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<title>The C Preprocessor: Function-like Macros</title>
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<a name="Function_002dlike-Macros"></a>
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<p>
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Next: <a href="Macro-Arguments.html#Macro-Arguments" accesskey="n" rel="next">Macro Arguments</a>, Previous: <a href="Object_002dlike-Macros.html#Object_002dlike-Macros" accesskey="p" rel="prev">Object-like Macros</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="Function_002dlike-Macros-1"></a>
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<h3 class="section">3.2 Function-like Macros</h3>
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<a name="index-function_002dlike-macros"></a>
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<p>You can also define macros whose use looks like a function call. These
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are called <em>function-like macros</em>. To define a function-like macro,
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you use the same ‘<samp>#define</samp>’ directive, but you put a pair of
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parentheses immediately after the macro name. For example,
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</p>
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<div class="smallexample">
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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></div>
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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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</p>
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<div class="smallexample">
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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></div>
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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>
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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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</p>
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<div class="smallexample">
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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></div>
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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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