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Node:<a name="Local%20Labels">Local Labels</a>,
Next:<a rel="next" accesskey="n" href="Labels-as-Values.html#Labels%20as%20Values">Labels as Values</a>,
Previous:<a rel="previous" accesskey="p" href="Statement-Exprs.html#Statement%20Exprs">Statement Exprs</a>,
Up:<a rel="up" accesskey="u" href="C-Extensions.html#C%20Extensions">C Extensions</a>
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<h3 class="section">Locally Declared Labels</h3>
<p>Each statement expression is a scope in which <dfn>local labels</dfn> can be
declared. A local label is simply an identifier; you can jump to it
with an ordinary <code>goto</code> statement, but only from within the
statement expression it belongs to.
<p>A local label declaration looks like this:
<pre class="example"> __label__ <var>label</var>;
</pre>
<p>or
<pre class="example"> __label__ <var>label1</var>, <var>label2</var>, /* <small class="dots">...</small> */;
</pre>
<p>Local label declarations must come at the beginning of the statement
expression, right after the <code>({</code>, before any ordinary
declarations.
<p>The label declaration defines the label <em>name</em>, but does not define
the label itself. You must do this in the usual way, with
<code></code><var>label</var><code>:</code>, within the statements of the statement expression.
<p>The local label feature is useful because statement expressions are
often used in macros. If the macro contains nested loops, a <code>goto</code>
can be useful for breaking out of them. However, an ordinary label
whose scope is the whole function cannot be used: if the macro can be
expanded several times in one function, the label will be multiply
defined in that function. A local label avoids this problem. For
example:
<pre class="example"> #define SEARCH(array, target) \
({ \
__label__ found; \
typeof (target) _SEARCH_target = (target); \
typeof (*(array)) *_SEARCH_array = (array); \
int i, j; \
int value; \
for (i = 0; i < max; i++) \
for (j = 0; j < max; j++) \
if (_SEARCH_array[i][j] == _SEARCH_target) \
{ value = i; goto found; } \
value = -1; \
found: \
value; \
})
</pre>
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