131 lines
5.3 KiB
HTML
131 lines
5.3 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1987-2016 Free Software Foundation, Inc.
|
|
|
|
Permission is granted to copy, distribute and/or modify this document
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
|
any later version published by the Free Software Foundation. A copy of
|
|
the license is included in the
|
|
section entitled "GNU Free Documentation License".
|
|
|
|
This manual contains no Invariant Sections. The Front-Cover Texts are
|
|
(a) (see below), and the Back-Cover Texts are (b) (see below).
|
|
|
|
(a) The FSF's Front-Cover Text is:
|
|
|
|
A GNU Manual
|
|
|
|
(b) The FSF's Back-Cover Text is:
|
|
|
|
You have freedom to copy and modify this GNU Manual, like GNU
|
|
software. Copies published by the Free Software Foundation raise
|
|
funds for GNU development. -->
|
|
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>The C Preprocessor: Function-like Macros</title>
|
|
|
|
<meta name="description" content="The C Preprocessor: Function-like Macros">
|
|
<meta name="keywords" content="The C Preprocessor: Function-like Macros">
|
|
<meta name="resource-type" content="document">
|
|
<meta name="distribution" content="global">
|
|
<meta name="Generator" content="makeinfo">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<link href="index.html#Top" rel="start" title="Top">
|
|
<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="Macros.html#Macros" rel="up" title="Macros">
|
|
<link href="Macro-Arguments.html#Macro-Arguments" rel="next" title="Macro Arguments">
|
|
<link href="Object_002dlike-Macros.html#Object_002dlike-Macros" rel="prev" title="Object-like Macros">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.indentedblock {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
|
|
div.smalllisp {margin-left: 3.2em}
|
|
kbd {font-style:oblique}
|
|
pre.display {font-family: inherit}
|
|
pre.format {font-family: inherit}
|
|
pre.menu-comment {font-family: serif}
|
|
pre.menu-preformatted {font-family: serif}
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
|
pre.smallexample {font-size: smaller}
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
|
pre.smalllisp {font-size: smaller}
|
|
span.nocodebreak {white-space:nowrap}
|
|
span.nolinebreak {white-space:nowrap}
|
|
span.roman {font-family:serif; font-weight:normal}
|
|
span.sansserif {font-family:sans-serif; font-weight:normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
|
|
<a name="Function_002dlike-Macros"></a>
|
|
<div class="header">
|
|
<p>
|
|
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>
|
|
</div>
|
|
<hr>
|
|
<a name="Function_002dlike-Macros-1"></a>
|
|
<h3 class="section">3.2 Function-like Macros</h3>
|
|
<a name="index-function_002dlike-macros"></a>
|
|
|
|
<p>You can also define macros whose use looks like a function call. These
|
|
are called <em>function-like macros</em>. To define a function-like macro,
|
|
you use the same ‘<samp>#define</samp>’ directive, but you put a pair of
|
|
parentheses immediately after the macro name. For example,
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">#define lang_init() c_init()
|
|
lang_init()
|
|
→ c_init()
|
|
</pre></div>
|
|
|
|
<p>A function-like macro is only expanded if its name appears with a pair
|
|
of parentheses after it. If you write just the name, it is left alone.
|
|
This can be useful when you have a function and a macro of the same
|
|
name, and you wish to use the function sometimes.
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">extern void foo(void);
|
|
#define foo() /* <span class="roman">optimized inline version</span> */
|
|
…
|
|
foo();
|
|
funcptr = foo;
|
|
</pre></div>
|
|
|
|
<p>Here the call to <code>foo()</code> will use the macro, but the function
|
|
pointer will get the address of the real function. If the macro were to
|
|
be expanded, it would cause a syntax error.
|
|
</p>
|
|
<p>If you put spaces between the macro name and the parentheses in the
|
|
macro definition, that does not define a function-like macro, it defines
|
|
an object-like macro whose expansion happens to begin with a pair of
|
|
parentheses.
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">#define lang_init () c_init()
|
|
lang_init()
|
|
→ () c_init()()
|
|
</pre></div>
|
|
|
|
<p>The first two pairs of parentheses in this expansion come from the
|
|
macro. The third is the pair that was originally after the macro
|
|
invocation. Since <code>lang_init</code> is an object-like macro, it does not
|
|
consume those parentheses.
|
|
</p>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|