c - Compare two floats

Question

#include <stdbool.h>

bool Equality(double a, double b, double epsilon)
{
  if (fabs(a-b) < epsilon) return true;
  return false;
}

I tried this method to compare two doubles, but I always get problems since I don't know how to chose the epsilon, actually I want to compare small numbers (6 6 digits after the decimal point) like 0.000001. I tried with some numbers, sometimes I get 0.000001 != 0.000001 and sometimes 0.000001 == 0.000002 Is there another method else than comparing with the epsilon?

My purpose is to compare two doubles (which represent the time in my case). The variable t which represents the time in milliseconds is a double. It is incremented by another function 0.000001 then 0.000002 etc. each time t changes, I want to check if it is equal to another variable of type double tt, in case tt == t, I have some instructions to execute..
Thanks for your help

Answer 1

Look here: http://floating-point-gui.de/errors/comparison/

Due to rounding errors, most floating-point numbers end up being slightly imprecise. As long as this imprecision stays small, it can usually be ignored. However, it also means that numbers expected to be equal (e.g. when calculating the same result through different correct methods) often differ slightly, and a simple equality test fails.

And, of course, What Every Computer Scientist Should Know About Floating-Point Arithmetic