Problems designing a signal handler in UNIX

I'm trying to create a signal handler thread to handle SIGALRM signals.

The main thread calls the following function before creating any other threads

Code:

---

pthread_t Timer::Init(long _clock_interval_musec)
{
        // Set interval.
        clock_interval = _clock_interval_musec;
        // This is called while in a single threaded environment.
        // Main thread masks alarm.
        sigset_t alarm_off_mask;
        if ((sigemptyset(&alarm_off_mask) == -1) || (sigaddset(&alarm_off_mask, SIGALRM) == -1))
          perror("Failed to initialize the signal set");
        if (pthread_sigmask(SIG_BLOCK, &alarm_off_mask, NULL) == -1)
          perror("Failed to block SIGALRM");
        // Now call a new thread which handles alarm signals.
        pthread_t retID;
        pthread_create(&retID, NULL, TimerHandlerThread, NULL);
        // Return id of thread responsible for managing alarm signals.
        return retID;
}

---

As you can see, I block the alarm signal.

The spawned signal handler thread has the following Run function

Code:

---

void* Timer::TimerHandlerThread(void* args)
{
        // Change granualirity of alarm.
        itimerval tv;
        tv.it_interval.tv_sec  = clock_interval / USECS_IN_SECS;
        tv.it_interval.tv_usec = clock_interval % USECS_IN_SECS;

        setitimer(ITIMER_REAL, &tv, NULL);
        // Unmask alarm signals in this thread.
        sigset_t sigset;
        if ((sigemptyset(&sigset) == -1) || (sigaddset(&sigset, SIGALRM) == -1))
        perror("Failed to add signal to wait for");

        Timer currentTime;
        int sig;
        while (1)
        {
                /* wait for a signal to arrive */
                sigwait(&sigset, &sig);
              std::cout << "we are here\n";
                if (sig == SIGALRM && g_timerQueue.size() > 0)
                {
                        currentTime.Set(0);
                        // Handle if needed
                        g_mutex.P();
                        if (*(g_timerQueue.front()) < currentTime)
                        {
                                g_timerQueue.front()->Event_Handler();
                                std::pop_heap(g_timerQueue.begin(), g_timerQueue.end());
                                g_timerQueue.pop_back();
                        }
                        g_mutex.V();
                }
        }

        return NULL;
}

---

Sigwait never returns ("we are here" never prints).

clock_interval is 50.

Is the timer not firing?
Did I not set my interval correctly?
Did I not proper block the signals ensuring the that the default handler would not take care of them?

Any help is welcome. :)

If the lack of replies is due to the question not being clear, please do say so. :)

Try setting both tv.it_interval and tv.it_value.

You should flush cout if you want to see the output immediately.

gg

I'm not familiar with Unix style threading (or the Posix pthreads library) - so I'm probably not the person to help, but don't you need to add the timer signal to your signal set? (sigaddset)

Signal handlers and threads don't mix well. There's little you can do in a signal handler, and there's no control of which thread (if any) the signal handler is executed on.

To have a thread deal with particular signals, you have to block that signal in all threads, then a single thread can use sigwait() to determine if the signal is pending and act on it.
See Examples section here: http://www.opengroup.org/onlinepubs/...d_sigmask.html

gg