ARC_HDRLEN
if (m->m_len < ARC_HDRLEN)
m = m_pullup(m, ARC_HDRLEN);/* gcc does structure padding */
hlen = ARC_HDRLEN;
M_PREPEND(m, ARC_HDRLEN, M_DONTWAIT);
m_adj(m, ARC_HDRLEN);
m_adj(m, ARC_HDRLEN);
ifp->if_hdrlen = ARC_HDRLEN;
bpf_attach(ifp, DLT_ARCNET, ARC_HDRLEN);