VMS Help POSIX Threads, TIS routines, tis_mutex_lock, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    This routine locks the specified mutex mutex. A deadlock can
    result if the owner of a mutex calls this routine in an attempt
    to lock the same mutex a second time. (The Threads Library may
    not detect or report the deadlock.)

    In a threaded environment, the thread that has locked a mutex
    becomes its current owner and remains the owner until the same
    thread has unlocked it. This routine returns with the mutex
    in the locked state and with the current thread as the mutex's
    current owner.