Node:Local Labels, Next:Labels as Values, Previous:Statement Exprs, Up:C Extensions

### Locally Declared Labels

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;                                          \
         })