Wow, I would never have known about stdbool. I have never seen it used (explicitly), even though it's been around since I was two years old. Thanks for bringing this to light! :- )

Nonetheless, bool or _Bool (aside from having implicit saturating arithmetic and assignment) is a one-bit integer type. It seems it mostly just formalizes the (implied, fictional) narrowing conversion involved in testing conditions, which is probably why I've not seen it used explicitly. The macros true and false seem to expand to 1 and 0 respectively.