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| 22 | <title>Operator Precedence Problems (GNU C Language Manual)</title>
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| 57 | <body lang="en">
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| 58 | <span id="Operator-Precedence-Problems"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Swallowing-the-Semicolon.html" accesskey="n" rel="next">Swallowing the Semicolon</a>, Previous: <a href="Misnesting.html" accesskey="p" rel="prev">Misnesting</a>, Up: <a href="Macro-Pitfalls.html" accesskey="u" rel="up">Macro Pitfalls</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Operator-Precedence-Problems-1"></span><h4 class="subsubsection">26.5.10.2 Operator Precedence Problems</h4>
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| 64 | <span id="index-parentheses-in-macro-bodies"></span>
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| 65 |
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| 66 | <p>You may have noticed that in most of the macro definition examples shown
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| 67 | above, each occurrence of a macro parameter name had parentheses around
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| 68 | it. In addition, another pair of parentheses usually surrounds the
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| 69 | entire macro definition. Here is why it is best to write macros that
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| 70 | way.
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| 71 | </p>
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| 72 | <p>Suppose you define a macro as follows,
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| 73 | </p>
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| 74 | <div class="example">
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| 75 | <pre class="example">#define ceil_div(x, y) (x + y - 1) / y
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| 76 | </pre></div>
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| 77 |
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| 78 | <p>whose purpose is to divide, rounding up. (One use for this operation is
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| 79 | to compute how many <code>int</code> objects are needed to hold a certain
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| 80 | number of <code>char</code> objects.) Then suppose it is used as follows:
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| 81 | </p>
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| 82 | <div class="example">
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| 83 | <pre class="example">a = ceil_div (b & c, sizeof (int));
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| 84 | → a = (b & c + sizeof (int) - 1) / sizeof (int);
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| 85 | </pre></div>
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| 86 |
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| 87 | <p>This does not do what is intended. The operator-precedence rules of
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| 88 | C make it equivalent to this:
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| 89 | </p>
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| 90 | <div class="example">
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| 91 | <pre class="example">a = (b & (c + sizeof (int) - 1)) / sizeof (int);
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| 92 | </pre></div>
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| 93 |
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| 94 | <p>What we want is this:
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| 95 | </p>
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| 96 | <div class="example">
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| 97 | <pre class="example">a = ((b & c) + sizeof (int) - 1)) / sizeof (int);
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| 98 | </pre></div>
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| 99 |
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| 100 | <p>Defining the macro as
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| 101 | </p>
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| 102 | <div class="example">
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| 103 | <pre class="example">#define ceil_div(x, y) ((x) + (y) - 1) / (y)
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| 104 | </pre></div>
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| 105 |
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| 106 | <p>provides the desired result.
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| 107 | </p>
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| 108 | <p>Unintended grouping can result in another way. Consider <code>sizeof
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| 109 | ceil_div(1, 2)</code>. That has the appearance of a C expression that would
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| 110 | compute the size of the type of <code>ceil_div (1, 2)</code>, but in fact it
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| 111 | means something very different. Here is what it expands to:
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| 112 | </p>
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| 113 | <div class="example">
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| 114 | <pre class="example">sizeof ((1) + (2) - 1) / (2)
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| 115 | </pre></div>
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| 116 |
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| 117 | <p>This would take the size of an integer and divide it by two. The
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| 118 | precedence rules have put the division outside the <code>sizeof</code> when it
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| 119 | was intended to be inside.
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| 120 | </p>
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| 121 | <p>Parentheses around the entire macro definition prevent such problems.
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| 122 | Here, then, is the recommended way to define <code>ceil_div</code>:
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| 123 | </p>
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| 124 | <div class="example">
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| 125 | <pre class="example">#define ceil_div(x, y) (((x) + (y) - 1) / (y))
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| 126 | </pre></div>
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| 127 |
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| 128 | <hr>
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| 129 | <div class="header">
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| 130 | <p>
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| 131 | Next: <a href="Swallowing-the-Semicolon.html" accesskey="n" rel="next">Swallowing the Semicolon</a>, Previous: <a href="Misnesting.html" accesskey="p" rel="prev">Misnesting</a>, Up: <a href="Macro-Pitfalls.html" accesskey="u" rel="up">Macro Pitfalls</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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