Checking if a file is closed

I want my program to detect when a file is closed in order to encrypt it again.

Is there a better way to do this than checking periodically the process list if the program opened the file is still open?

Does your decrypt/encrypt program run say the editor as a sub-process?

No, not sub-process, just opens a file simply.

But you gave me an idea, maybe if I open it up from a thread I could check for the thread if its still running.