154 lines
6.4 KiB
HTML
154 lines
6.4 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: Operator Precedence Problems</title>
|
|
|
|
<meta name="description" content="The C Preprocessor: Operator Precedence Problems">
|
|
<meta name="keywords" content="The C Preprocessor: Operator Precedence Problems">
|
|
<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="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon" rel="next" title="Swallowing the Semicolon">
|
|
<link href="Misnesting.html#Misnesting" rel="prev" title="Misnesting">
|
|
<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="Operator-Precedence-Problems"></a>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon" accesskey="n" rel="next">Swallowing the Semicolon</a>, Previous: <a href="Misnesting.html#Misnesting" accesskey="p" rel="prev">Misnesting</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="Operator-Precedence-Problems-1"></a>
|
|
<h4 class="subsection">3.10.2 Operator Precedence Problems</h4>
|
|
<a name="index-parentheses-in-macro-bodies"></a>
|
|
|
|
<p>You may have noticed that in most of the macro definition examples shown
|
|
above, each occurrence of a macro argument name had parentheses around
|
|
it. In addition, another pair of parentheses usually surround the
|
|
entire macro definition. Here is why it is best to write macros that
|
|
way.
|
|
</p>
|
|
<p>Suppose you define a macro as follows,
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">#define ceil_div(x, y) (x + y - 1) / y
|
|
</pre></div>
|
|
|
|
<p>whose purpose is to divide, rounding up. (One use for this operation is
|
|
to compute how many <code>int</code> objects are needed to hold a certain
|
|
number of <code>char</code> objects.) Then suppose it is used as follows:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">a = ceil_div (b & c, sizeof (int));
|
|
→ a = (b & c + sizeof (int) - 1) / sizeof (int);
|
|
</pre></div>
|
|
|
|
<p>This does not do what is intended. The operator-precedence rules of
|
|
C make it equivalent to this:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">a = (b & (c + sizeof (int) - 1)) / sizeof (int);
|
|
</pre></div>
|
|
|
|
<p>What we want is this:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">a = ((b & c) + sizeof (int) - 1)) / sizeof (int);
|
|
</pre></div>
|
|
|
|
<p>Defining the macro as
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">#define ceil_div(x, y) ((x) + (y) - 1) / (y)
|
|
</pre></div>
|
|
|
|
<p>provides the desired result.
|
|
</p>
|
|
<p>Unintended grouping can result in another way. Consider <code>sizeof
|
|
ceil_div(1, 2)</code>. That has the appearance of a C expression that would
|
|
compute the size of the type of <code>ceil_div (1, 2)</code>, but in fact it
|
|
means something very different. Here is what it expands to:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">sizeof ((1) + (2) - 1) / (2)
|
|
</pre></div>
|
|
|
|
<p>This would take the size of an integer and divide it by two. The
|
|
precedence rules have put the division outside the <code>sizeof</code> when it
|
|
was intended to be inside.
|
|
</p>
|
|
<p>Parentheses around the entire macro definition prevent such problems.
|
|
Here, then, is the recommended way to define <code>ceil_div</code>:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">#define ceil_div(x, y) (((x) + (y) - 1) / (y))
|
|
</pre></div>
|
|
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon" accesskey="n" rel="next">Swallowing the Semicolon</a>, Previous: <a href="Misnesting.html#Misnesting" accesskey="p" rel="prev">Misnesting</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>
|