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149 lines
6.4 KiB
HTML
149 lines
6.4 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>Swallowing the Semicolon (The C Preprocessor)</title>
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<meta name="description" content="Swallowing the Semicolon (The C Preprocessor)">
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<meta name="keywords" content="Swallowing the Semicolon (The C Preprocessor)">
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<meta name="distribution" content="global">
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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="Macro-Pitfalls.html#Macro-Pitfalls" rel="up" title="Macro Pitfalls">
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<link href="Duplication-of-Side-Effects.html#Duplication-of-Side-Effects" rel="next" title="Duplication of Side Effects">
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<link href="Operator-Precedence-Problems.html#Operator-Precedence-Problems" rel="prev" title="Operator Precedence Problems">
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<style type="text/css">
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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="Swallowing-the-Semicolon"></a>
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<div class="header">
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<p>
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Next: <a href="Duplication-of-Side-Effects.html#Duplication-of-Side-Effects" accesskey="n" rel="next">Duplication of Side Effects</a>, Previous: <a href="Operator-Precedence-Problems.html#Operator-Precedence-Problems" accesskey="p" rel="prev">Operator Precedence Problems</a>, Up: <a href="Macro-Pitfalls.html#Macro-Pitfalls" accesskey="u" rel="up">Macro Pitfalls</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="Swallowing-the-Semicolon-1"></a>
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<h4 class="subsection">3.10.3 Swallowing the Semicolon</h4>
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<a name="index-semicolons-_0028after-macro-calls_0029"></a>
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<p>Often it is desirable to define a macro that expands into a compound
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statement. Consider, for example, the following macro, that advances a
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pointer (the argument <code>p</code> says where to find it) across whitespace
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characters:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define SKIP_SPACES(p, limit) \
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{ char *lim = (limit); \
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while (p < lim) { \
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if (*p++ != ' ') { \
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p--; break; }}}
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</pre></div>
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<p>Here backslash-newline is used to split the macro definition, which must
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be a single logical line, so that it resembles the way such code would
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be laid out if not part of a macro definition.
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</p>
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<p>A call to this macro might be <code>SKIP_SPACES (p, lim)</code>. Strictly
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speaking, the call expands to a compound statement, which is a complete
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statement with no need for a semicolon to end it. However, since it
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looks like a function call, it minimizes confusion if you can use it
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like a function call, writing a semicolon afterward, as in
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<code>SKIP_SPACES (p, lim);</code>
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</p>
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<p>This can cause trouble before <code>else</code> statements, because the
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semicolon is actually a null statement. Suppose you write
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</p>
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<div class="smallexample">
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<pre class="smallexample">if (*p != 0)
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SKIP_SPACES (p, lim);
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else …
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</pre></div>
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<p>The presence of two statements—the compound statement and a null
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statement—in between the <code>if</code> condition and the <code>else</code>
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makes invalid C code.
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</p>
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<p>The definition of the macro <code>SKIP_SPACES</code> can be altered to solve
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this problem, using a <code>do … while</code> statement. Here is how:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define SKIP_SPACES(p, limit) \
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do { char *lim = (limit); \
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while (p < lim) { \
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if (*p++ != ' ') { \
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p--; break; }}} \
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while (0)
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</pre></div>
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<p>Now <code>SKIP_SPACES (p, lim);</code> expands into
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</p>
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<div class="smallexample">
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<pre class="smallexample">do {…} while (0);
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</pre></div>
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<p>which is one statement. The loop executes exactly once; most compilers
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generate no extra code for it.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Duplication-of-Side-Effects.html#Duplication-of-Side-Effects" accesskey="n" rel="next">Duplication of Side Effects</a>, Previous: <a href="Operator-Precedence-Problems.html#Operator-Precedence-Problems" accesskey="p" rel="prev">Operator Precedence Problems</a>, Up: <a href="Macro-Pitfalls.html#Macro-Pitfalls" accesskey="u" rel="up">Macro Pitfalls</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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</body>
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</html>
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