What should i know when using floats/doubles between different machines?

I've heard that there are many problems with floats/doubles on different CPU's.

If i want to make a game that uses floats for everything, how can i be sure the float calculations are exactly the same on every machine so that my simulation will look exactly same on every machine?

I am also concerned about writing/reading file开发者_C百科s or sending/receiving the float values to different computers. What conversions there must be done, if any?

I need to be 100% sure that my float values are computed exactly the same, because even a slight difference in the calculations will result in a totally different future. Is this even possible ?

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Standard C++ does not prescribe any details about floating point types other than range constraints, and possibly that some of the maths functions (like sine and exponential) have to be correct up to a certain level of accuracy.

Other than that, at that level of generality, there's really nothing else you can rely on!

That said, it is quite possible that you will not actually require binarily identical computations on every platform, and that the precision and accuracy guarantees of the float or double types will in fact be sufficient for simulation purposes.

Note that you cannot even produce a reliable result of an algebraic expression inside your own program when you modify the order of evaluation of subexpressions, so asking for the sort of reproducibility that you want may be a bit unrealistic anyway. If you need real floating point precision and accuracy guarantees, you might be better off with an arbitrary precision library with correct rounding, like MPFR - but that seems unrealistic for a game.

Serializing floats is an entirely different story, and you'll have to have some idea of the representations used by your target platforms. If all platforms were in fact to use IEEE 754 floats of 32 or 64 bit size, you could probably just exchange the binary representation directly (modulo endianness). If you have other platforms, you'll have to think up your own serialization scheme.

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What every programmer should know: http://docs.sun.com/source/806-3568/ncg_goldberg.html