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| 22 | <title>Duplication of Side Effects (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Macro-Pitfalls.html" rel="up" title="Macro Pitfalls">
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| 57 | <body lang="en">
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| 58 | <span id="Duplication-of-Side-Effects"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Macros-and-Auto-Type.html" accesskey="n" rel="next">Macros and Auto Type</a>, Previous: <a href="Swallowing-the-Semicolon.html" accesskey="p" rel="prev">Swallowing the Semicolon</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="Duplication-of-Side-Effects-1"></span><h4 class="subsubsection">26.5.10.4 Duplication of Side Effects</h4>
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| 64 |
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| 65 | <span id="index-side-effects-_0028in-macro-arguments_0029"></span>
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| 66 | <span id="index-unsafe-macros"></span>
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| 67 | <p>Many C programs define a macro <code>min</code>, for “minimum”, like this:
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| 68 | </p>
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| 69 | <div class="example">
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| 70 | <pre class="example">#define min(X, Y) ((X) < (Y) ? (X) : (Y))
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| 71 | </pre></div>
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| 72 |
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| 73 | <p>When you use this macro with an argument containing a side effect,
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| 74 | as shown here,
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| 75 | </p>
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| 76 | <div class="example">
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| 77 | <pre class="example">next = min (x + y, foo (z));
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| 78 | </pre></div>
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| 79 |
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| 80 | <p>it expands as follows:
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| 81 | </p>
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| 82 | <div class="example">
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| 83 | <pre class="example">next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));
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| 84 | </pre></div>
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| 85 |
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| 86 | <p>where <code>x + y</code> has been substituted for <code>X</code> and <code>foo (z)</code>
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| 87 | for <code>Y</code>.
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| 88 | </p>
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| 89 | <p>The function <code>foo</code> is used only once in the statement as it
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| 90 | appears in the program, but the expression <code>foo (z)</code> has been
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| 91 | substituted twice into the macro expansion. As a result, <code>foo</code>
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| 92 | might be called twice when the statement is executed. If it has side
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| 93 | effects or if it takes a long time to compute, that may be
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| 94 | undesirable. We say that <code>min</code> is an <em>unsafe</em> macro.
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| 95 | </p>
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| 96 | <p>The best solution to this problem is to define <code>min</code> in a way that
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| 97 | computes the value of <code>foo (z)</code> only once. In general, that requires
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| 98 | using <code>__auto_type</code> (see <a href="Auto-Type.html">Auto Type</a>). How to use it for this
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| 99 | is described in the following section. See <a href="Macros-and-Auto-Type.html">Macros and Auto Type</a>.
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| 100 | </p>
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| 101 | <p>Otherwise, you will need to be careful when <em>using</em> the macro
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| 102 | <code>min</code>. For example, you can calculate the value of <code>foo
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| 103 | (z)</code>, save it in a variable, and use that variable in <code>min</code>:
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| 104 | </p>
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| 105 | <div class="example">
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| 106 | <pre class="example">#define min(X, Y) ((X) < (Y) ? (X) : (Y))
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| 107 | /* <span class="roman">…</span> */
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| 108 | {
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| 109 | int tem = foo (z);
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| 110 | next = min (x + y, tem);
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| 111 | }
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| 112 | </pre></div>
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| 113 |
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| 114 | <p>(where we assume that <code>foo</code> returns type <code>int</code>).
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| 115 | </p>
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| 116 | <p>When the repeated value appears as the condition of the <code>?:</code>
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| 117 | operator and again as its <var>iftrue</var> expression, you can avoid
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| 118 | repeated execution by omitting the <var>iftrue</var> expression, like this:
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| 119 | </p>
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| 120 | <div class="example">
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| 121 | <pre class="example">#define x_or_y(X, Y) ((X) ? : (Y))
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| 122 | </pre></div>
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| 123 |
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| 124 | <p>In GNU C, this expands to use the first macro argument’s value if that
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| 125 | isn’t zero. If that’s zero, it compiles the second argument and uses
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| 126 | that value. See <a href="Conditional-Expression.html">Conditional Expression</a>.
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| 127 | </p>
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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="Macros-and-Auto-Type.html" accesskey="n" rel="next">Macros and Auto Type</a>, Previous: <a href="Swallowing-the-Semicolon.html" accesskey="p" rel="prev">Swallowing the Semicolon</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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