How to trace a 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 the calculations, the resulting value is 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 an exception when a NaN value appears.

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

Could anyone suggest a method for tracing the NaN or turning a NaN into an exception?

Toth

Related: Can I make gcc tell me when a calculation results in NaN or inf at runtime? – Millsaps

Also #5394497 – Margravine

Answer 1

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.

Nitty

Hi, thank you for your answer. Unfortunately my platform is GCC4.1 , Centos 5.5 Linux. I am still trying to figure out how to achieve the same thing in Linux with GCC – Toth

feenableexcept() seems working, I am now trying to trace my program with this. gcc.gnu.org/onlinedocs/gcc-3.1.1/g77/… – Toth

Answer 2

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

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

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

   feenableexcept(FE_ALL_EXCEPT & ~FE_INEXACT);  // Enable all floating point exceptions but FE_INEXACT
   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!)

Baroscope

Thanks for this answer, I just want to point out that enabling all the floating point exceptions enables also FE_INEXACT which happens very often (even for: float f = 0.1) and the debug becomes impossible. Is much better to use feenableexcept(FE_DIVBYZERO| FE_INVALID|FE_OVERFLOW); – Tichon

Here's a useful page for OS X, which doesn't include feenableexcept: www-personal.umich.edu/~williams/archive/computation/… – Landmark

Answer 3

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.

Excitable

Answer 4

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();

Usefulness of signaling NaN?

Dorton

That expression evaluates to a signaling NaN, but I think pagedown is more interested in forcing a floating point exception for operations that return NaN, which would probably be a platform-specific compiler option, or runtime configuration of the FP processor. – Baroscope

Ah sorry. I understood it as making all NaN signaling. – Dorton

Thank you very much. My platform is GCC4.1 , Centos 5.5 Linux I am going to take a look at the compiler options :) – Toth