Creating a Non-Blocking Socket

have a look at the following link

https://www.experts-exchange.com/questions/21125017/Winsock-Blocking-sockets-to-non-blocking.html

also

http://www.codeproject.com/internet/nonblocking_syncsocket.asp

Good links above.

Also: if you cannot create a nonblocking socket for some reason (there are plenty), then you can use connect() in a different thread.

As a rule, you shouldn't be doing anything that might be time consuming in the GUI thread, if you do it will lead to the "dead dialog" syndrome you describe. Create a worker thread instead.

hope this helps

Thanks for tips, but I still have problems.
I've tried to used ioctlsocket function. See the source:

  if(ioctlsocket(s, FIONBIO, (u_long FAR*)&intNonBlock ) == SOCKET_ERROR) {
    MessageBox(&WSERROR5[0], &WSTITLE[0], MB_OK | MB_ICONSTOP);
    return;
  }

  if(connect(s,(struct sockaddr*)&sin,sizeof(sin)) == SOCKET_ERROR) {
    MessageBox(&WSERROR3[0], &WSTITLE[0], MB_OK | MB_ICONSTOP);
    WSACleanup();
    return;
  }

So when I use it like this, I don't even get the socket connect if the cable is connected to board. When I don't use ioctlsocket() and cable is connected to board, communication is working ok. But I would like to use nonblocking mode, in case the cable is not connected.
Do you have any suggestion?

Hmm.. i'm not 100% sure what you mean. But let me see if i understand: connect() returns SOCKET_ERROR, and WSAGetLastError returns  WSAEWOULDBLOCK.

then you would follow these steps (quoted from MSDN)

there are three possible scenarios:

- Use the select function to determine the completion of the connection request by checking to see if the socket is writeable.

- If the application is using WSAAsyncSelect to indicate interest in connection events, then the application will receive an FD_CONNECT notification indicating that the connect operation is complete (successfully or not).

-If the application is using WSAEventSelect to indicate interest in connection events, then the associated event object will be signaled indicating that the connect operation is complete (successfully or not).

dunno if that helps

Thanks for help.
I found in MSDN a functioin "select", which would be appropriate for my application. But I don't know how to set it correct. The source would be something like this:

  if(ioctlsocket(s, FIONBIO, (u_long FAR*)&intNonBlock ) == SOCKET_ERROR) {
    MessageBox(&WSERROR5[0], &WSTITLE[0], MB_OK | MB_ICONSTOP);
    return;
  }
  if(connect(s,(struct sockaddr*)&sin,sizeof(sin)) == SOCKET_ERROR) {
    if (select(0, 0, writefds, exceptfds, timeout) == ?)

The parameters in select() are not correct. See the MSDN. Return from select shoud be one of next:
 - connection has succeeded or
 - connection attempt failed or
 - timeout in 5 sec

Thanks