Question

I want to repeatedly set up a SIGALRM and wait until it goes off, then do it again. In my handler function I have:

void handler(int signum)
{
    struct itimerval timer;
    struct sigaction sa; 

    sa.sa_handler = handler;

    timer.it_interval.tv_sec = 0;
    timer.it_interval.tv_usec = 0;
    timer.it_value.tv_usec = 0;
    timer.it_value.tv_sec = 1000;

    sigaction(SIGALRM, &sa, 0);
    setitimer(ITIMER_REAL, &timer, 0);

    printf("pause for me\n");
    pause();
}

and in my main I have:

int main(void)
{
    handler(0);
    return 0;
}

However, all it does is repeat twice then stalls. I want it to continue repeating. What’s the problem?

Answer 1

struct sigaction has several fields which you are not initializing. The stack garbage you happen to get the first few times works and then you hit a case where it does not and your handler is not installed. You should test the sigaction() return. When you get -1 print a message including errno.

You should call sigaction() once (from main() or similar). It will “stick” if you don’t set the SA_RESETHAND flag. Then your other existing code will probably work.

For more accurate repeating timing you should take advantage of struct itimerval‘s it_interval which lets you set the next timer value so it will commence immediately when the current one fires.