Question

I’m using epoll to manage about 20 to 30 sockets. I figure out that epoll_wait can be used to wait for some data to arrive over one of the socket but I’m missing how do I implement timeouts on socket level. I can use timeout on epoll_wait but it not very useful in my case. For example, if I need to every close a socket where no activity is recorded for > 500 ms orr may be send some data to a socket every 200 ms no matter what. How can these socket level timeout be implemented using epoll? Any suggestion and idea would be appreciated!

Thanks,
Shivam Kalra

Answer 1

Sounds like you’re trying to write an event loop (if so have a look at libev btw). epoll will not help you there, you have to keep track of socket inactivity yourself (clock_gettime() or gettimeofday() for instance), then wake up several times a second and check everything you need.

Some pseudo code

while (1) {
    n = epoll_wait(..., 5);
    if (n > 0) {
        /* process activity */
    } else {
        /* process inactivity */
    }
}

This will wake you up 200 times a second if all sockets are inactive.

The inactivity check requires a list of the sockets to be examined along with timestamps of the last inactivity:

struct sockstamp_s {
    /* socket descriptor */
    int sockfd;
    /* last active */
    struct timeval tv;
};

/* check which socket has been inactive */
for (struct sockstamp_s *i = socklist; ...; i = next(i)) {
    if (diff(s->tv, now()) > 500) {
        /* socket s->sockfd was inactive for more than 500 ms */
        ...
    }
}

where diff() gives you the difference of 2 struct timevals and now() gives you the current timestamp.