pthread_sigmask(3C) does for a thread what sigprocmask(2) does for a process. pthread_sigmask() sets the thread's signal mask. When a new thread is created, its initial mask is inherited from its creator.
The call to sigprocmask() in a multithreaded process is equivalent to a call to pthread_sigmask(). See the sigprocmask(2) man page for more information.