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; \
})