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<title>Swallowing the Semicolon - The C Preprocessor</title>
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Next: <a rel="next" accesskey="n" href="Duplication-of-Side-Effects.html#Duplication-of-Side-Effects">Duplication of Side Effects</a>,
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Previous: <a rel="previous" accesskey="p" href="Operator-Precedence-Problems.html#Operator-Precedence-Problems">Operator Precedence Problems</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">3.10.3 Swallowing the Semicolon</h4>
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<p><a name="index-semicolons-_0028after-macro-calls_0029-75"></a>
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Often it is desirable to define a macro that expands into a compound
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statement. Consider, for example, the following macro, that advances a
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pointer (the argument <code>p</code> says where to find it) across whitespace
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characters:
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<pre class="smallexample"> #define SKIP_SPACES(p, limit) \
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{ char *lim = (limit); \
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while (p < lim) { \
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if (*p++ != ' ') { \
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p--; break; }}}
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</pre>
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<p class="noindent">Here backslash-newline is used to split the macro definition, which must
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be a single logical line, so that it resembles the way such code would
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be laid out if not part of a macro definition.
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<p>A call to this macro might be <code>SKIP_SPACES (p, lim)</code>. Strictly
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speaking, the call expands to a compound statement, which is a complete
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statement with no need for a semicolon to end it. However, since it
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looks like a function call, it minimizes confusion if you can use it
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like a function call, writing a semicolon afterward, as in
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<code>SKIP_SPACES (p, lim);</code>
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<p>This can cause trouble before <code>else</code> statements, because the
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semicolon is actually a null statement. Suppose you write
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<pre class="smallexample"> if (*p != 0)
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SKIP_SPACES (p, lim);
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else ...
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</pre>
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<p class="noindent">The presence of two statements—the compound statement and a null
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statement—in between the <code>if</code> condition and the <code>else</code>
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makes invalid C code.
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<p>The definition of the macro <code>SKIP_SPACES</code> can be altered to solve
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this problem, using a <code>do ... while</code> statement. Here is how:
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<pre class="smallexample"> #define SKIP_SPACES(p, limit) \
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do { char *lim = (limit); \
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while (p < lim) { \
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if (*p++ != ' ') { \
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p--; break; }}} \
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while (0)
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</pre>
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<p>Now <code>SKIP_SPACES (p, lim);</code> expands into
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<pre class="smallexample"> do {...} while (0);
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</pre>
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<p class="noindent">which is one statement. The loop executes exactly once; most compilers
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generate no extra code for it.
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