qtail
if ((m = m0 = qtail(q)) == NULL)
qtail(q) = NULL;
qtail(q) = prev;
if ((m = qtail(q)) == NULL)
qtail(q) = NULL;
if (m == qtail(q))
qtail(q) = prev;
m0 = qtail(q);
qtail(q) = NULL;
else if (qtail(q) == m)
qtail(q) = prev;
if (k != sc->qlen || k > MAX_MBUFQ || *mp || mp != sc->qtail) \
sc->qtail = &sc->qhead;
sc->qtail = &sc->qhead;
if (sc->qtail == &sc->qhead)
sc->qtail = &m->m_nextpkt;
*sc->qtail = m;
sc->qtail = &m->m_nextpkt;
struct mbuf *qhead, **qtail; /* Queue of outgoing mbuf's */