Can't figure out epoll example in manual - c

Question

Recently, I am study system call epoll. I already have basic concept about this topic, but I stuck in example given by manual.
/* Code to set up listening socket, 'listen_sock',
(socket(), bind(), listen()) omitted */
epollfd = epoll_create1(0);
if (epollfd == -1) {
perror("epoll_create1");
exit(EXIT_FAILURE);
}
ev.events = EPOLLIN;
ev.data.fd = listen_sock;
if (epoll_ctl(epollfd, EPOLL_CTL_ADD, listen_sock, &ev) == -1) {
perror("epoll_ctl: listen_sock");
exit(EXIT_FAILURE);
}
for (;;) {
nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1);
if (nfds == -1) {
perror("epoll_wait");
exit(EXIT_FAILURE);
}
for (n = 0; n < nfds; ++n) {
if (events[n].data.fd == listen_sock) {
conn_sock = accept(listen_sock,
(struct sockaddr *) &addr, &addrlen);
if (conn_sock == -1) {
perror("accept");
exit(EXIT_FAILURE);
}
setnonblocking(conn_sock);
ev.events = EPOLLIN | EPOLLET;
ev.data.fd = conn_sock;
if (epoll_ctl(epollfd, EPOLL_CTL_ADD, conn_sock,
&ev) == -1) {
perror("epoll_ctl: conn_sock");
exit(EXIT_FAILURE);
}
} else {
do_use_fd(events[n].data.fd);
}
}
}
Following is my assumption:
Above is server code.
listen_sock is file descriptor return from socket() in server.
My question is:
What did file descriptor events[n].data.fd mean? Is server's fd or client's?
Why if events[n].data.fd == listen_sock , then we need to create a new connection?

Answer 1

You can look at the man page for epoll_wait for details on struct epoll_event and its data member and to help make sense of the code.
Any given events[n].data.fd refers to an fd that had some kind of event on it. It could either be the listening socket, or it could be a client socket if there are active client connections.
If the fd with activity is the listen_sock listening socket, this means that a new client is attempting a connection. At this point we accept the connection and add the new conn_sock fd to the set of epoll's fds.