m_pktdat
((m)->m_flags & M_PKTHDR) ? &(m)->m_pktdat[0] : \
#define MPKTHSIZE __offsetof(struct mbuf, m_pktdat)
CTASSERT(MSIZE - offsetof(struct mbuf, m_pktdat) == MHLEN);
CTASSERT(offsetof(struct mbuf, m_pktdat) % 8 == 0);
m->m_data = m->m_pktdat;
to->m_data = to->m_pktdat;
to->m_data = to->m_pktdat;
n->m_data = n->m_pktdat + (m->m_data - m->m_pktdat );