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

    This routine unlocks a read-write lock that was acquired for
    read access. If there are no other holders of the lock for read
    access and another thread is waiting to acquire the lock for
    write access, that lock acquisition is granted.