场景

        boost库中的asio模块，在阻塞模式下，调用connect进行服务器的连接，如果服务器没有打开，默认情况下需要20秒的超时连接，才会返回。目前尝试设置套接字的连接超时时间，都不奏效。估计只能修改boost源码中的超时时间

//设置成非阻塞，然后调用select等待超时，无效
int TestBoostAsioConnectTimeout()
{
 boost::asio::io_service ios;
 boost::asio::ip::tcp::socket sock(ios);
 boost::system::error_code ec;

 sock.open(boost::asio::ip::tcp::v4());
 // 设为非阻塞 
 sock.io_control(boost::asio::ip::tcp::socket::non_blocking_io(true));
 sock.connect(
  boost::asio::ip::tcp::endpoint(
   boost::asio::ip::address::from_string("192.168.1.1"), 80)
  , ec);
 fd_set fdWrite;
 FD_ZERO(&fdWrite);
 FD_SET(sock.native(), &fdWrite);
 timeval tv = { 5 };    // 5秒超时 
 if (select(0, NULL, &fdWrite, NULL, &tv) <= 0
  || !FD_ISSET(sock.native(), &fdWrite))
 {
  std::cout << "连接超时" << std::endl;
  sock.close();
  return 0;
 }
 // 设回阻塞 
 sock.io_control(boost::asio::ip::tcp::socket::non_blocking_io(false));
 std::cout << "连接成功" << std::endl;
 sock.close();

 return 0;
}

//获取原始的套接字，进行属性的设置

int TestBoostAsioConnectOverTime()
{
 boost::asio::io_service ios;
 boost::asio::ip::tcp::socket sock(ios);
 boost::system::error_code ec;

 sock.open(boost::asio::ip::tcp::v4());
 // 设为非阻塞 
 sock.io_control(boost::asio::ip::tcp::socket::non_blocking_io(true));
 int timeout = 3000;
 int nRet = setsockopt(sock.native(), SOL_SOCKET, SO_CONNECT_TIME, (const char*)&timeout, sizeof(timeout));
 nRet = setsockopt(sock.native(), SOL_SOCKET, SO_RCVTIMEO, (const char*)&timeout, sizeof(timeout));
 sock.connect(
  boost::asio::ip::tcp::endpoint(
   boost::asio::ip::address::from_string("192.168.1.1"), 80)
  , ec);
 fd_set fdWrite;
 FD_ZERO(&fdWrite);
 FD_SET(sock.native(), &fdWrite);
 timeval tv;
 tv.tv_sec = 1;   // 5秒超时
 tv.tv_usec = 0;
 if (select(0, NULL, &fdWrite, NULL, &tv) <= 0
  || !FD_ISSET(sock.native(), &fdWrite))
 {
  std::cout << "超时/出错啦" << std::endl;
  sock.close();
  return 0;
 }
 // 设回阻塞 
 sock.io_control(boost::asio::ip::tcp::socket::non_blocking_io(false));
 std::cout << "连接成功" << std::endl;
 sock.close();

 return 0;
}


设置超时的代码：
windows:
 int nTvlen=1000;  //1s
 int retOpt=setsockopt(m_Socket,SOL_SOCKET, SO_RCVTIMEO, (const char*)&nTvlen,sizeof(nTvlen));
linux:
 struct timeval nTvlen={1,0}; //1S
 int retOpt=setsockopt(m_Socket,SOL_SOCKET, SO_RCVTIMEO, (const char*)&nTvlen,sizeof(nTvlen));