Cancelling thread that is stuck on epoll_wait

I'm doing some event handling with C++ and pthreads. I have a main thread that reads from event queue I defined, and a worker thread that fills the event queue. The queue is of course thread safe.

The worker thread have a list of file descriptors and create an epoll system call to get events on those file descriptors. It uses epoll_wait to wait for events on the fd's.

Now the problem. Assuming I want to terminate my application cleanly, how can I cancel the worker thread properly? epoll_wait is not one of the cancellation points of pthread(7) so it cannot react properly on pthread_cancel.

The worker thread main() looks like this

while(m_WorkerRunning) {
    epoll_wait(m_EpollDescriptor, events, MAXEVENTS, -1);
    //handle events and insert to queue
}

The m_WorkerRunning is set to true when the thread starts and it looks like I can interrupt the thread by settings m_WorkerRunning to false from the main thread. The problem is that epoll_wait theoretically can wait forever.

Other solution I though about is: instead of waiting forever (-1) I can wait for example X time slots, then handle properly no-events case and if m_WorkerRunning == false then exit the loop and terminate the worker thread cleanly. The main thread then sets m_WorkerRunning to false, and sleeps X. However I'm not sure about the performance of such epoll_wait and also not sure what would be the correct X? 500ms? 1s? 10s?

I'd like to hear some experienced advises!

More relevant information: the fd's I'm waiting events on, are devices in /dev/input so technically I'm doing some sort of input subsystem. The targeted OS is Linux (latest kernel) on ARM architecture.

Thanks!

You could send the thread a signal which would interupt the blocking call to epoll_wait(). If doing so modify your code like this:

while(m_WorkerRunning) 
{
  int result = epoll_wait(m_EpollDescriptor, events, MAXEVENTS, -1);
  if (-1 == result)
  {
    if (EINTR == errno)
    {
      /* Handle shutdown request here. */ 
      break;
    }
    else
    {
      /* Error handling goes here. */
    }
  }

  /* Handle events and insert to queue. */
}

---

A way to add a signal handler:

#include <signal.h>

/* A generic signal handler doing nothing */
void signal_handler(int sig)
{
  sig = sig; /* Cheat compiler to not give a warning about an unused variable. */
}

/* Wrapper to set a signal handler */
int signal_handler_set(int sig, void (*sa_handler)(int))
{
  struct sigaction sa = {0};
  sa.sa_handler = sa_handler;
  return sigaction(sig, &sa, NULL);
}

To set this handler for the signal SIGUSR1 do:

if (-1 == signal_handler_set(SIGUSR1, signal_handler))
{
  perror("signal_handler_set() failed");
}

To send a signal SIGUSR1 from another process:

if (-1 == kill(<target process' pid>, SIGUSR1))
{
  perror("kill() failed");
}

To have a process send a signal to itself:

if (-1 == raise(SIGUSR1))
{
  perror("raise() failed");
}