How to code an epoll based sockets client in C

Question

All the examples I can find online are servers. I want to build a basic web crawler using epoll. So I need a basic client example to get me started.

When I say basic I really mean a complete example that demonstrates multiple connections with sending and receiving of data to live web hosts. A simple HEAD request and its response for example.

Answer 1

Here is a sample c code for client socket with epoll.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <sys/socket.h>
#include <resolv.h>
#include <sys/epoll.h>
#include <arpa/inet.h>
#include <unistd.h>

#define PORT 22
#define SERVER "127.0.0.1"
#define MAXBUF 1024
#define MAX_EPOLL_EVENTS 64

int main() {
    int sockfd;
    struct sockaddr_in dest;
    char buffer[MAXBUF];
    struct epoll_event events[MAX_EPOLL_EVENTS];
    int i, num_ready;

    /*---Open socket for streaming---*/
    if ( (sockfd = socket(AF_INET, SOCK_STREAM|SOCK_NONBLOCK, 0)) < 0 ) {
        perror("Socket");
        exit(errno);
    }

    /*---Add socket to epoll---*/
    int epfd = epoll_create(1);
    struct epoll_event event;
    event.events = EPOLLIN; // Can append "|EPOLLOUT" for write events as well
    event.data.fd = sockfd;
    epoll_ctl(epfd, EPOLL_CTL_ADD, sockfd, &event);

    /*---Initialize server address/port struct---*/
    bzero(&dest, sizeof(dest));
    dest.sin_family = AF_INET;
    dest.sin_port = htons(PORT);
    if ( inet_pton(AF_INET, SERVER, &dest.sin_addr.s_addr) == 0 ) {
        perror(SERVER);
        exit(errno);
    }

    /*---Connect to server---*/
    if ( connect(sockfd, (struct sockaddr*)&dest, sizeof(dest)) != 0 ) {
        if(errno != EINPROGRESS) {
            perror("Connect ");
            exit(errno);
        }
    }

    /*---Wait for socket connect to complete---*/
    num_ready = epoll_wait(epfd, events, MAX_EPOLL_EVENTS, 1000/*timeout*/);
    for(i = 0; i < num_ready; i++) {
        if(events[i].events & EPOLLIN) {
            printf("Socket %d connected\n", events[i].data.fd);
        }
    }

    /*---Wait for data---*/
    num_ready = epoll_wait(epfd, events, MAX_EPOLL_EVENTS, 1000/*timeout*/);
    for(i = 0; i < num_ready; i++) {
        if(events[i].events & EPOLLIN) {
            printf("Socket %d got some data\n", events[i].data.fd);
            bzero(buffer, MAXBUF);
            recv(sockfd, buffer, sizeof(buffer), 0);
            printf("Received: %s", buffer);
        }
    }

    close(sockfd);
    return 0;
}