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<h3 class="section">6.2 Locally Declared Labels</h3>
<p><a name="index-local-labels-2873"></a><a name="index-macros_002c-local-labels-2874"></a>
GCC allows you to declare <dfn>local labels</dfn> in any nested block
scope. A local label is just like an ordinary label, but you can
only reference it (with a <code>goto</code> statement, or by taking its
address) within the block in which it is declared.
<p>A local label declaration looks like this:
<pre class="smallexample"> __label__ <var>label</var>;
</pre>
<p class="noindent">or
<pre class="smallexample"> __label__ <var>label1</var>, <var>label2</var>, /* <span class="roman">...</span> */;
</pre>
<p>Local label declarations must come at the beginning of the block,
before any ordinary declarations or statements.
<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
<var>label</var><code>:</code>, within the statements of the statement expression.
<p>The local label feature is useful for complex macros. If a 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 is multiply defined in that function. A
local label avoids this problem. For example:
<pre class="smallexample"> #define SEARCH(value, array, target) \
do { \
__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:; \
} while (0)
</pre>
<p>This could also be written using a statement expression:
<pre class="smallexample"> #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>
<p>Local label declarations also make the labels they declare visible to
nested functions, if there are any. See <a href="Nested-Functions.html#Nested-Functions">Nested Functions</a>, for details.
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