mtod
# define mtod(m, t) ((t)(m)->data)
#define mtod(_v, _t) (_t)((_v)->b_rptr)
#define mtod(_a, _t) ((_t)((_a)->b_rptr))
#define mtod(m, t) ((t)((m)->b_rptr))
#define mtod(m, t) ((t)(m)->m_data)
#define mtod(m, t) ((t)((m)->m_data))