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 <a name="Swallowing-the-Semicolon"></a>
 <div class="header">
 <p>
 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> &nbsp; [<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>
 </div>
 <hr>
 <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 &lt; 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 &hellip;
 </pre></div>

 <p>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>
 <p>The definition of the macro <code>SKIP_SPACES</code> can be altered to solve
 this problem, using a <code>do &hellip; while</code> statement.  Here is how:
 </p>
 <div class="smallexample">
 <pre class="smallexample">#define SKIP_SPACES(p, limit)     \
 do { char *lim = (limit);         \
      while (p &lt; 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 {&hellip;} while (0);
 </pre></div>

 <p>which is one statement.  The loop executes exactly once; most compilers
 generate no extra code for it.
 </p>
 <hr>
 <div class="header">
 <p>
 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> &nbsp; [<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"></a>
	<div class="header">
	<p>
	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>
	</div>
	<hr>
	<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>
	<hr>
	<div class="header">
	<p>
	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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