If an error condition occurs, this routine returns an integer
    value indicating the type of error. Possible return values are as
    follows:

    Return      Description

    0           Successful completion; the read-write lock object was
                acquired for read access.
    [EINVAL]    The value specified by rwlock does not refer to an
                initialized read-write lock object.
    [EDEADLCK]  The calling thread already owns the specified read-
                write lock object for write access.
    [EAGAIN]    The lock for read access could not be acquired
                because the maximum number of read lock acquisitions
                for rwlock has been exceeded.