Is it possible to get window-pointer of another application?

[Clemens Eisserer]

Hello,

I would like to change the size of other applications by a C-program
which just gets the pointer of the foreign window and sets its size,
however I do not have any clue howto get access to windows that were
not created inside the application itself?

Is this possible by normal X-client applications at all?

Thanks a lot, lg Clemens