unblockconnect.c

//
//  main.c
//  C_lang
//
//  Created by 方朋 on 16/4/21.
//  Copyright © 2016年 方朋. All rights reserved.
//

#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdlib.h>
#include <assert.h>
#include <time.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <string.h>

#define BUFFER_SIZE 1023

int setnonblocking(int fd);
int unblock_connection(const char *, int, int);

int main(int argc, const char * argv[]) {
    if (argc <= 2) {
        fprintf(stderr, "Usage: %s ip_address port.\n", argv[0]);
        return 1;
    }
    const char *ip = argv[1];
    int port = atoi(argv[2]);
    int sockfd = unblock_connection(ip, port, 10);
    if (sockfd < 0) return 1;
    close(sockfd);
    return 0;
}

/**
 *
 * @param:  fd  文件描述符
 * return:  flags 原先的文件描述符状态flags
 **/
int setnonblocking(int fd)
{
    int flags = fcntl(fd, F_GETFL, 0);
    //设置为非阻塞
    if (fcntl(fd, F_SETFL, flags|O_NONBLOCK) < 0) {
        perror("fcntl error");
    }
    return flags;
}


int unblock_connection(const char *ip, int port, int time)
{
    int ret;
    struct sockaddr_in sockaddr;
    bzero(&sockaddr, sizeof(sockaddr));
    sockaddr.sin_family = AF_INET;
    sockaddr.sin_port = htons(port);
    inet_pton(AF_INET, ip, &sockaddr.sin_addr);
    
    int sockfd = socket(PF_INET, SOCK_STREAM, 0);
    
    // 设置非阻塞
    int fdopt = setnonblocking(sockfd);
    
    ret = connect(sockfd, (struct sockaddr*)&sockaddr, sizeof(sockaddr));
    if (ret == 0) {
        //如果连接成功则恢复sockfd的属性，并立即返回之
        printf("connect with server immediately\n");
        fcntl(sockfd, F_SETFL, fdopt);     // 为什么要恢复sockfd的原来属性呢？？
        return sockfd;
    } else if (errno != EINPROGRESS) {
        // 如果errno不是EINPROGRESS则表示出错，为EINPROGRESS表示连接还在进行
        fprintf(stderr, "unblock connect not support\n");
        return -1;
    }
    
    // 那么接下来的情况就是EINPROGRESS了
    fd_set reads;
    fd_set writes;
    FD_ZERO(&reads);      //清空可读文件描述符的所有位
    FD_SET(sockfd, &writes);
    struct timeval timeout;
    timeout.tv_sec = time;
    timeout.tv_usec = 0;
    ret = select(sockfd+1, &reads, &writes, NULL, &timeout);
    if (ret <= 0) {
        // select 超时或出错
        fprintf(stderr, "select timeout or error\n");
        close(sockfd);
        return -1;
    }
    if(!FD_ISSET(sockfd, &writes)) {
        fprintf(stderr, "no event on select found\n");
        close(sockfd);
        return -1;
    }
    
    int error = 0;
    socklen_t length = sizeof(error);
    //调用getsockopt来获取并清除sockfd上的错误
    if(getsockopt(sockfd, SOL_SOCKET, SO_ERROR, &error, &length) < 0) {
        fprintf(stderr, "get socket option failed\n");
        close(sockfd);
        return -1;
    }
    // 如果错误号非0表示连接出错
    if (error != 0) {
        fprintf(stderr, "connection failed after select with error:%d\n", errno);
        close(sockfd);
        return -1;
    }
    //连接成功
    printf("connection ready after select with the socket:%d\n", sockfd);
    fcntl(sockfd, F_SETFL, fdopt);
    return sockfd;
}