Why is my signal handler not working?

Question

I want to handle the signal SIGTSTP (18) on Linux. This is my code:

void handler(int signum){
    printf("Signal %d has tried to stop me.", signum);
}

int main(void){
    struct sigaction act;
    sigset_t mask;

    act.sa_handler = handler;
    act.sa_flags = 0;

    sigemptyset(&mask);
    sigaddset(&mask, SIGTSTP);
    sigprocmask(SIG_SETMASK, &mask, NULL);

    sigaction(SIGTSTP, &act, NULL);

    pause();

}

When I send a signal from another terminal this way:

$ kill -18 PID

the handler() function does not run. I've tried replacing SIGTSTP with 18 in the call to sigaction(). It does not work.

Any ideas? Thanks for your time.

nullptr nullptr · Accepted Answer · 2015-01-31T02:31:55

You are deliberately blocking delivery of SIGTSTP to your process by doing

sigaddset(&mask, SIGTSTP);
sigprocmask(SIG_SETMASK, &mask, NULL);

sigprocmask with SIG_SETMASK sets a mask to block signals. Remove the call to sigprocmask, and the signal should be delivered.

One other thing to do is to zero act before filling it in:

memset(&act, 0, sizeof(act));

Why is my signal handler not working?

2 Answers