what does C/C++ handler SIGFPE?

Question

well, I have searched the articles about SIGFPE ,then I wrote a few tests but it's behavoir is strange. Then I have to post it here to ask for help. Is the GCC/G++ or ISO C++ clearly defined what happens if divide by zero?

1) I searched the article : Division by zero does not throw SIGFPE it sames the output is inf

2) If I rewrite it as the following:

void signal_handler (int signo) {
    if(signo == SIGFPE) {
      std::cout << "Caught FPE\n";
    }
}

int main (void) {
  signal(SIGFPE,(*signal_handler));

  int b = 1;
  int c = 0;
  int d = b/c;
  //fprintf(stderr,"d number is %d\n,d);
  return 0;
}

then signal_handler will not happens. but if I uncomment the line

//fprintf(stderr,"d number is %d\n,d);

then signal_handler keeps calling.

can someone explains it ?

Answer 1

This is interesting: With the fprintf commented out, the compiler has determined that the calculated result: d = b/c is an unused local expression and can be optimized away.

Clearly though, it is not side-effect free in its execution, but the compiler can't determine anything about the runtime environment at this stage. I'm surprised that static analysis doesn't pick this up as a warning (at least) in a modern compiler.

@vonbrand is right. You got lucky with what you're doing in the (asynchronous) signal handler.

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Edit: when you say "signal_handler keeps calling", do you mean it's repeating indefinitely? If so, there could be issues with underlying system calls restarting. Try: siginterrupt(SIGFPE, 1); (assuming it's available).