28.17 Rounding Control

Here we describe how to specify the rounding mode at run time. System header file fenv.h provides the prototypes for these functions. See Rounding in The GNU C Library Reference Manual.

That header file also provides constant names for the four rounding modes: FE_DOWNWARD, FE_TONEAREST, FE_TOWARDZERO, and FE_UPWARD.

The function fegetround examines and returns the current rounding mode. On a platform with IEEE 754 floating point, the value will always equal one of those four constants. On other platforms, it may return a negative value. The function fesetround sets the current rounding mode.

Changing the rounding mode can be slow, so it is useful to minimize the number of changes. For interval arithmetic, we seem to need three changes for each operation, but we really only need two, because we can write code like this example for interval addition of two reals:

{
  struct interval_double
    {
      double hi, lo;
    } v;
  extern volatile double x, y;
  int rule;

  rule = fegetround ();

  if (fesetround (FE_UPWARD) == 0)
    {
      v.hi = x + y;
      v.lo = -(-x - y);
    }
  else
    fatal ("ERROR: failed to change rounding rule");

  if (fesetround (rule) != 0)
    fatal ("ERROR: failed to restore rounding rule");
}

The volatile qualifier (see volatile) is essential on x86 platforms to prevent an optimizing compiler from producing the same value for both bounds.