M_SIZE
explicit_bzero(M_DATABUF(m), M_SIZE(m));
KASSERT(M_DATABUF(m) + M_SIZE(m) >= (m->m_data + m->m_len));
return M_DATABUF(m) + M_SIZE(m) - (m->m_data + m->m_len);
KASSERT(((len + sizeof(long) - 1) &~ (sizeof(long) - 1)) <= M_SIZE(m));
m->m_data = M_DATABUF(m) + ((M_SIZE(m) - (len)) &~ (sizeof(long) - 1));
size += M_SIZE(m);
size += M_SIZE(n);
tail = head + M_SIZE(m0);