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 <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> &nbsp; [<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 &amp; c, sizeof (int));
      &rarr; a = (b &amp; 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 &amp; (c + sizeof (int) - 1)) / sizeof (int);
 </pre></div>

 <p>What we want is this:
 </p>
 <div class="smallexample">
 <pre class="smallexample">a = ((b &amp; 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> &nbsp; [<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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	<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>
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