C89 compatible but not written in C89, which is the point. And almost certainly just C89 compatible because C99 didn't change the ABI, so that happened "for fre

C89 compatible but not written in C89, which is the point. And almost certainly just C89 compatible because C99 didn't change the ABI, so that happened "for free". But the actual source, the thing people actually write in, is not C89. Linux heavily leverages new language and compiler extensions. It doesn't handicap itself with only using C89, because that's just silly.