That is not "pure C". "pthread.h" (POSIX threads) is not part of the C language standard, it is a system-specific header commonly found on UNIX systems. Windows has a different threading system for example.
To be fair, you'll probably have better luck finding people who can compile it if you use pthreads.h vs. threads.h. IIRC glibc doesn't have threads.h implemented, and I don't know of any C standard libraries that do. pthreads is fairly universal even if it's not standard C, and at the very least it's much more universal then threads.h is at the moment.
Correct, I was just going to chime in about this. Musl is very impressive with its compact size, correctness, Drepper-less-ness =P, and speed of implementing new standards.
That is not "pure C". "pthread.h" (POSIX threads) is not part of the C language standard, it is a system-specific header commonly found on UNIX systems. Windows has a different threading system for example.