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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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