Question

#include <iostream>
#include <signal.h>
#include <fenv.h>
#include <string.h>

void signal_handler(int sig, siginfo_t *siginfo, void* context) 
{ 
  std::cout << " signal_handler " << fetestexcept(FE_ALL_EXCEPT) << std::endl;
  throw "exception"; 
}

void divide() {
  float a = 1000., b = 0.,  c, f = 1e-300;
  c = a / b;
  
  std::cout << c << " and f = " << f << std::endl;  
}

void init_sig_hanlder() {
  feenableexcept(FE_ALL_EXCEPT);
  
  struct sigaction sa, initial_sa;
  
  sa.sa_sigaction   = &signal_handler ;
  sigemptyset( &sa.sa_mask ) ;
  sa.sa_flags   = SA_SIGINFO;   // man sigaction(3) // allows for void(*)(int,siginfo_t*,void*) handler
  
  sigaction(SIGFPE, &sa, &initial_sa);

}

int main(int argc, char** argv) {
  init_sig_hanlder();
  
  while(true)
    {
      try {
    sleep(1);
    divide();
      }
      catch(const char * a) {
    std::cout << "Exception in catch: " << a << std::endl;
      }    
      catch(...) {
    std::cout << "Exception in ..." << std::endl;
      }    
    }
  
  return 0;
}

Produce the following results on Linux/g++4.2:

signal_handler 0
Exception in catch: exception
inf and f = 0
inf and f = 0
inf and f = 0
inf and f = 0

So, signal handler is executed the first time but the next fp exception does not trigger the handler again. Where am I wrong ?

Answer 1

I don’t think throwing an exception inside a signal handler is good practice. The operating system expects the signal handler to return, because the signal is blocked while the handler for it is called. By throwing an exception you prevent the system from unblocking the signal.