For selection, you'll use a combination of global functions. First, use the acedSSGet() function to obtain the selection set. Then, use the acedSSNameX() function to obtain a subentity GS marker for the selected entity.
int acedSSGet( const char * str, const void * pt1, const void * pt2, const struct resbuf * filter, ads_name ss); int acedSSNameX( struct resbuf ** rbpp, const ads_name ss, const long i);