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 lock was acquired.
    [EBUSY]     The lock is being held for write access. The lock for
                read access was not acquired.
    [EINVAL]    The value specified by lock is not a valid read-write
                lock.