Question

这个问题与In a non blocking socket connect, select() always returns 1非常相似（或几乎完全相同）;但是，我似乎无法找到代码蹒跚的地方。

我正在使用非阻塞套接字，并希望在将客户端连接到服务器时使用select（）来检查超时/成功。问题是select（）总是几乎立即返回1，即使我甚至没有服务器运行也没有任何东西要连接。在此先感谢您的帮助，代码段如下：

//Loop through the addrinfo structs and try to connect to the first one we can
for(p = serverinfo; p != NULL; p = p->ai_next) {
    if ((sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) == -1) 
    {
        //We couldn't create the socket, try again
        perror("client: socket");
        continue;
    }

    //Set the socket to non-blocking
    int flags = fcntl(sockfd, F_GETFL, 0);
    fcntl(sockfd, F_SETFL, flags | O_NONBLOCK);

    if (connect(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
        //The error was something other than non-block/in progress, try next addrinfo
        if(errno != EINPROGRESS) 
        {
            close(sockfd);
            perror("client: connect");
            continue;
        }

        fd_set write_fds;
        FD_ZERO(&write_fds);            //Zero out the file descriptor set
        FD_SET(sockfd, &write_fds);     //Set the current socket file descriptor into the set

        //We are going to use select to wait for the socket to connect
        struct timeval tv;              //Time value struct declaration
        tv.tv_sec = 5;                  //The second portion of the struct
        tv.tv_usec = 0;                 //The microsecond portion of the struct

        //DEBUG: This is ALWAYS 1
        int select_ret = select(sockfd + 1, NULL, &write_fds, NULL, &tv);
        cout << select_ret << endl;

        //Check return, -1 is error, 0 is timeout
        if(select_ret == -1 || select_ret == 0)
        {
            //We had an error connecting
            cout << "Error Connecting\n";
            close(sockfd);
            continue;
        }
    }

    //We successfully connected, break out of loop
    break;
}

Answer 1

你期待什么 select（）返回？考虑到select（）通常用于等待多个文件描述符 - 如果你连接两个，你怎么知道哪一个成功/失败纯粹基于select的返回值？你不会，显然。

这就是为什么select（）只是告诉你哪些文件描述符以某种方式发生了变化，你应该独立确定它是什么。在connect（）的情况下，您应该调用getsockopt（）来检索连接尝试的结果。请参阅this answer，其中我解释了如何进行非阻塞连接（）。

Answer 2

当以非阻塞模式连接并且select（）表示连接是可写的时，您应该再次调用connect（）。这样做将使用errno == ECONNRESET或其他任何内容返回-1。

使用select（）进行非阻塞套接字连接始终返回1

2 个答案: