Sure, you'll need to handle special cases yourself. But perhaps you don't have to handle all the cases yourself?

As I understand it, one of the new things in IEEE 754 is the idea of a "context", which stores this information. This can be global, but does not need to be. With Python's decimal module it is a thread-local variable.

If you are concerned about, say, mixing thread-local and async, you can also use context methods directly, like:

  >>> import decimal
  >>> x = decimal.Decimal("54.1234")
  >>> y = decimal.Decimal("987.340")
  >>> x+y
  Decimal('1041.4634')
  >>> decimal.getcontext()
  Context(prec=28, rounding=ROUND_HALF_EVEN, Emin=-999999, Emax=999999,
  capitals=1, clamp=0, flags=[], traps=[InvalidOperation, DivisionByZero,
  Overflow])
  >>>
  >>> c1 = decimal.Context(prec=4)
  >>> c1.add(x, y)
  Decimal('1041')
  >>> c2 = decimal.Context(prec=5)
  >>> c2.add(x, y)
  Decimal('1041.5')
  >>> c3 = decimal.Context(prec=5, rounding=decimal.ROUND_DOWN)
  >>> c3.add(x, y)
  Decimal('1041.4')

I don't know what the C or C++ API proposals are.