Most useful GCC extensions to C language; avoid by using -pedantic.

A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.

#define maxint(a,b) \
  ({int _a = (a), _b = (b); _a > _b ? _a : _b; })

GCC allows you to declare local labels in any nested block scope. A local label is just like an ordinary label, but you can only reference it (with a goto statement, or by taking its address) within the block in which it is declared.

__label__ label;