m_dat
&(m)->m_dat[0])
#define MHSIZE __offsetof(struct mbuf, m_dat)
CTASSERT(MSIZE - offsetof(struct mbuf, m_dat) == MLEN);
CTASSERT(offsetof(struct mbuf, m_dat) % 8 == 0);
CTASSERT(offsetof(struct mbuf, m_dat) == 32);
CTASSERT(offsetof(struct mbuf, m_dat) == 24);
n->m_data + len < &n->m_dat[MLEN] && n->m_next) {
space = &m->m_dat[MLEN] - (m->m_data + m->m_len);
space = &m->m_dat[MLEN] - (m->m_data + m->m_len);
m->m_data = m->m_dat;