Problems designing a signal handler in UNIX

[19bernardo87]

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

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

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.

[19bernardo87]

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

[Codeplug]

Try setting both tv.it_interval and tv.it_value.

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

gg

[majic79]

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)

[Codeplug]

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