157 lines
6.8 KiB
HTML
157 lines
6.8 KiB
HTML
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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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<!-- Copyright (C) 1987-2016 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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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>The C Preprocessor: Duplication of Side Effects</title>
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<meta name="description" content="The C Preprocessor: Duplication of Side Effects">
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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="Self_002dReferential-Macros.html#Self_002dReferential-Macros" rel="next" title="Self-Referential Macros">
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<link href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon" rel="prev" title="Swallowing the Semicolon">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Duplication-of-Side-Effects"></a>
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<div class="header">
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<p>
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Next: <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros" accesskey="n" rel="next">Self-Referential Macros</a>, Previous: <a href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon" accesskey="p" rel="prev">Swallowing the Semicolon</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="Duplication-of-Side-Effects-1"></a>
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<h4 class="subsection">3.10.4 Duplication of Side Effects</h4>
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<a name="index-side-effects-_0028in-macro-arguments_0029"></a>
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<a name="index-unsafe-macros"></a>
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<p>Many C programs define a macro <code>min</code>, for “minimum”, like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define min(X, Y) ((X) < (Y) ? (X) : (Y))
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</pre></div>
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<p>When you use this macro with an argument containing a side effect,
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as shown here,
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</p>
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<div class="smallexample">
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<pre class="smallexample">next = min (x + y, foo (z));
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</pre></div>
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<p>it expands as follows:
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</p>
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<div class="smallexample">
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<pre class="smallexample">next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));
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</pre></div>
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<p>where <code>x + y</code> has been substituted for <code>X</code> and <code>foo (z)</code>
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for <code>Y</code>.
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</p>
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<p>The function <code>foo</code> is used only once in the statement as it appears
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in the program, but the expression <code>foo (z)</code> has been substituted
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twice into the macro expansion. As a result, <code>foo</code> might be called
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two times when the statement is executed. If it has side effects or if
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it takes a long time to compute, the results might not be what you
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intended. We say that <code>min</code> is an <em>unsafe</em> macro.
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</p>
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<p>The best solution to this problem is to define <code>min</code> in a way that
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computes the value of <code>foo (z)</code> only once. The C language offers
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no standard way to do this, but it can be done with GNU extensions as
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follows:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define min(X, Y) \
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({ typeof (X) x_ = (X); \
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typeof (Y) y_ = (Y); \
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(x_ < y_) ? x_ : y_; })
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</pre></div>
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<p>The ‘<samp>({ … })</samp>’ notation produces a compound statement that
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acts as an expression. Its value is the value of its last statement.
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This permits us to define local variables and assign each argument to
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one. The local variables have underscores after their names to reduce
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the risk of conflict with an identifier of wider scope (it is impossible
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to avoid this entirely). Now each argument is evaluated exactly once.
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</p>
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<p>If you do not wish to use GNU C extensions, the only solution is to be
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careful when <em>using</em> the macro <code>min</code>. For example, you can
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calculate the value of <code>foo (z)</code>, save it in a variable, and use
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that variable in <code>min</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define min(X, Y) ((X) < (Y) ? (X) : (Y))
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…
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{
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int tem = foo (z);
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next = min (x + y, tem);
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}
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</pre></div>
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<p>(where we assume that <code>foo</code> returns type <code>int</code>).
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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="Self_002dReferential-Macros.html#Self_002dReferential-Macros" accesskey="n" rel="next">Self-Referential Macros</a>, Previous: <a href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon" accesskey="p" rel="prev">Swallowing the Semicolon</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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