I should also note that after a lively discussion, it's probably not accurate for me to say that pthread_join() is "interruptable", as the man page doesn't claim that it can be, technically, interrupted (returning EINTR). That being said, it can't be disputed that the C program is "suspending" the pthread_join(), or something, in order to call the ALRM signal handler, and then goes back to waiting in the pthread_join().