Signal handler for all signal

Question

How can I register a signal handler for ALL signal, available on the running OS, using signal(3)?

My code looks like this:

void sig_handler(int signum)
{
    printf("Received signal %d\n", signum);
}


int main()
{
    signal(ALL_SIGNALS_??, sig_handler);
    while (1) {
        sleep(1);
    };
    return 0;
}

Answer 1

Most systems have a macro NSIG or _NSIG (the former would not be available in standards-conformance mode since it violates the namespace) defined in signal.h such that a loop for (i=1; i<_NSIG; i++) will walk all signals. Also, on POSIX systems that have signal masks, CHAR_BIT*sizeof(sigset_t) is an upper bound on the number of signals which you could use as a fallback if neither NSIG nor _NSIG is defined.

Answer 2

Signal handlers have to deal with reentrancy concerns and other problems. In practice, it's often more convenient to mask signals and then retrieve them from time to time. You can mask all signals (except SIGSTOP and SIGKILL, which you can't handle anyway) with this:

sigset_t all_signals;
sigfillset(&all_signals);
sigprocmask(SIG_BLOCK, &all_signals, NULL);

The code is slightly different if you're using pthreads. Call this in every thread, or (preferably) in the main thread before you create any others:

sigset_t all_signals;
sigfillset(&all_signals);
pthread_sigmask(SIG_BLOCK, &all_signals, NULL);

Once you've done that, you should periodically call sigtimedwait(2) like this:

struct timespec no_time = {0, 0};
siginfo_t result;
int rc = sigtimedwait(&all_signals, &result, &no_time);

If there is a signal pending, information about it will be placed in result and rc will be the signal number; if not, rc will be -1 and errno will be EAGAIN. If you're already calling select(2)/poll(2) (e.g. as part of some event-driven system), you may want to create a signalfd(2) instead and attach it to your event loop. In this case, you still need to mask the signals as shown above.