Is fmod() exact when y is an integer?

Question

In using double fmod(double x, double y) and y is an integer, the result appears to be always exact.

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Answer 1

The result of fmod is always exact; whether y is an integer is irrelevant. Of course, if x and/or y are already approximations of some real numbers a and b, then fmod(x,y) is unlikely to be exactly equal to a mod b.