int m_copy(struct mbuf *n, struct mbuf *m, int off, int len)
{
    if (len > M_FREEROOM(n))
        return -1;

    memcpy((n->m_data + n->m_len), (m->m_data + off), len);
    n->m_len += len;
    return 0;
}
int m_copy(struct mbuf *n, struct mbuf *m, int off, int len)
{
    if (len > M_FREEROOM(n))
        return -1;

    memcpy((n->m_data + n->m_len), (m->m_data + off), len);
    n->m_len += len;
    return 0;
}