C ++应用程序中NaN的原因没有引发浮点异常

Question

To find the cause of floating point variables beeing set to NaN in my C++ program I enabled floating point exceptions like this:

#include <fenv.h>
feenableexcept(FE_INVALID | FE_OVERFLOW);

I know it works because when I write:

int val = 0.0/0.0;

in my program a floating point exception is risen. But NaNs are "spreading" through the floating point calculations of my program and I have no idea which variable is set to NaN first.

What causes for a variable beeing set to NaN exist, that would not cause a floating point exception?

Answer 1

If any input has a quiet NaN value, it will cause the result of nearly any floating-point operation that consumes it to be NaN without raising the invalid exception .

The most likely explanation is that some data is being read from the wrong address, and that the read data (which may not even be floating-point data) happens to match a quiet NaN encoding. This can happen pretty easily, because the relatively common pattern 0xffff... encodes a quiet NaN.

Answer 2

Are you doing any square root operation? If you try to compute a square root for a negative number as in sqrt(-1) then you will get a Nan. In this case you should get an 'Invalid Operation' exception though. Are you sure you are trapping correctly all exceptions? It seems like a numeric exception is untrapped somewhere in your code.