c - 如果未连接到网络，套接字连接超时挂起

Question

如果给出不存在的 IP 地址，以下例程将超时，这很好。但是，如果 linux 系统（它正在运行）没有连接到 eth0 或 wlan0 上的任何网络，那么它就会挂起。即使网络再次连接到 eth0，仍然没有响应。即使没有连接到网络，有没有办法让超时应用？或者，是否可以事先进行检查以确定是否存在与网络的连接？谢谢。

int connect_to_host()
{

    u_short port;                /* user specified port number */
    char *addr;                  /* will be a pointer to the address */
    struct sockaddr_in address;  /* the libc network address data structure */
    short int sock = -1;         /* file descriptor for the network socket */
    fd_set fdset;
    struct timeval tv;
    int connected = 0;

    port = 22;
    addr = "192.168.2.5";

    address.sin_family = AF_INET;
    address.sin_addr.s_addr = inet_addr(addr); /* assign the address */
    address.sin_port = htons(port);            /* translate int2port num */

    sock = socket(AF_INET, SOCK_STREAM, 0);
    fcntl(sock, F_SETFL, O_NONBLOCK);

    connect(sock, (struct sockaddr *)&address, sizeof(address));

    FD_ZERO(&fdset);
    FD_SET(sock, &fdset);
    tv.tv_sec = 3;             /* 10 second timeout */
    tv.tv_usec = 0;

    if (select(sock + 1, NULL, &fdset, NULL, &tv) == 1)
    {
        int so_error;
        socklen_t len = sizeof so_error;

        getsockopt(sock, SOL_SOCKET, SO_ERROR, &so_error, &len);

        if (so_error == 0) {
            printf("%s:%d is open\n", addr, port);
        connected = 1;
        }
    }

    close(sock);
    return connected;

}