Those memory barriers are in the function implementation. They don't exist at the call site. Again, from the perspective of the compiler it's a regular function call.

> However setjmp()/longjmp() are different beasts entirely

Yes, exactly, they are not "equivalent" to pthread_mutex_lock() at all, which is what you suggested in the beginning. A call to pthread_mutex_lock() is a regular function call as far as the compiler is concerned.

Nope! The memory synchronizing properties of those functions are at the specification level. POSIX says so, and so the implementation has to make it so, somehow. That could involve recognizing those functions in the compiler. Usually external function calls are good enough to have a compiler barrier (the compiler won't reorder accesses around those locking calls), so that the function then just has to contain the hardware memory barriers.

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.