[epoll] How to use epoll in socket program?

When we do socket program, the most popular way is of PPC/TPC mode ( Process Per Connection / Thread Per Connection ). But the mode has some drawbacks, for instance, it will consum too much memory and CPU for context switch when creating more than hundreds of connections. epoll is good solution to deal with that.

The following URL is a good explaintion about epoll.
How to use epoll? A complete example in C

And here is a complete examle of using epoll in socket program.

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/epoll.h>
#include <sys/wait.h>
#define SERVPORT 9527 /* Server Port Number */
#define BACKLOG 10 /* The Max of Client Number */

void setnonblocking(int sock)
{
    int opts;
    opts = fcntl(sock,F_GETFL);
    if(opts<0)
    {
        perror("fcntl(sock,GETFL)");
        exit(1);
    }
    opts = opts|O_NONBLOCK;
    if(fcntl(sock,F_SETFL,opts)<0)
    {
        perror("fcntl(sock,SETFL,opts)");
        exit(1);
    }  
}

void do_use_fd(int client_fd)
{
    const char str[] = "God bless you!\n";
    if (send(client_fd,  str,  sizeof(str),  0) == -1)
        perror("send");
    close(client_fd);
}

int main()
{
    int sockfd;
    struct sockaddr_in my_addr;
    struct sockaddr_in remote_addr;
    if ((sockfd = socket(AF_INET,  SOCK_STREAM,  0)) == -1) {
        perror("socket");
        exit(1);
    }
    my_addr.sin_family = AF_INET;
    my_addr.sin_port = htons(SERVPORT);
    my_addr.sin_addr.s_addr = INADDR_ANY;
    bzero(&(my_addr.sin_zero), 8);
    if (bind(sockfd,  (struct sockaddr *)&my_addr,  sizeof(struct sockaddr)) == -1) {
        perror("bind");
        exit(1);
    }
    printf("bind ok\n");
    if (listen(sockfd,  BACKLOG) == -1) {
        perror("listen");
        exit(1);
    }
    printf("listen ok\b");
    size_t sin_size = sizeof(struct sockaddr_in);

#define MAX_EVENTS 10
    struct epoll_event ev, events[MAX_EVENTS];
    int conn_sock, nfds, epollfd;

    epollfd = epoll_create(10);
    if (epollfd == -1) {
        perror("epoll_create");
        exit(EXIT_FAILURE);
    }
    printf("epoll_create\n");

    ev.events = EPOLLIN;
    ev.data.fd = sockfd;
    if (epoll_ctl(epollfd, EPOLL_CTL_ADD, sockfd, &ev) == -1) {
        perror("epoll_ctl: sockfd");
        exit(EXIT_FAILURE);
    }
    printf("epoll_ctl ok\n");

    for (;;) {
        printf("start epoll_wait\n");
        nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1);
        if (nfds == -1) {
            perror("epoll_pwait");
            exit(EXIT_FAILURE);
        }
        printf("epoll_wait returns, nfds = %d\n", nfds);

        for (int n = 0; n < nfds; ++n) {
            if (events[n].data.fd == sockfd) {
                conn_sock = accept(sockfd,
                        (struct sockaddr *) &remote_addr, &sin_size);
                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);
            }
        }
    }
}

[Socket] How to use he function fcntl()

When server is running to accept() and no client connection request comes, what it happens? When the server stops waiting for the arrival of the connection request on accept(). Similarly, when the program is running to receive(), ifno data can be read, the program will also stop receiving statements. This situation is called blocking. 
If you want to only pay attention to check whether there are customers waiting for a connection server to accept connection, otherwise the program continues to do other things, through the Socket set for non-blocking way: non-blocking socket when no customers waiting for accept calls return immediately.
 
# include <unistd.h>
# include <fcntl.h>
....
sockfd = socket (AF_INET, SOCK_STREAM, 0);
fcntl (sockfd, F_SETFL, O_NONBLOCK);
....
 
Set socket to non-blocking mode, "polling" the Socket. When no data is waiting to be processed from a non-blocking Socket ( for read data ), the function will return immediately, and the return value is set to -1 and errno set of EWOULDBLOCK. 
But this "polling" causes the CPU is busy waiting mode, thereby reducing performance.


P.S:
There is another way to deal with non-blocking socket.
ioctl(sockfd, FIONBIO, 0); // 0 is about non-blocking mode