using window's handle or process id to get the path from an opened file.

Hallo,

My Application will have to shell (launch) notepad to open a file.
I would like that only one instance of this file can be open.
I can use FindWindow to check if notepad is running and has open a file that has the same name as that one I want to open. But I can’t be sure if it is going on the same file I want to open.
Is it a way to get the path from an opened file using a window’s handle or a process id?

Thanks a lot for your response