a problem with fmod() c++

Question

Hi everyone!
I need to calculate the remainder of a number when divided by 0.001 for a code.
fmod(x,0.001) works perfectly when the remainder is not 0 (like the number 2.1503) but when the remainder is zero (like the number 3.73 or an integer like 3) it returns 0.001.
Why is that?
And how can I prevent it?

Answer 1

floating point arithmetic is not precise in general, so every floating point modulus function will have its downsides.

Answer 2

fmod is not precise.

Answer 3

So how can I calculate the remainder?