echo_server_by_epoll.c

int sock = socket(PF_INET, SOCK_STREAM);
bind(sock, addr);
listen(sock);

struct epoll_event ev;
struct epoll_event events[1024];

int epfd = epoll_create(1024);
ev.events = EPOLLIN;
ev.data.fd = sock;
epoll_ctl(epfd, EPOLL_CTL_ADD, sock, &ev);

while ( 1 ) {
    int nfd = epoll_wait(epfd, events, 1024, -1);

     for (i = 0; i < nfd; i++) {
         if ( events[i].data.fd == sock ) {
             int client = accept(sock, &addr);
             ev.events = EPOLLIN;
             ev.data.fd = client;
             epoll_ctl(epfd, EPOLL_CTL_ADD, client, &ev);    
         }
         else {
             int client = events[i].data.fd;
             int n = read(client, buffer, sizeof buffer);
             if (n < 0) {
                 perror("read");
                 epoll_ctl(epfd, EPOLL_CTL_DEL, client, &ev);
                 close(client);
             } else if (n == 0) {
                 epoll_ctl(epfd, EPOLL_CTL_DEL, client, &ev);
                 close(client);
             } else {
                 write(client, buffer, n);
             }
         }
     }
}