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130 lines
5.2 KiB
HTML
130 lines
5.2 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1987-2018 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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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Function-like Macros (The C Preprocessor)</title>
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<meta name="description" content="Function-like Macros (The C Preprocessor)">
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<meta name="keywords" content="Function-like Macros (The C Preprocessor)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Macros.html#Macros" rel="up" title="Macros">
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<link href="Macro-Arguments.html#Macro-Arguments" rel="next" title="Macro Arguments">
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<link href="Object_002dlike-Macros.html#Object_002dlike-Macros" rel="prev" title="Object-like Macros">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Function_002dlike-Macros"></a>
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<div class="header">
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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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</div>
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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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</p>
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</body>
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</html>
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