefining-Macros-1"></span><h4 class="subsection">26.5.8 Undefining and Redefining Macros</h4>
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| 64 | <span id="index-undefining-macros"></span>
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| 65 | <span id="index-redefining-macros"></span>
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| 66 | <span id="index-_0023undef"></span>
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| 67 |
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| 68 | <p>You can <em>undefine</em> a macro with the <code>#undef</code> directive.
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| 69 | <code>#undef</code> takes a single argument, the name of the macro to
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| 70 | undefine. You use the bare macro name, even if the macro is
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| 71 | function-like. It is an error if anything appears on the line after
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| 72 | the macro name. <code>#undef</code> has no effect if the name is not a
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| 73 | macro.
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| 74 | </p>
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| 75 | <div class="example">
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| 76 | <pre class="example">#define FOO 4
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| 77 | x = FOO; → x = 4;
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| 78 | #undef FOO
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| 79 | x = FOO; → x = FOO;
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| 80 | </pre></div>
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| 81 |
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| 82 | <p>Once a macro has been undefined, that identifier may be <em>redefined</em>
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| 83 | as a macro by a subsequent <code>#define</code> directive. The new definition
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| 84 | need not have any resemblance to the old definition.
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| 85 | </p>
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| 86 | <p>You can define a macro again without first undefining it only if
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| 87 | the new definition is <em>effectively the same</em> as the old one.
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| 88 | Two macro definitions are effectively the same if:
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| 89 | </p>
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| 90 | <ul>
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| 91 | <li> Both are the same type of macro (object- or function-like).
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| 92 | </li><li> All the tokens of the replacement list are the same.
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| 93 | </li><li> If there are any parameters, they are the same.
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| 94 | </li><li> Whitespace appears in the same places in both. It need not be
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| 95 | exactly the same amount of whitespace, though. Remember that comments
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| 96 | count as whitespace.
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| 97 | </li></ul>
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| 98 |
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| 99 | <p>These definitions are effectively the same:
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| 100 | </p><div class="example">
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| 101 | <pre class="example">#define FOUR (2 + 2)
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| 102 | #define FOUR (2 + 2)
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| 103 | #define FOUR (2 /* <span class="roman">two</span> */ + 2)
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| 104 | </pre></div>
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| 105 | <p>but these are not:
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| 106 | </p><div class="example">
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| 107 | <pre class="example">#define FOUR (2 + 2)
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| 108 | #define FOUR ( 2+2 )
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| 109 | #define FOUR (2 * 2)
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| 110 | #define FOUR(score,and,seven,years,ago) (2 + 2)
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| 111 | </pre></div>
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| 112 |
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| 113 | <p>This allows two different header files to define a common macro.
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| 114 | </p>
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| 115 | <p>You can redefine an existing macro with #define, but redefining an
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| 116 | existing macro name with a different definition results in a warning.
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| 117 | </p>
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| 118 | <hr>
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| 119 | <div class="header">
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| 120 | <p>
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| 121 | Next: <a href="Directives-Within-Macro-Arguments.html" accesskey="n" rel="next">Directives Within Macro Arguments</a>, Previous: <a href="Predefined-Macros.html" accesskey="p" rel="prev">Predefined Macros</a>, Up: <a href="Macros.html" accesskey="u" rel="up">Macros</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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