Choosing a tolerance is hard enough when you know the floating point model you're using, but it seems like an impossible task to try to support all possible flo

Choosing a tolerance is hard enough when you know the floating point model you're using, but it seems like an impossible task to try to support all possible floating point hardware. I couldn't tell you what guarantees you have when __STDC_IEC_559__ remains undefined. These days I write my floating point code assuming float is a 32-bit IEEE 754 floating point number, and double is 64-bit. You can get those guarantees on any desktop hardware with the right flags, and the same semantics are commonly available on other platforms too. Picking a well-defined implementation makes it much easier to reason about conversions between integer and floating point types. In fact, it allows you to reason about a lot of operations, e.g. 1.0 + 2.0 == 3.0 is true; (float)183 == 183.0 is true; 0.1 == (double)0.1f is false; etc.