Document: WG14 N1633

Possible defect report: fmod(0.,NaN) and fmod(NaN,infinity)

Submitter: Fred J. Tydeman (USA)
Submission Date: 2012-9-13
Related:WG14 N1497
Subject:Possible defect report: fmod(0.,NaN) and fmod(NaN,infinity)

First question. When Annex F is in effect, what should the value of fmod(0.,NaN) be? The two choices are 0. or NaN.

Annex F.10.7.1 The fmod functions has:

• fmod(+/-0, y) returns +/-0 for y not zero.
• fmod(x, y) returns a NaN and raises the ''invalid'' floating-point exception for x infinite or y zero (and neither is a NaN).

So, that first bullet item says fmod(0.,NaN) is 0.

Elsewhere in that annex (F.10 Mathematics, paragraph 11), we have:

Functions with a NaN argument return a NaN result and raise no floating-point exception, except where stated otherwise.

That says that fmod(0.,NaN) is NaN.

One idea is to explicitly add words about a NaN to the first bullet item in F.10.7.1, such as:

• fmod(+/-0, y) returns +/-0 for y not zero nor NaN.

However, if F.10#11 covers NaN arguments before any other arguments are considered, then words about NaN could be removed from the second case in F.10.7.1, such as:

• fmod(x, y) returns a NaN and raises the ''invalid'' floating-point exception for x infinite or y zero.

I believe that takes us back to before N1497 was done.

Second question: what should fmod(NaN,infinity) be? Must it be the same NaN argument, or may it be any NaN?

Annex F.10.7.1 The fmod functions has:

• fmod(x, +/-infinity) returns x for x not infinite.

Which says fmod(NaN,infinity) must be the same NaN argument.

But, if F.10#11 covers this NaN argument, then this case is just some NaN.

It appears that the third bullet should either be left alone or changed to:

• fmod(x, +/-infinity) returns x for finite x.

Some other functions that discuss NaN arguments in Annex F are: frexp, ilogb, modf, hypot, pow, fmax, fmin, and fma. Of those, only hypot, pow, fmax, and fmin have exceptions on NaN in implies NaN out.