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| <title>The C Preprocessor: Swallowing the Semicolon</title> |
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| <a name="Swallowing-the-Semicolon"></a> |
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| Next: <a href="Duplication-of-Side-Effects.html#Duplication-of-Side-Effects" accesskey="n" rel="next">Duplication of Side Effects</a>, Previous: <a href="Operator-Precedence-Problems.html#Operator-Precedence-Problems" accesskey="p" rel="prev">Operator Precedence Problems</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="Swallowing-the-Semicolon-1"></a> |
| <h4 class="subsection">3.10.3 Swallowing the Semicolon</h4> |
| <a name="index-semicolons-_0028after-macro-calls_0029"></a> |
| |
| <p>Often it is desirable to define a macro that expands into a compound |
| statement. Consider, for example, the following macro, that advances a |
| pointer (the argument <code>p</code> says where to find it) across whitespace |
| characters: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define SKIP_SPACES(p, limit) \ |
| { char *lim = (limit); \ |
| while (p < lim) { \ |
| if (*p++ != ' ') { \ |
| p--; break; }}} |
| </pre></div> |
| |
| <p>Here backslash-newline is used to split the macro definition, which must |
| be a single logical line, so that it resembles the way such code would |
| be laid out if not part of a macro definition. |
| </p> |
| <p>A call to this macro might be <code>SKIP_SPACES (p, lim)</code>. Strictly |
| speaking, the call expands to a compound statement, which is a complete |
| statement with no need for a semicolon to end it. However, since it |
| looks like a function call, it minimizes confusion if you can use it |
| like a function call, writing a semicolon afterward, as in |
| <code>SKIP_SPACES (p, lim);</code> |
| </p> |
| <p>This can cause trouble before <code>else</code> statements, because the |
| semicolon is actually a null statement. Suppose you write |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">if (*p != 0) |
| SKIP_SPACES (p, lim); |
| else … |
| </pre></div> |
| |
| <p>The presence of two statements—the compound statement and a null |
| statement—in between the <code>if</code> condition and the <code>else</code> |
| makes invalid C code. |
| </p> |
| <p>The definition of the macro <code>SKIP_SPACES</code> can be altered to solve |
| this problem, using a <code>do … while</code> statement. Here is how: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define SKIP_SPACES(p, limit) \ |
| do { char *lim = (limit); \ |
| while (p < lim) { \ |
| if (*p++ != ' ') { \ |
| p--; break; }}} \ |
| while (0) |
| </pre></div> |
| |
| <p>Now <code>SKIP_SPACES (p, lim);</code> expands into |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">do {…} while (0); |
| </pre></div> |
| |
| <p>which is one statement. The loop executes exactly once; most compilers |
| generate no extra code for it. |
| </p> |
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