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101 lines
4.1 KiB
HTML
101 lines
4.1 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Function-like Macros - 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="Macros.html#Macros" title="Macros">
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<link rel="prev" href="Object_002dlike-Macros.html#Object_002dlike-Macros" title="Object-like Macros">
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<link rel="next" href="Macro-Arguments.html#Macro-Arguments" title="Macro Arguments">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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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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funds for GNU development.
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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="Function-like-Macros"></a>
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<a name="Function_002dlike-Macros"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Macro-Arguments.html#Macro-Arguments">Macro Arguments</a>,
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Previous: <a rel="previous" accesskey="p" href="Object_002dlike-Macros.html#Object_002dlike-Macros">Object-like Macros</a>,
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Up: <a rel="up" accesskey="u" href="Macros.html#Macros">Macros</a>
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<hr>
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</div>
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<h3 class="section">3.2 Function-like Macros</h3>
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<p><a name="index-function_002dlike-macros-45"></a>
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You can also define macros whose use looks like a function call. These
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are called <dfn>function-like macros</dfn>. To define a function-like macro,
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you use the same ‘<samp><span class="samp">#define</span></samp>’ directive, but you put a pair of
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parentheses immediately after the macro name. For example,
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<pre class="smallexample"> #define lang_init() c_init()
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lang_init()
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==> c_init()
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</pre>
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<p>A function-like macro is only expanded if its name appears with a pair
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of parentheses after it. If you write just the name, it is left alone.
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This can be useful when you have a function and a macro of the same
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name, and you wish to use the function sometimes.
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<pre class="smallexample"> extern void foo(void);
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#define foo() /* <span class="roman">optimized inline version</span> */
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...
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foo();
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funcptr = foo;
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</pre>
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<p>Here the call to <code>foo()</code> will use the macro, but the function
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pointer will get the address of the real function. If the macro were to
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be expanded, it would cause a syntax error.
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<p>If you put spaces between the macro name and the parentheses in the
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macro definition, that does not define a function-like macro, it defines
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an object-like macro whose expansion happens to begin with a pair of
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parentheses.
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<pre class="smallexample"> #define lang_init () c_init()
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lang_init()
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==> () c_init()()
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</pre>
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<p>The first two pairs of parentheses in this expansion come from the
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macro. The third is the pair that was originally after the macro
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invocation. Since <code>lang_init</code> is an object-like macro, it does not
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consume those parentheses.
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</body></html>
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