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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 &lt; 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&mdash;the compound statement and a null
statement&mdash;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 &lt; 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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