Visual Studio C++ 2008 / 2010 - break on float NaN

Question

Is there any way to set up Visual Studio (just upgraded from 2008 to 2010) to break, as if an assertion failed, whenever any floating point number becomes NaN, QNAN, INF, etc?

Up until now I have just been using the assert(x == x) trick, but I would rather something implicit, so that I dont have to add assertions everywhere.

Quite surprised I can't find an answer to this via google. Some stuff about 'floating point exceptions', but I'm not sure if they are the same thing, and I've tried enabling them in Visual Studio, but the program doesn't break until something catastrophic happens because of the NaN later on in execution.

Answer 1

1) Go to project option and enable /fp:strict (C/C++ -> Code Generation -> Floating Pint Model).

2) Use _controlfp to set the floating-point control word (see code below).

#include <float.h>
unsigned int fp_control_state = _controlfp(_EM_INEXACT, _MCW_EM);

#include <math.h>

int main () {

    sqrtf(-1.0);    // floating point exception

    double x = 0.0;
    double y = 1.0/x;   // floating point exception

    return 0;
}

Answer 2

I am not sure if this is possible the way you want it, but You could create an macro which wraps the code in the marked line into an assert or which sets a breakpoint for this.

Hope this helps

Answer 3

At least on x86, when you generate an NaN etc, one of the FPU status register bits is set. There's a way you can set so that it throws a H/W exception on the next subsequent FP operation occurs, but that's not quite as soon as you hoped for. I can't recall the reference though.

Answer 4

Try enabling fp exceptions