RT_ADVANCE
RT_ADVANCE(cp, sa);
RT_ADVANCE(cp, sa[i]);
#define RT_XADVANCE(a,b) RT_ADVANCE(a,b)
RT_ADVANCE(cp, ((const struct sockaddr *)&s)); \
RT_ADVANCE(cp, sa);
RT_ADVANCE(cp, (struct sockaddr *)(void *)&s); \
RT_ADVANCE(sa, sa);