BSD

    int connectTimeout = 30;
    setsockopt(sock, IPPROTO_TCP, TCP_CONNECTIONTIMEOUT,(char *)&connectTimeout, sizeof(connectTimeout));

Linux

socket选项TCP_SYNCNT可以控制TCP连接SYN重传次数，默认为0，为0时SYN重传次数由系统参数 net.ipv4.tcp_syn_retries 控制，该系统参数默认值为6。

SYN重传次数影响connect超时时间，当重传次数为6时，超时时间为1+2+4+8+16+32+64=127秒。

    int syncnt = 4;
    setsockopt(sock, IPPROTO_TCP, TCP_SYNCNT, &syncnt, sizeof(syncnt));

使用select(Windows，Linux，BSD都可以用)

1. 创建socket，将socket设置为非阻塞模式。
2. 调用connect连接，如果能立即连接则返回0，不能立即连接返回-1，这个时候判断错误码是否表示暂时不能完成，是的话继续下一步。
3. 接着调用select()在指定的时间内检测socket是否可写，如果可写表明connect()连接成功，0表示超时，-1表示出现了错误。

windows下代码：

void attemptConnect(const char* ip,unsigned short port,int timeout)
{
    //初始化
    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != NO_ERROR) {
        printf("WSAStartup function failed\n");
        return;
    }

    SOCKET connectSocket= INVALID_SOCKET;
    do {
        //创建socket
        connectSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
        if (connectSocket == INVALID_SOCKET) {
            printf("socket function failed with error: %ld\n", WSAGetLastError());
            break;
        }

        //socket设置为非阻塞 
        unsigned long on = 1;
        if (ioctlsocket(connectSocket, FIONBIO, &on) < 0) {
            printf("ioctlsocket failed\n");
            break;
        }

        //尝试连接
        sockaddr_in clientService;
        clientService.sin_family = AF_INET;
        clientService.sin_addr.s_addr = inet_addr(ip);
        clientService.sin_port = htons(port);
        int ret = connect(connectSocket, (struct sockaddr*)&clientService, sizeof(clientService));
        if (ret == 0) {
            printf("connect success1\n");
            return;
        }

        //因为是非阻塞的，这个时候错误码应该是WSAEWOULDBLOCK，Linux下是EINPROGRESS
        if (ret < 0 && WSAGetLastError() != WSAEWOULDBLOCK) {
            printf("connect failed with error: %ld\n", WSAGetLastError());
            return;
        }


        fd_set writeset;
        FD_ZERO(&writeset);
        FD_SET(connectSocket, &writeset);
        timeval tv;
        tv.tv_sec = timeout;
        tv.tv_usec = 0;
        ret = select(connectSocket + 1, NULL, &writeset, NULL, &tv);
        if (ret == 0) {
            printf("connect timeout\n");
        } else if (ret < 0) {
            printf("connect failed with error: %ld\n", WSAGetLastError());
        } else {
            printf("connect success2\n");
        }
    } while (false);
    if (connectSocket != INVALID_SOCKET) {
        closesocket(connectSocket);
    }
    WSACleanup();
}
 
int main()
{
    attemptConnect("127.0.0.1",80,15);
    system("pause");
    return 0;
}