question about esp Register usage in ATL Windowing Thunking

Question

It's well know that ATL hooks a window to it's wndproc by thunking, which actually replace hwnd with this pointer in place and and jumps to the wndproc so that a call to wndproc(hwnd, ...) is actually wndproc(this, ...) .

Below is the assembly code the thunk constructs:

mov dword ptr [esp+0x4], pThis (esp+0x4 is hWnd)

My question is, since this thunk only executes once, how can we be sure that [esp+0x4] will not be overwritten by CPU for calling another procedure, and next time wndproc(...) is called, hwnd is passed in again? My understanding is that [esp+0x4] is a reusable general-purpose register for storing a first parameter of any procedure.

What am wrong here? how the modification of hwnd is guaranteed to be perpetual?

Thanks.

Answer 1

I was wrong about the

this thunk only executes once

part, it actually runs each time a message arrives.

this is because the thunk itself , not CWindowImplBaseT<>::WindowProc(), is set to be the wndproc of the window class by the following lines CWindowImplBaseT<>::StartWindowProc in :

WNDPROC pProc = (WNDPROC)&(pThis->m_thunk.thunk);
::SetWindowLong(hWnd, GWL_WNDPROC, (LONG)pProc);