Why and when should one call _fpreset( )?

Question

The only documentation I can find (on MSDN or otherwise) is that a call to _fpreset() "resets the floating-point package." What is the "floating point package?" Does this also clear the FPU status word? I see documentation that says to call _fpreset() when recovering from a SIGFPE, but doesn't _clearfp() do this as well? Do I need to call both?

I am working on an application that unmasks some FP exceptions (using _controlfp()). When I want to reset the FPU to the default state (say, when calling to .NET code), should I just call _clearfp(), _fpreset(), or both. This is performance critical code, so I don't want to call both if I don't have to...

Answer 1

_fpreset() resets the state of the floating-point unit. It resets the FPU precision to its default and clears the FPU status word. The two occasitions I see to use it are when recovering from an FPE (as you said) and when getting control back from library code (e.g. a DLL that you have no control about) that has screwed the FPU in any way, like changing the precision.