void m_adj(struct mbuf *m, int len)
{
    if (m == NULL)
        return;
    if (len >= 0) {
        /* Trim from head */
        m->m_data += len;
        m->m_len -= len;
    } else {
        /* Trim from tail */
        len = -len;
        m->m_len -= len;
    }
}
void m_adj(struct mbuf *m, int len)
{
    if (m == NULL)
        return;
    if (len >= 0) {
         
        m->m_data += len;
        m->m_len -= len;
    } else {
         
        len = -len;
        m->m_len -= len;
    }
}