// create Tuple from values
HeapTuple
heap_form_tuple(TupleDesc tupDesc, Datum *values, bool *isnull)

// return Datum given Tuple, attr and descriptor
//   sets isnull to true if value is NULL
#define heap_getattr(tup, attnum, tupleDesc, isnull) ...

// returns true if attribute has no value
bool heap_attisnull(HeapTuple tup, int attnum) ...

// produce a modified tuple from an existing one
HeapTuple
heap_modify_tuple(HeapTuple tuple, TupleDesc tupleDesc,
                  Datum *replValues, bool *replIsnull,
                  bool *doReplace)