c++ - Rounding differences on Windows vs Unix based system in sprintf

Question

I have problem on UNIX based systems sprintf does not round up properly value.

For example

double tmp = 88888888888885.875
char out[512];

Thats 88,888,888,888,885.875 just to be easier on eyes. I am giving such specific and big example because it seems it works fine on smaller numbers.

I am trying to use it in following way

sprintf(out, "%021.2f", tmp);
printf("out = %s
", tmp);

On windows this results in:

out = 000088888888888885.88

On for example AIX, but shows in Linux as well:

out = 000088888888888885.87

Why is this happening? Any ideas and how to make it behave same way on Win/Unix

Thanks

Answer 1

What floating point representations are used by your processor and your compiler?

Not all processors use the same way to represent floating-point values and even compilers may choose different floating-point representation methods (I think the Microsoft C++ compiler even has options to choose the representation).

The page http://www.quadibloc.com/comp/cp0201.htm gives an overview of some of the floating-point representations (although they seem to be rather old architectures shown there).

http://msdn.microsoft.com/en-us/library/0b34tf65.aspx describes how Microsoft Visual C++ stores floating-point values. I couldn't find immediately what representation is used by AIX or Linux.

Additinally, every compiler has options that let you indicate how you want to work with floating-point operations. Do you want them to be correct as possible (but possibly somewhat slower)? Or do you want floating-point operations to be fast as possible (but possibly less correct)?