m_hdr
struct m_hdr mh, md;
struct m_hdr m_hdr; \
#define m_next m_hdr.mh_next
#define m_len m_hdr.mh_len
#define m_data m_hdr.mh_data
#define m_owner m_hdr.mh_owner
#define m_type m_hdr.mh_type
#define m_flags m_hdr.mh_flags
#define m_nextpkt m_hdr.mh_nextpkt
#define m_paddr m_hdr.mh_paddr
m = malloc(sizeof(struct m_hdr) + len + off);
m->m_data = (char *)m + sizeof(struct m_hdr) + off;