You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
207 lines
8.2 KiB
HTML
207 lines
8.2 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1987-2018 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 6.4, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>Object-like Macros (The C Preprocessor)</title>
|
|
|
|
<meta name="description" content="Object-like Macros (The C Preprocessor)">
|
|
<meta name="keywords" content="Object-like Macros (The C Preprocessor)">
|
|
<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="Function_002dlike-Macros.html#Function_002dlike-Macros" rel="next" title="Function-like Macros">
|
|
<link href="Macros.html#Macros" rel="prev" title="Macros">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
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.nolinebreak {white-space: nowrap}
|
|
span.roman {font-family: initial; font-weight: normal}
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<a name="Object_002dlike-Macros"></a>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Function_002dlike-Macros.html#Function_002dlike-Macros" accesskey="n" rel="next">Function-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="Object_002dlike-Macros-1"></a>
|
|
<h3 class="section">3.1 Object-like Macros</h3>
|
|
<a name="index-object_002dlike-macro"></a>
|
|
<a name="index-symbolic-constants"></a>
|
|
<a name="index-manifest-constants"></a>
|
|
|
|
<p>An <em>object-like macro</em> is a simple identifier which will be replaced
|
|
by a code fragment. It is called object-like because it looks like a
|
|
data object in code that uses it. They are most commonly used to give
|
|
symbolic names to numeric constants.
|
|
</p>
|
|
<a name="index-_0023define"></a>
|
|
<p>You create macros with the ‘<samp>#define</samp>’ directive. ‘<samp>#define</samp>’ is
|
|
followed by the name of the macro and then the token sequence it should
|
|
be an abbreviation for, which is variously referred to as the macro’s
|
|
<em>body</em>, <em>expansion</em> or <em>replacement list</em>. For example,
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">#define BUFFER_SIZE 1024
|
|
</pre></div>
|
|
|
|
<p>defines a macro named <code>BUFFER_SIZE</code> as an abbreviation for the
|
|
token <code>1024</code>. If somewhere after this ‘<samp>#define</samp>’ directive
|
|
there comes a C statement of the form
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">foo = (char *) malloc (BUFFER_SIZE);
|
|
</pre></div>
|
|
|
|
<p>then the C preprocessor will recognize and <em>expand</em> the macro
|
|
<code>BUFFER_SIZE</code>. The C compiler will see the same tokens as it would
|
|
if you had written
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">foo = (char *) malloc (1024);
|
|
</pre></div>
|
|
|
|
<p>By convention, macro names are written in uppercase. Programs are
|
|
easier to read when it is possible to tell at a glance which names are
|
|
macros.
|
|
</p>
|
|
<p>The macro’s body ends at the end of the ‘<samp>#define</samp>’ line. You may
|
|
continue the definition onto multiple lines, if necessary, using
|
|
backslash-newline. When the macro is expanded, however, it will all
|
|
come out on one line. For example,
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">#define NUMBERS 1, \
|
|
2, \
|
|
3
|
|
int x[] = { NUMBERS };
|
|
→ int x[] = { 1, 2, 3 };
|
|
</pre></div>
|
|
|
|
<p>The most common visible consequence of this is surprising line numbers
|
|
in error messages.
|
|
</p>
|
|
<p>There is no restriction on what can go in a macro body provided it
|
|
decomposes into valid preprocessing tokens. Parentheses need not
|
|
balance, and the body need not resemble valid C code. (If it does not,
|
|
you may get error messages from the C compiler when you use the macro.)
|
|
</p>
|
|
<p>The C preprocessor scans your program sequentially. Macro definitions
|
|
take effect at the place you write them. Therefore, the following input
|
|
to the C preprocessor
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">foo = X;
|
|
#define X 4
|
|
bar = X;
|
|
</pre></div>
|
|
|
|
<p>produces
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">foo = X;
|
|
bar = 4;
|
|
</pre></div>
|
|
|
|
<p>When the preprocessor expands a macro name, the macro’s expansion
|
|
replaces the macro invocation, then the expansion is examined for more
|
|
macros to expand. For example,
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">#define TABLESIZE BUFSIZE
|
|
#define BUFSIZE 1024
|
|
TABLESIZE
|
|
→ BUFSIZE
|
|
→ 1024
|
|
</pre></div>
|
|
|
|
<p><code>TABLESIZE</code> is expanded first to produce <code>BUFSIZE</code>, then that
|
|
macro is expanded to produce the final result, <code>1024</code>.
|
|
</p>
|
|
<p>Notice that <code>BUFSIZE</code> was not defined when <code>TABLESIZE</code> was
|
|
defined. The ‘<samp>#define</samp>’ for <code>TABLESIZE</code> uses exactly the
|
|
expansion you specify—in this case, <code>BUFSIZE</code>—and does not
|
|
check to see whether it too contains macro names. Only when you
|
|
<em>use</em> <code>TABLESIZE</code> is the result of its expansion scanned for
|
|
more macro names.
|
|
</p>
|
|
<p>This makes a difference if you change the definition of <code>BUFSIZE</code>
|
|
at some point in the source file. <code>TABLESIZE</code>, defined as shown,
|
|
will always expand using the definition of <code>BUFSIZE</code> that is
|
|
currently in effect:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">#define BUFSIZE 1020
|
|
#define TABLESIZE BUFSIZE
|
|
#undef BUFSIZE
|
|
#define BUFSIZE 37
|
|
</pre></div>
|
|
|
|
<p>Now <code>TABLESIZE</code> expands (in two stages) to <code>37</code>.
|
|
</p>
|
|
<p>If the expansion of a macro contains its own name, either directly or
|
|
via intermediate macros, it is not expanded again when the expansion is
|
|
examined for more macros. This prevents infinite recursion.
|
|
See <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros">Self-Referential Macros</a>, for the precise details.
|
|
</p>
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Function_002dlike-Macros.html#Function_002dlike-Macros" accesskey="n" rel="next">Function-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>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|