round.c

#include <stdio.h>
#include <math.h>
#include <fenv.h>
#include <assert.h>
#include <caml/mlvalues.h>
#include <caml/alloc.h>

static void change_round_mode(int mode) {
  switch (mode) {
    case NearestTiesToEven:
      fesetround(FE_TONEAREST);
      break;
    case ToZero:
      fesetround(FE_TOWARDZERO);
      break;
    case Up:
      fesetround(FE_UPWARD);
      break;
    case Down:
      fesetround(FE_DOWNWARD);
      break;
    default: assert(0); break;
  }
}

CAMLprim value addc(value mode, value x, value y) {
    volatile double r, x1 = Double_val(x), y1 = Double_val(y);
    change_round_mode(Int_val(mode));
    r = x1 + y1;
    change_round_mode(NearestTiesToEven);
    return caml_copy_double(r);
}