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| <title>The C Preprocessor: Duplication of Side Effects</title> |
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| <a name="Duplication-of-Side-Effects"></a> |
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| Next: <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros" accesskey="n" rel="next">Self-Referential Macros</a>, Previous: <a href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon" accesskey="p" rel="prev">Swallowing the Semicolon</a>, Up: <a href="Macro-Pitfalls.html#Macro-Pitfalls" accesskey="u" rel="up">Macro Pitfalls</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> |
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| <a name="Duplication-of-Side-Effects-1"></a> |
| <h4 class="subsection">3.10.4 Duplication of Side Effects</h4> |
| |
| <a name="index-side-effects-_0028in-macro-arguments_0029"></a> |
| <a name="index-unsafe-macros"></a> |
| <p>Many C programs define a macro <code>min</code>, for “minimum”, like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define min(X, Y) ((X) < (Y) ? (X) : (Y)) |
| </pre></div> |
| |
| <p>When you use this macro with an argument containing a side effect, |
| as shown here, |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">next = min (x + y, foo (z)); |
| </pre></div> |
| |
| <p>it expands as follows: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">next = ((x + y) < (foo (z)) ? (x + y) : (foo (z))); |
| </pre></div> |
| |
| <p>where <code>x + y</code> has been substituted for <code>X</code> and <code>foo (z)</code> |
| for <code>Y</code>. |
| </p> |
| <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 <em>unsafe</em> macro. |
| </p> |
| <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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define min(X, Y) \ |
| ({ typeof (X) x_ = (X); \ |
| typeof (Y) y_ = (Y); \ |
| (x_ < y_) ? x_ : y_; }) |
| </pre></div> |
| |
| <p>The ‘<samp>({ … })</samp>’ 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> |
| <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>: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define min(X, Y) ((X) < (Y) ? (X) : (Y)) |
| … |
| { |
| int tem = foo (z); |
| next = min (x + y, tem); |
| } |
| </pre></div> |
| |
| <p>(where we assume that <code>foo</code> returns type <code>int</code>). |
| </p> |
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| Next: <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros" accesskey="n" rel="next">Self-Referential Macros</a>, Previous: <a href="Swallowing-the-Semicolon.html#Swallowing-the-Semicolon" accesskey="p" rel="prev">Swallowing the Semicolon</a>, Up: <a href="Macro-Pitfalls.html#Macro-Pitfalls" accesskey="u" rel="up">Macro Pitfalls</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> |
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