This routine attempts to acquire a read-write lock for read
    access. If the lock cannot be granted, the routine returns
    without waiting.

    When a thread calls this routine, an attempt is made to
    immediately acquire the lock for read access. If the lock is
    acquired, zero (0) is returned. If a holder of the lock for write
    access exists, [EBUSY] is returned.

    If the lock cannot be acquired for read access immediately, the
    calling program does not wait for the lock to be released.