There are at least 5 or 6 major Lisp implementations that follow the standard very closely. They might have the odd quirk, and they all have extensions, but ANS

There are at least 5 or 6 major Lisp implementations that follow the standard very closely. They might have the odd quirk, and they all have extensions, but ANSI CL also provides a standardised way to handle implementation-specific parts. The situation there isn't any different from C. Edit: Not to mention, C only got threads in the most recent revision of the standard, and I can't say anything about their implementation because I've yet to encounter a project that uses C11.