How to trace NaN in C++

Question

I am going to do some math calculations using C++ .

The input floating point number is a valid number, but after calculations , the resulting value becomes NaN.

I would like to trace the point where NaN value appears (possibly using gdb), instead of inserting a lot of isNan() into the code.

But, I found that even code like this will not trigger any exceptions when NaN value appears.

    double dirty = 0.0;
    double nanvalue = 0.0/dirty;

Could anyone suggest some methods for tracing the NaN or turning Nan to exceptions?

Thank you very much :)

Answer 1

One can enable so-called "signaling NaN". That should make it easily possible to make the debugger find the correct position.

Via google, I found this for enabling signaling NaNs in C++, no idea if it works:

std::numeric_limits::signaling_NaN();

http://stackoverflow.com/questions/2247447/usefulness-of-signaling-nan

Answer 2

In Visual Studio you can use the _controlfp function to set the behavior of floating-point calculations (see http://msdn.microsoft.com/en-us/library/e9b52ceh(VS.80).aspx). Maybe there is a similar variant for your platform.

Answer 3

Since you mention using gdb, here's a solution that works with gcc -- you want the functions defined in fenv.h :

#include <fenv.h>
#include <stdio.h>

int main(int argc, char **argv)
{
   double dirty = 0.0;

   feenableexcept(-1);  // Enable all floating point exceptions
   double nanval=0.0/dirty;
   printf("Succeeded! dirty=%lf, nanval=%lf\n",dirty,nanval);
}

Running the above program produces the output "Floating point exception". Without the call to feenableexcept, the "Succeeded!" message is printed.

If you were to write a signal handler for SIGFPE, that might be a good place to set a breakpoint and get the traceback you want. (Disclaimer: haven't tried it!)

Answer 4

Some notes on floating point programming can be found on http://ds9a.nl/fp/ including the difference between 1/0 and 1.0/0 etc, and what a NaN is and how it acts.