It's not even clear what part you're dismissively replying "Nope!" to. I'll be explicit: on POSIX systems that implement the POSIX threads extension there is a

It's not even clear what part you're dismissively replying "Nope!" to. I'll be explicit: on POSIX systems that implement the POSIX threads extension there is a header file called pthread.h that declares a regular function called pthread_mutex_lock() and that function can be called as a regular function by an ISO C compliant compiler. (POSIX also allows defining macros that can achieve the same effect, possibly more efficiently, but pthread_mutex_lock() et al. have to exist also as regular function definitions.) So the point remains: pthread_mutex_lock() works not because the C compiler treats it specially. That makes sense since the C standard doesn't even mention it. Unlike setjmp() it's not part of the C standard, and it doesn't need to be, because none of its behavior requires compiler support, beyond what is already required by the platform ABI.