Problem with hook

I'm making a plugin for an old program for personal use.

I set a WH_CALLWNDPROC hook because I need to inspect some messages. The
problem appears when I try to analyze WM_WINDOWPOSCHANGED.

I can do:
if (PCWPSTRUCT(lp).message = WM_WINDOWPOSCHANGED) then
  ...

But how can I get the WINDOWPOS structure?

The help for WM_WINDOWPOSCHANGED says:

Parameters
lpwp   Value of lParam. Points to a WINDOWPOS structure that contains
information about the window's new size and position.


Thanks