The classic select() and poll() have O(n) scan overhead — with 10,000 fds, every call scans all 10,000 even if only 1 is ready. epoll maintains a kernel-side data structure and returns only the fds that are actually ready — O(1) per event, O(k) where k is ready events.

#include <sys/epoll.h>
#include <fcntl.h>

/* Set fd to non-blocking */
void set_nonblocking(int fd) {
    int flags = fcntl(fd, F_GETFL, 0);
    fcntl(fd, F_SETFL, flags | O_NONBLOCK);
}

/* Create epoll instance and event loop */
int epoll_server(int lfd) {
    int epfd = epoll_create1(0);

    /* Add listener to epoll */
    struct epoll_event ev = { .events = EPOLLIN, .data.fd = lfd };
    epoll_ctl(epfd, EPOLL_CTL_ADD, lfd, &ev);

    struct epoll_event events[1024];
    while (1) {
        int n = epoll_wait(epfd, events, 1024, -1);  /* -1 = block forever */
        for (int i = 0; i if (events[i].data.fd == lfd) {
                /* New connection */
                int cfd = accept(lfd, NULL, NULL);
                set_nonblocking(cfd);
                struct epoll_event cev = {
                    .events = EPOLLIN | EPOLLET,  /* edge-triggered */
                    .data.fd = cfd
                };
                epoll_ctl(epfd, EPOLL_CTL_ADD, cfd, &cev);
            } else {
                /* Data ready on existing connection */
                char buf[4096];
                ssize_t nr;
                /* Edge-triggered: MUST read until EAGAIN */
                while ((nr = recv(events[i].data.fd, buf, sizeof(buf), 0)) > 0)
                    process(buf, nr);
                if (nr == 0) {  /* connection closed */
                    epoll_ctl(epfd, EPOLL_CTL_DEL, events[i].data.fd, NULL);
                    close(events[i].data.fd);
                }
                /* nr == -1 && errno == EAGAIN: no more data right now */
            }
        }
    }
}

/* Edge-triggered vs Level-triggered */
# EPOLLET (edge): notify ONCE when state changes (unread→readable)
#   Must read ALL data immediately or it won't be reported again
#   Higher performance (fewer epoll_wait wakeups)
# Level (default): notify every time data is available
#   Easier to code correctly; acceptable for most applications