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<title>Duplication of Side Effects - The C Preprocessor</title>
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<a name="Duplication-of-Side-Effects"></a>
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Next: <a rel="next" accesskey="n" href="Self_002dReferential-Macros.html#Self_002dReferential-Macros">Self-Referential Macros</a>,
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Previous: <a rel="previous" accesskey="p" href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon">Swallowing the Semicolon</a>,
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Up: <a rel="up" accesskey="u" href="Macro-Pitfalls.html#Macro-Pitfalls">Macro Pitfalls</a>
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<h4 class="subsection">3.10.4 Duplication of Side Effects</h4>
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<p><a name="index-side-effects-_0028in-macro-arguments_0029-76"></a><a name="index-unsafe-macros-77"></a>Many C programs define a macro <code>min</code>, for “minimum”, like this:
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<pre class="smallexample"> #define min(X, Y) ((X) < (Y) ? (X) : (Y))
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</pre>
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<p>When you use this macro with an argument containing a side effect,
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as shown here,
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<pre class="smallexample"> next = min (x + y, foo (z));
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</pre>
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<p class="noindent">it expands as follows:
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<pre class="smallexample"> next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));
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</pre>
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<p class="noindent">where <code>x + y</code> has been substituted for <code>X</code> and <code>foo (z)</code>
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for <code>Y</code>.
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<p>The function <code>foo</code> is used only once in the statement as it appears
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in the program, but the expression <code>foo (z)</code> has been substituted
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twice into the macro expansion. As a result, <code>foo</code> might be called
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two times when the statement is executed. If it has side effects or if
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it takes a long time to compute, the results might not be what you
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intended. We say that <code>min</code> is an <dfn>unsafe</dfn> macro.
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<p>The best solution to this problem is to define <code>min</code> in a way that
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computes the value of <code>foo (z)</code> only once. The C language offers
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no standard way to do this, but it can be done with GNU extensions as
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follows:
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<pre class="smallexample"> #define min(X, Y) \
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({ typeof (X) x_ = (X); \
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typeof (Y) y_ = (Y); \
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(x_ < y_) ? x_ : y_; })
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</pre>
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<p>The ‘<samp><span class="samp">({ ... })</span></samp>’ notation produces a compound statement that
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acts as an expression. Its value is the value of its last statement.
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This permits us to define local variables and assign each argument to
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one. The local variables have underscores after their names to reduce
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the risk of conflict with an identifier of wider scope (it is impossible
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to avoid this entirely). Now each argument is evaluated exactly once.
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<p>If you do not wish to use GNU C extensions, the only solution is to be
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careful when <em>using</em> the macro <code>min</code>. For example, you can
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calculate the value of <code>foo (z)</code>, save it in a variable, and use
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that variable in <code>min</code>:
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<pre class="smallexample"> #define min(X, Y) ((X) < (Y) ? (X) : (Y))
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...
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{
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int tem = foo (z);
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next = min (x + y, tem);
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}
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</pre>
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<p class="noindent">(where we assume that <code>foo</code> returns type <code>int</code>).
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