executing default signal handler

Question

I have written an application where i have registered number of signal handler for different signals in linux . After process receives the signal the control is transferred to the signal handler i had registered. In this signal handler i do some work which i need to do, and then i would like to call the default signal hander i.e SIF_DFL or SIG_IGN . However, SIG_DFL and SIG_ING are both macros which expand to numeric values 0 and 1 respectively, which are invalid function addresses.

IS there any way i can call default actions i.e SIG_DFL or SIG_IGN ?

In order to achieve the effect of SIG_DFL or SIG_ING i call exit(1) and do nothing , respectively . But for signals like SIGSEGV i also would like to have core dump . In general i would want to my default behavior to be same as SIG_DFL and ignore behavior same SIG_IGN , the way Operating system would do .

Answer 1

The GNU C Library Reference Manual has a whole chapter explaining everything about signal handling.

You always get the previously set signal handler (a function pointer) when you install your own handler (see manpages for signal() or sigaction()).

previous_handler = signal(SIGINT, myhandler);

The general rule is, that you can always reset to the previous handler and raise() the signal again.

void myhandler(int sig) {
  /* own stuff .. */
  signal(sig, previous_handler);
  raise(sig);
  /* when it returns here .. set our signal handler again */
  signal(sig, myhandler);
}

There is one disadvantage of the general rule: Hardware exceptions which are mapped to signals are usually assigned to a certain instruction which caused the exception. So, when you raise a signal again, the associated instruction is not the same as originally. This can but should not harm other signal handlers.

Another disadvantage is, that each raised signal causes a lot of processing time. To prevent excessive use of raise() you can use the following alternatives:

1. In case of SIG_DFL the function pointer points to address 0 (which is obviously no valid address). Thus, you have to reset the handler and raise() the signal again.

   if (previous_handler == SIG_DFL)
   {
     signal(sig, SIG_DFL);
     raise(sig);
     signal(sig, myhandler);
   } 

2. SIG_IGN has value 1 (also an invalid address). Here you can just return (do nothing).

   else if (previous_handler == SIG_IGN)
   {
     return;
   } 

3. Otherwise (neither SIG_IGN nor SIG_DFL) you have received a valid function pointer and you can call the handler directly,

   else
   {
     previous_handler(sig);
   }

Of course, you have to consider the different APIs as well (see manpages for signal() and sigaction()).

Answer 2

You can save the previous handler and then call it when the time is right.

Install handler. Make sure you save old handler

static struct sigaction new_sa, old_sa;

new_sa.sa_handler = my_handler;
sigemptyset(&new_handler.sa_mask);

if (sigaction(signo, &new_sa, &old_sa) == -1) {
    /* handle sigaction error */
}

In your new handler, call the old handler

(*old_sa.sa_handler)(signo)

You don't need to raise it again or do any messy stuff; simply call the old handler (of course, since you saved the sigaction you have acces to the old disposition and so on).

Answer 3

The usual approach is to reset the signal handler and then raise() the signal again:

Here's an example SIGINT handler:

void sigint_handler(int num)
{
    /* handle SIGINT */

    // call default handler
    signal(SIGINT, SIG_DFL);
    raise(SIGINT);
}

Answer 4

Given the signal handlers are implemented in kernel, the only way I see is to

• reset the handler and
• raise() the signal again