A sample program of how epoll works.

epoll_sample.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/epoll.h>
#include <errno.h>

#define MAXEVENTS 64

static int make_socket_non_blocking(int sfd)
{
    int flags, s;

    flags = fcntl(sfd, F_GETFL, 0);
    if(flags == -1)
    {
        perror("fcntl");
        return -1;
    }

    flags |= O_NONBLOCK;
    s = fcntl(sfd, F_SETFL, flags);
    if(s == -1)
    {
        perror("fcntl");
        return -1;
    }

    return 0;
}

static int create_and_bind(char *port)
{
    struct addrinfo hints;
    struct addrinfo *result, *rp;
    int s, sfd;

    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE;

    s = getaddrinfo(NULL, port, &hints, &result);
    if(s != 0)
    {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
        return -1;
    }

    for(rp=result;rp!=NULL;rp=rp->ai_next)
    {
        sfd = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol);
        if(sfd == -1)
        {
            continue;
        }

        s = bind(sfd, rp->ai_addr, rp->ai_addrlen);
        if(s == 0)
        {
            break;
        }

        close(sfd);
    }

    if(rp == NULL)
    {
        fprintf(stderr, "Could not bind\n");
        return -1;
    }

    freeaddrinfo(result);
    return sfd;
}

int main(int argc, char *argv[])
{
    int sfd, s;
    int efd;
    struct epoll_event event;
    struct epoll_event *events;

    if(argc != 2)
    {
        fprintf(stderr, "Usage: %s [port]\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    sfd = create_and_bind(argv[1]);
    if(sfd == -1)
        abort();

    s = make_socket_non_blocking(sfd);
    if(s == -1)
        abort();

    s = listen(sfd, SOMAXCONN);
    if(s == -1)
    {
        perror("listen");
        abort();
    }

    efd = epoll_create1(0);
    if(efd == -1)
    {
        perror("epoll_create");
        abort();
    }

    event.data.fd = sfd;
    event.events = EPOLLIN | EPOLLET;
    s = epoll_ctl(efd, EPOLL_CTL_ADD, sfd, &event);
    if(s == -1)
    {
        perror("epoll_ctl");
        abort();
    }

    /* Buffer where events are returned */
    events = calloc(MAXEVENTS, sizeof(event));

    /* event loop */
    while(1)
    {
        int n, i;
        
        fprintf(stdout, "Blocking and waiting for epoll event...\n");
        n = epoll_wait(efd, events, MAXEVENTS, -1);
        fprintf(stdout, "Received epoll event\n");
        for(i=0; i<n; i++)
        {
            if((events[i].events & EPOLLERR) ||
               (events[i].events & EPOLLHUP) ||
               (!(events[i].events & EPOLLIN)))
            {
                fprintf(stderr, "epoll error\n");
                close(events[i].data.fd);
                continue;
            }
            else if(sfd == events[i].data.fd)
            {
                while(1)
                {
                    struct sockaddr in_addr;
                    socklen_t in_len;
                    int infd;
                    char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];

                    in_len = sizeof(in_addr);
                    infd = accept(sfd, &in_addr, &in_len);
                    if(infd == -1)
                    {
                        if((errno == EAGAIN) ||
                           (errno == EWOULDBLOCK))
                        {
                            break;
                        }
                        else
                        {
                            perror("accept");
                            break;
                        }
                    }
                    s = getnameinfo(&in_addr, in_len,
                                    hbuf, sizeof(hbuf),
                                    sbuf, sizeof(sbuf),
                                    NI_NUMERICHOST | NI_NUMERICSERV);
                    if(s == 0)
                    {
                        printf("Accepted connection on descriptor %d"
                              "(host=%s, port=%s)\n", infd, hbuf, sbuf);
                    }

                    s = make_socket_non_blocking(infd);
                    if(s == -1)
                        abort();

                    event.data.fd = infd;
                    event.events = EPOLLIN | EPOLLET;
                    s = epoll_ctl(efd, EPOLL_CTL_ADD, infd, &event);
                    if(s == -1)
                    {
                        perror("epoll_ctl");
                        abort();
                    }
                }
                continue;
            }
            else
            {
                int done = 0;

                while(1)
                {
                    ssize_t count;
                    char buf[512];

                    count = read(events[i].data.fd, buf, sizeof(buf));
                    if(count == -1)
                    {
                        if(errno != EAGAIN)
                        {
                            perror("read");
                            done =1;
                        }
                        break;
                    }
                    else if(count == 0)
                    {
                        done = 1;
                        break;
                    }

                    buf[count] = '\n';
                    s = write(1, buf, count+1);
                    if(s == -1)
                    {
                        perror("write");
                        abort();
                    }

                    s = write(events[i].data.fd, buf, count+1);
                    if(s == -1)
                    {
                        perror("socket write");
                        abort();
                    }
                }
                if(done)
                {
                    printf("Closed connection on descriptor %d\n",
                           events[i].data.fd);
                    close(events[i].data.fd);
                }
            }
        }
    }

    free(events);
    close(sfd);

    return EXIT_SUCCESS;
}

How to trigger an event for epoll_wait to release.