is it possible to have an SDI application without MFC?

Of course.

OK, I think I see how it is done.

The problem I have now is that I am going to do it in a dll that is loaded from a seperately running program with its own interface already (this is part of a big client server application).

So the first think I am going to need to do is to launch a seperate thread just for this SDI gui.

This is a little embarassing asking this because I used to know this. How do you create a seperate thread in C++ and have mutexes, semiphores and all that. I have forgotten and I wonder if anyone knows a good example.

You don't necessarily need an own thread for the separate window - this would require (as you already noticed by yourself) a fair amount of synchronizing if the new thread / UI to communicate or exchange data in any senseful way with the existing part of the application. Instead, you should rather find a way to feed the new window from the same central message loop already present in the existing application.

Thanks.

I will give that a shot or keep it in mind.

One thing for sure is, as I can see, I cannot do this in MFC since MFC requires to be the main thread of execution and because of the way this is designed, I cannot have the gui be the main thread of execution.

It is a strange and complicated program. But the way it is written is that the gui has to be called and popped up from another app that does not have a gui.