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