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(gcc-295.info)Local Labels

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Locally Declared Labels
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   Each statement expression is a scope in which "local labels" can be
declared.  A local label is simply an identifier; you can jump to it
with an ordinary `goto' statement, but only from within the statement
expression it belongs to.

   A local label declaration looks like this:

     __label__ LABEL;

or

     __label__ LABEL1, LABEL2, ...;

   Local label declarations must come at the beginning of the statement
expression, right after the `({', before any ordinary declarations.

   The label declaration defines the label *name*, but does not define
the label itself.  You must do this in the usual way, with `LABEL:',
within the statements of the statement expression.

   The local label feature is useful because statement expressions are
often used in macros.  If the macro contains nested loops, a `goto' 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:

     #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;                                          \
     })