Inter-program communication

I wrote the other day about inter-program communication using a user-defined message which is broadcast to all windows. This technique is sound because only the programs involved recognise the user-defined message. Today I was reading about the wm_copydata message, which allows one program to pass data to another program. Here, broadcasting the message to all windows is strictly forbidden, as any program which has a wm_copydata handler will try to handle the message, sometimes with disastrous results. Here, the technique has to be to identify in advance (via the findwindow function) the receiving program. My use was limited only to passing a trigger: in plain English, 'when you receive this message, you'll know that I've finished'. If I ever use this technique in the future, I could pass a parameter in the 'wparam' field which has an agreed meaning. Here's a link to a program written in Delphi which uses wm_copydata. This isn't quite as robust as the one shown in the first link, although the article uses 'findwindow' and thus glosses over certain problems.