M_PKTHDR
if (m->m_flags & M_PKTHDR) {
if (m->m_flags & M_PKTHDR) {
if (((m = m0)->m_flags & M_PKTHDR) && (m->m_pkthdr.len < totlen))
#define M_COPYFLAGS (M_PKTHDR|M_EOR|M_BCAST|M_MCAST)
(m)->m_flags = M_PKTHDR; \
((m)->m_flags & M_PKTHDR) ? &(m)->m_pktdat[0] : \
((m)->m_flags & M_PKTHDR) ? MHLEN : MLEN)