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diff --git a/share/doc/cpp/Duplication-of-Side-Effects.html b/share/doc/cpp/Duplication-of-Side-Effects.html new file mode 100644 index 0000000..3486ba5 --- /dev/null +++ b/share/doc/cpp/Duplication-of-Side-Effects.html @@ -0,0 +1,156 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1987-2023 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. 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Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>The C Preprocessor: Duplication of Side Effects</title> + +<meta name="description" content="The C Preprocessor: Duplication of Side Effects"> +<meta name="keywords" content="The C Preprocessor: Duplication of Side Effects"> +<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="Macro-Pitfalls.html#Macro-Pitfalls" rel="up" title="Macro Pitfalls"> +<link href="Self_002dReferential-Macros.html#Self_002dReferential-Macros" rel="next" title="Self-Referential Macros"> +<link href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon" rel="previous" title="Swallowing the Semicolon"> +<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="Duplication-of-Side-Effects"></a> +<div class="header"> +<p> +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="previous">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> +</div> +<hr> +<a name="Duplication-of-Side-Effects-1"></a> +<h4 class="subsection">3.10.4 Duplication of Side Effects</h4> + +<a name="index-side-effects-_0028in-macro-arguments_0029"></a> +<a name="index-unsafe-macros"></a> +<p>Many C programs define a macro <code>min</code>, for “minimum”, like this: +</p> +<div class="smallexample"> +<pre class="smallexample">#define min(X, Y) ((X) < (Y) ? (X) : (Y)) +</pre></div> + +<p>When you use this macro with an argument containing a side effect, +as shown here, +</p> +<div class="smallexample"> +<pre class="smallexample">next = min (x + y, foo (z)); +</pre></div> + +<p>it expands as follows: +</p> +<div class="smallexample"> +<pre class="smallexample">next = ((x + y) < (foo (z)) ? (x + y) : (foo (z))); +</pre></div> + +<p>where <code>x + y</code> has been substituted for <code>X</code> and <code>foo (z)</code> +for <code>Y</code>. +</p> +<p>The function <code>foo</code> is used only once in the statement as it appears +in the program, but the expression <code>foo (z)</code> has been substituted +twice into the macro expansion. As a result, <code>foo</code> might be called +two times when the statement is executed. If it has side effects or if +it takes a long time to compute, the results might not be what you +intended. We say that <code>min</code> is an <em>unsafe</em> macro. +</p> +<p>The best solution to this problem is to define <code>min</code> in a way that +computes the value of <code>foo (z)</code> only once. The C language offers +no standard way to do this, but it can be done with GNU extensions as +follows: +</p> +<div class="smallexample"> +<pre class="smallexample">#define min(X, Y) \ +({ typeof (X) x_ = (X); \ + typeof (Y) y_ = (Y); \ + (x_ < y_) ? x_ : y_; }) +</pre></div> + +<p>The ‘<samp>({ … })</samp>’ notation produces a compound statement that +acts as an expression. Its value is the value of its last statement. +This permits us to define local variables and assign each argument to +one. The local variables have underscores after their names to reduce +the risk of conflict with an identifier of wider scope (it is impossible +to avoid this entirely). Now each argument is evaluated exactly once. +</p> +<p>If you do not wish to use GNU C extensions, the only solution is to be +careful when <em>using</em> the macro <code>min</code>. For example, you can +calculate the value of <code>foo (z)</code>, save it in a variable, and use +that variable in <code>min</code>: +</p> +<div class="smallexample"> +<pre class="smallexample">#define min(X, Y) ((X) < (Y) ? (X) : (Y)) +… +{ + int tem = foo (z); + next = min (x + y, tem); +} +</pre></div> + +<p>(where we assume that <code>foo</code> returns type <code>int</code>). +</p> +<hr> +<div class="header"> +<p> +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="previous">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> +</div> + + + +</body> +</html> |