This routine acquires a read-write lock for read access. If no
    thread already holds the lock for write access and there are no
    writers waiting to acquire the lock, the lock for read access
    is granted to the calling thread and this routine returns. If
    a thread already holds the lock for read access, the lock is
    granted and this routine returns.

    A thread can hold multiple, concurrent locks for read access on
    the same read-write lock. In a given thread, for each call to
    this routine that successfully acquires the same read-write lock
    for read access, a corresponding call to pthread_rwlock_unlock
    must be issued.

    If some thread already holds the lock for write access, the
    calling thread will not acquire the read lock. If the read lock
    is not acquired, the calling thread blocks until it can acquire
    the lock for read access. Results are undefined if the calling
    thread has already acquired a lock for write access on rwlock
    when this routine is called.

    If the read-write lock object referenced by rwlock is not
    initialized, the results of calling this routine are undefined.

    If a thread is interrupted (via a Tru64 UNIX signal or an OpenVMS
    AST) while waiting for a read-write lock for read access, upon
    return from the interrupt routine the thread resumes waiting for
    the lock as if it had not been interrupted.