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<title>Local Labels (Using the GNU Compiler Collection (GCC))</title>

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<a name="Local-Labels"></a>
<div class="header">
<p>
Next: <a href="Labels-as-Values.html#Labels-as-Values" accesskey="n" rel="next">Labels as Values</a>, Previous: <a href="Statement-Exprs.html#Statement-Exprs" accesskey="p" rel="prev">Statement Exprs</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Locally-Declared-Labels"></a>
<h3 class="section">6.2 Locally Declared Labels</h3>
<a name="index-local-labels"></a>
<a name="index-macros_002c-local-labels"></a>

<p>GCC allows you to declare <em>local labels</em> 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>
<p>A local label declaration looks like this:
</p>
<div class="smallexample">
<pre class="smallexample">__label__ <var>label</var>;
</pre></div>

<p>or
</p>
<div class="smallexample">
<pre class="smallexample">__label__ <var>label1</var>, <var>label2</var>, /* <span class="roman">&hellip;</span> */;
</pre></div>

<p>Local label declarations must come at the beginning of the block,
before any ordinary declarations or statements.
</p>
<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><var>label</var>:</code>, within the statements of the statement expression.
</p>
<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:
</p>
<div class="smallexample">
<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 &lt; max; i++)                       \
    for (j = 0; j &lt; max; j++)                     \
      if (_SEARCH_array[i][j] == _SEARCH_target)  \
        { (value) = i; goto found; }              \
  (value) = -1;                                   \
 found:;                                          \
} while (0)
</pre></div>

<p>This could also be written using a statement expression:
</p>
<div class="smallexample">
<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 &lt; max; i++)                       \
    for (j = 0; j &lt; max; j++)                     \
      if (_SEARCH_array[i][j] == _SEARCH_target)  \
        { value = i; goto found; }                \
  value = -1;                                     \
 found:                                           \
  value;                                          \
})
</pre></div>

<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.
</p>
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Next: <a href="Labels-as-Values.html#Labels-as-Values" accesskey="n" rel="next">Labels as Values</a>, Previous: <a href="Statement-Exprs.html#Statement-Exprs" accesskey="p" rel="prev">Statement Exprs</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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