sm_off
db->sm_off = mb->sm_off ;
memcpy(((char *)(db->sm_data+db->sm_off)),(char *)sm,
(struct smt_header *)(db->sm_data+db->sm_off),
short sm_off ; /* offset in m_data */
#define smtod(x,t) ((t)((x)->sm_data + (x)->sm_off))
mb->sm_off-- ; /* set to fc */
mb->sm_off = 8 ;