好教程推荐系列：TCP面试常见题-张小方的知乎Live-轻松搞定技术面试中常见的网络通信问题（一）

typedef union epoll_data 
{ void *ptr;
 int fd; 
 uint32_t u32;
 uint64_t u64;
 } epoll_data_t; 
struct epoll_event 
{ uint32_t events; /* Epoll events */ 
 epoll_data_t data; /* User data variable */
 };

//关于tcp连接的异步connect实现流程如下：  
//（1）设置socket连接为非阻塞.  
//（2）调用connect函数.返回0表明连接成功.如果返回-1,同时errno为EINPROGRESS表明正在建立连接.  
//（3）使用select , epoll等 , 当描述符可写的时候检查连接状态.  
#include <stdio.h>  
#include <fcntl.h>  
#include <sys/types.h>  
#include <sys/socket.h>  
#include <errno.h>  
#include <sys/epoll.h>  
#include <netinet/in.h>  
#include <string.h>  
void setnonblock(int fd)  
{  
    int flags = fcntl(fd, F_GETFL);  
    fcntl(fd, F_SETFL, flags | O_NONBLOCK);  
}  
int main(){  
    const char* ip = "127.0.0.1";  
    short port = 9999;  
    //设置连接地址  
    struct sockaddr_in addr;  
    socklen_t socklen = sizeof(struct sockaddr_in);  
    memset(&addr , 0 , sizeof(struct sockaddr_in));  
    addr.sin_family = AF_INET;  
    addr.sin_addr.s_addr = inet_addr(ip);  
    addr.sin_port = htons(port);  
    //创建描述符  
    int fd = socket( AF_INET , SOCK_STREAM , 0);  
    if (fd < 0){  
        printf("socket() error\n");  
        return -1;  
    }  
    //设置描述符为非阻塞  
    setnonblock(fd);  
    //连接  
    int res;  
    res = connect(fd , (struct sockaddr*)&addr , socklen);  
    if (res == 0){  
        printf("connect ok(1)\n");  
    } else if (res == -1 && errno != EINPROGRESS){  
        printf("connect err(1)\n");  
        close(fd);  
        return -1;  
    } else {  
        int epfd;  
        //创建epoll描述符  
        epfd = epoll_create(1024);  
        if ( (epfd = epoll_create(1024) ) == -1){  
            printf("epoll_create() err\n");  
            close(fd);  
            return -1;  
        }  
        //添加关注事件  
        struct epoll_event ev;  
        ev.events = EPOLLOUT;  
        ev.data.fd = fd;  
        epoll_ctl( epfd , EPOLL_CTL_ADD , fd , &ev);  
        //编写网络程序的时候,epoll是程序的主循环.我们这里为了测试,连接上或connect超时(75秒)就break掉.  
        //正常的流程是写一个处理connect结果的回调函数.  
        int event_max = 1;  
        struct epoll_event events[event_max];  
        int i;  
        while (1){  
            res = epoll_wait( epfd , events , event_max , -1);  
            if (res > 0){  
                for ( i = 0 ; i < res ; i++){  
                    if ( events[i].data.fd == fd && ( events[i].events & EPOLLOUT) ){ //29(EPOLLOUT|EPOLLERR|EPOLLHUP)  //4(EPOLLOUT)  
                        //检查是否连接成功  
                        int optval;  
                        socklen_t optlen = sizeof(optval);  
                        int res1 = getsockopt(fd, SOL_SOCKET, SO_ERROR, &optval, &optlen);  
                        if ( res1 < 0 || optval){  
                            close(fd);  
                            close(epfd);  
                            printf("connect err(2)\n");  
                            return -1;  
                        } else {  
                            printf("connect ok(2)\n");  
                        }  
                    }  
                }  
                break;  
            }  
        }  
        close(fd);  
        close(epfd);  
    }  
}