How to raise a float point exception

Question

guys. I am doing some work around float point operations. The 0.1 is inexact represented by binary float point format. Thus I wrote down this

float i = 0.1f;

and expecting the inexact exception to arise. I turnned on the -fp-trap-all=all option, set fp-mode to be strict and installed SIGFPE signal handler in my code. But nothing happened. Then I tried

float i = 0.1f,j = 0.2f, c;
c = i + j;

still can not catch any exceptions! It drive my crazy.

Sorry to mention that I am using intel c++ compiler on Linux at last.

Answer 1

You have to test for exceptions yourself. The following code works for me:

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

#ifndef FE_INEXACT
#  error No FP Exception handling!
#endif

int main()
{
    double a = 4.0;
    a /= 3.0;

    if (fetestexcept(FE_INEXACT) & FE_INEXACT)
    {
        printf("Exception occurred\n");
    }
    else
    {
        printf("No exception.\n");
    }
}

If you replace 4.0 by 3.0, you will not get the exception.

You can do something similar with double a = 0.0; a = sin(a);.

---

Trapping exceptions are only supported conditionally. To check, use the macros described in the documentation:

#define _GNU_SOURCE
#include <fenv.h>

#ifndef FE_NOMASK_ENV
#  warning Cannot raise FP exceptions!
#else
#  warning Trapping FP exceptions enabled.
feenableexcept(FE_INEXACT);
#endif

Answer 2

According to this answer, inexact exception is only raised if the rounded version of the float isn't the same as the mathematically exact amount. In your case, the rounded response is the same, so no exception raised.