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| <h4 class="subsection">3.10.3 Swallowing the Semicolon</h4> |
| |
| <p><a name="index-semicolons-_0028after-macro-calls_0029-75"></a> |
| 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: |
| |
| <pre class="smallexample"> #define SKIP_SPACES(p, limit) \ |
| { char *lim = (limit); \ |
| while (p < lim) { \ |
| if (*p++ != ' ') { \ |
| p--; break; }}} |
| </pre> |
| <p class="noindent">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>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>This can cause trouble before <code>else</code> statements, because the |
| semicolon is actually a null statement. Suppose you write |
| |
| <pre class="smallexample"> if (*p != 0) |
| SKIP_SPACES (p, lim); |
| else ... |
| </pre> |
| <p class="noindent">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>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: |
| |
| <pre class="smallexample"> #define SKIP_SPACES(p, limit) \ |
| do { char *lim = (limit); \ |
| while (p < lim) { \ |
| if (*p++ != ' ') { \ |
| p--; break; }}} \ |
| while (0) |
| </pre> |
| <p>Now <code>SKIP_SPACES (p, lim);</code> expands into |
| |
| <pre class="smallexample"> do {...} while (0); |
| </pre> |
| <p class="noindent">which is one statement. The loop executes exactly once; most compilers |
| generate no extra code for it. |
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