C++: how to throw EXCEPTION_FLT_UNDERFLOW?

Question

Ok, it sounds strange, but I need a sample code, that throws EXCEPTION_FLT_UNDERFLOW. I already have code to handle that exception. Now I need sample, that throws this damn EXCEPTION_FLT_UNDERFLOW. Any advises?

Answer 1

Try:

RaiseException(EXCEPTION_FLT_UNDERFLOW, EXCEPTION_NONCONTINUABLE, 0, NULL);

Answer 2

throw EXCEPTION_FLT_UNDERFLOW;

Answer 3

Even in C99 there is no portable way to do this. This works on Linux:

#define _GNU_SOURCE
#include <fenv.h>
int
main(void)
{
  fesetenv(FE_NOMASK_ENV);
  feraiseexcept(FE_UNDERFLOW);
  return 0;
} 

but without the #define _GNU_SOURCE and the fesetenv call (which are not portable), the exception is not thrown (== does not trigger SIGFPE, in Unix terms), it just sets a flag. And no iteration of MSVC supports <fenv.h>, so you're stuck with completely nonstandard on Windows.

That said, there does appear to be a Windows-specific equivalent:

#include <float.h>
#pragma fenv_access (on)
int
main(void)
{
  _controlfp_s(0, 0, _MCW_EM); /* throw all floating point exceptions */
  /* trigger floating-point underflow here */
}

You will have to cause an underflow condition using actual floating point operations. I don't know how to do that off the top of my head. It would probably be best to do it by hand in assembly language, to avoid interference from the compiler (even more non-portability, yes, but if you're on Windows you probably don't care about any CPU but x86).

Answer 4

Assuming you want actual code that will trigger this:

#include <float.h>

int main()
{
    _controlfp_s(NULL, 0, _MCW_EM); // enable all floating point exceptions

    float f= 1.0f;

    while (f)
    {
        f/=2.0f;
        // __asm fwait; // optional, if you want to trap the underflow sooner
    }

    return 0;
}