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| <h4 class="subsection">3.10.2 Operator Precedence Problems</h4> |
| |
| <p><a name="index-parentheses-in-macro-bodies-74"></a> |
| 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>Suppose you define a macro as follows, |
| |
| <pre class="smallexample"> #define ceil_div(x, y) (x + y - 1) / y |
| </pre> |
| <p class="noindent">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: |
| |
| <pre class="smallexample"> a = ceil_div (b & c, sizeof (int)); |
| ==> a = (b & c + sizeof (int) - 1) / sizeof (int); |
| </pre> |
| <p class="noindent">This does not do what is intended. The operator-precedence rules of |
| C make it equivalent to this: |
| |
| <pre class="smallexample"> a = (b & (c + sizeof (int) - 1)) / sizeof (int); |
| </pre> |
| <p class="noindent">What we want is this: |
| |
| <pre class="smallexample"> a = ((b & c) + sizeof (int) - 1)) / sizeof (int); |
| </pre> |
| <p class="noindent">Defining the macro as |
| |
| <pre class="smallexample"> #define ceil_div(x, y) ((x) + (y) - 1) / (y) |
| </pre> |
| <p class="noindent">provides the desired result. |
| |
| <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: |
| |
| <pre class="smallexample"> sizeof ((1) + (2) - 1) / (2) |
| </pre> |
| <p class="noindent">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>Parentheses around the entire macro definition prevent such problems. |
| Here, then, is the recommended way to define <code>ceil_div</code>: |
| |
| <pre class="smallexample"> #define ceil_div(x, y) (((x) + (y) - 1) / (y)) |
| </pre> |
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