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| 22 | <title>Function-like Macros (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Function-like Macros (GNU C Language Manual)">
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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="Macro-Arguments.html" rel="next" title="Macro Arguments">
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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="Function_002dlike-Macros"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Macro-Arguments.html" accesskey="n" rel="next">Macro Arguments</a>, Previous: <a href="Object_002dlike-Macros.html" accesskey="p" rel="prev">Object-like 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="Function_002dlike-Macros-1"></span><h4 class="subsection">26.5.2 Function-like Macros</h4>
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| 64 | <span id="index-function_002dlike-macros"></span>
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| 65 |
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| 66 | <p>You can also define macros whose use looks like a function call.
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| 67 | These are called <em>function-like macros</em>. To define one, use the
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| 68 | <code>#define</code> directive with a pair of parentheses immediately after
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| 69 | the macro name. For example,
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| 70 | </p>
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| 71 | <div class="example">
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| 72 | <pre class="example">#define lang_init() c_init()
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| 73 | lang_init()
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| 74 | → c_init()
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| 75 | </pre></div>
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| 76 |
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| 77 | <p>A function-like macro is expanded only when its name appears with a
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| 78 | pair of parentheses after it. If you write just the name, without
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| 79 | parentheses, it is left alone. This can be useful when you have a
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| 80 | function and a macro of the same name, and you wish to use the
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| 81 | function sometimes. Whitespace and line breaks before or between the
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| 82 | parentheses are ignored when the macro is called.
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| 83 | </p>
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| 84 | <div class="example">
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| 85 | <pre class="example">extern void foo(void);
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| 86 | #define foo() /* <span class="roman">optimized inline version</span> */
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| 87 | /* <span class="roman">…</span> */
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| 88 | foo();
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| 89 | funcptr = foo;
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| 90 | </pre></div>
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| 91 |
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| 92 | <p>Here the call to <code>foo()</code> expands the macro, but the function
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| 93 | pointer <code>funcptr</code> gets the address of the real function
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| 94 | <code>foo</code>. If the macro were to be expanded there, it would cause a
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| 95 | syntax error.
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| 96 | </p>
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| 97 | <p>If you put spaces between the macro name and the parentheses in the
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| 98 | macro definition, that does not define a function-like macro, it defines
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| 99 | an object-like macro whose expansion happens to begin with a pair of
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| 100 | parentheses. Here is an example:
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| 101 | </p>
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| 102 | <div class="example">
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| 103 | <pre class="example">#define lang_init () c_init()
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| 104 | lang_init()
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| 105 | → () c_init()()
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| 106 | </pre></div>
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| 107 |
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| 108 | <p>The first two pairs of parentheses in this expansion come from the
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| 109 | macro. The third is the pair that was originally after the macro
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| 110 | invocation. Since <code>lang_init</code> is an object-like macro, it does not
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| 111 | consume those parentheses.
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| 112 | </p>
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| 113 | <p>Any name can have at most one macro definition at a time. Thus,
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| 114 | you can’t define the same name as an object-like macro and a
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| 115 | function-like macro at once.
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| 116 | </p>
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| 117 | <hr>
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| 118 | <div class="header">
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| 119 | <p>
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| 120 | Next: <a href="Macro-Arguments.html" accesskey="n" rel="next">Macro Arguments</a>, Previous: <a href="Object_002dlike-Macros.html" accesskey="p" rel="prev">Object-like 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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