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 is acquired for write
                access.
    [EBUSY]     The lock was not acquired for write access, as it is
                already held by another thread.
    [EINVAL]    The value specified by lock is not a valid read-write
                lock.