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<title>Operator Precedence Problems - The C Preprocessor</title>
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Next: <a rel="next" accesskey="n" href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon">Swallowing the Semicolon</a>,
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Previous: <a rel="previous" accesskey="p" href="Misnesting.html#Misnesting">Misnesting</a>,
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<h4 class="subsection">3.10.2 Operator Precedence Problems</h4>
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<p><a name="index-parentheses-in-macro-bodies-74"></a>
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You may have noticed that in most of the macro definition examples shown
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above, each occurrence of a macro argument name had parentheses around
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it. In addition, another pair of parentheses usually surround the
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entire macro definition. Here is why it is best to write macros that
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way.
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<p>Suppose you define a macro as follows,
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<pre class="smallexample"> #define ceil_div(x, y) (x + y - 1) / y
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</pre>
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<p class="noindent">whose purpose is to divide, rounding up. (One use for this operation is
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to compute how many <code>int</code> objects are needed to hold a certain
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number of <code>char</code> objects.) Then suppose it is used as follows:
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<pre class="smallexample"> a = ceil_div (b & c, sizeof (int));
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==> a = (b & c + sizeof (int) - 1) / sizeof (int);
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</pre>
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<p class="noindent">This does not do what is intended. The operator-precedence rules of
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C make it equivalent to this:
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<pre class="smallexample"> a = (b & (c + sizeof (int) - 1)) / sizeof (int);
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</pre>
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<p class="noindent">What we want is this:
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<pre class="smallexample"> a = ((b & c) + sizeof (int) - 1)) / sizeof (int);
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</pre>
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<p class="noindent">Defining the macro as
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<pre class="smallexample"> #define ceil_div(x, y) ((x) + (y) - 1) / (y)
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</pre>
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<p class="noindent">provides the desired result.
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<p>Unintended grouping can result in another way. Consider <code>sizeof
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ceil_div(1, 2)</code>. That has the appearance of a C expression that would
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compute the size of the type of <code>ceil_div (1, 2)</code>, but in fact it
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means something very different. Here is what it expands to:
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<pre class="smallexample"> sizeof ((1) + (2) - 1) / (2)
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</pre>
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<p class="noindent">This would take the size of an integer and divide it by two. The
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precedence rules have put the division outside the <code>sizeof</code> when it
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was intended to be inside.
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<p>Parentheses around the entire macro definition prevent such problems.
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Here, then, is the recommended way to define <code>ceil_div</code>:
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<pre class="smallexample"> #define ceil_div(x, y) (((x) + (y) - 1) / (y))
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</pre>
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