
// Copied from
// https://banu.com/blog/2/how-to-use-epoll-a-complete-example-in-c/epoll-example.c

// Meant to be used for edge triggered epoll

#include <errno.h>
#include <fcntl.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/epoll.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.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;   /* Return IPv4 and IPv6 choices */
    hints.ai_socktype = SOCK_STREAM; /* We want a TCP socket */
    hints.ai_flags    = AI_PASSIVE;  /* All interfaces */

    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)
            /* We managed to bind successfully! */
            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);

    /* The event loop */
    while (1) {
        int n, i;

        n = epoll_wait(efd, events, MAXEVENTS, -1);
        for (i = 0; i < n; i++) {
            if ((events[i].events & EPOLLERR) || (events[i].events & EPOLLHUP) ||
                (!(events[i].events & EPOLLIN))) {
                /* An error has occured on this fd, or the socket is not
                   ready for reading (why were we notified then?) */
                fprintf(stderr, "epoll error\n");
                close(events[i].data.fd);
                continue;
            }

            else if (sfd == events[i].data.fd) {
                /* We have a notification on the listening socket, which
                   means one or more incoming connections. */
                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)) {
                            /* We have processed all incoming
                               connections. */
                            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);

                    /* Make the incoming socket non-blocking and add it to the
                       list of fds to monitor. */
                    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 {
                /* We have data on the fd waiting to be read. Read and
                   display it. We must read whatever data is available
                   completely, as we are running in edge-triggered mode
                   and won't get a notification again for the same
                   data. */
                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, that means we have read all
                           data. So go back to the main loop. */
                        if (errno != EAGAIN) {
                            perror("read");
                            done = 1;
                        }
                        break;
                    } else if (count == 0) {
                        /* End of file. The remote has closed the
                           connection. */
                        done = 1;
                        break;
                    }

                    /* Write the buffer to standard output */
                    s = write(1, buf, count);
                    if (s == -1) {
                        perror("write");
                        abort();
                    }
                }

                if (done) {
                    printf("Closed connection on descriptor %d\n", events[i].data.fd);

                    /* Closing the descriptor will make epoll remove it
                       from the set of descriptors which are monitored. */
                    close(events[i].data.fd);
                }
            }
        }
    }

    free(events);
    close(sfd);

    return EXIT_SUCCESS;
}
