m_xword
uint32_t m_xword(const struct mbuf *, uint32_t, int *);
#define xword(p, k, err) m_xword((const struct mbuf *)(p), (k), (err))
uint32_t m_xword(const struct mbuf *, uint32_t, int *);
ret0, ret0_size, ret0_maxsize, &m_xword);