Thread: How to raise different floating point exceptions in C?

I am interested in raising floating point exceptions like Division by zero, Overflow, Underflow, etc.

I am using functions in fenv.h and signal handling. Whenever, any floating point exception occurs,

The problem seems like when inside the function catch_fpe ,the flags set by feraiseexcept somehow gets cleared and no condition is true.

Code:

 
#define _GNU_SOURCE
#include <fenv.h>
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
void catch_fpe(int sig)
{
  //#pragma STDC FENV_ACCESS ON 
  printf("i am in catch_fpe\n");
  if (fetestexcept(FE_OVERFLOW))
    printf("OVERFLOW\n");
  else if (fetestexcept(FE_UNDERFLOW))
    printf("UNDERFLOW\n");
  else if (fetestexcept(FE_DIVBYZERO))
    printf("DIVBYZERO\n");
  else if (fetestexcept(FE_INVALID))
    printf("INVALID OPERATION\n");
  else if (fetestexcept(FE_INEXACT))
    printf("INEXACT RESULT\n");
  exit(0);
}

int main()
{
  feclearexcept(FE_ALL_EXCEPT);
  feenableexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW);
  signal(SIGFPE, catch_fpe);
  feraiseexcept(FE_DIVBYZERO);
}

To deal with floating point exceptions is something very machine dependent... Notice the signal is being catched, but the kernel do not keep the floating point context in context switching, so the "default" fenv (in theory, available only when you declare the pragma STD FENV_ACESS ON), isn't available to the signal routine (I think, not sure).

I suggest you avoid catching SIGFPE, since this is one of those "fatal error" signals. Deal with NaNs and Infs in your code to check the validity of your calculations...

PS: POSIX standard (or is it SysV?) recommends to add 128 to the signal number, on exit. Your exit() call in the signal handler should be:

Code:

exit(128+sig);