How to get window proc parameters? - winapi

Question

I try to create the simplest WinAPI window using assembly NASM.
I have problem with Window Proc. Look at commented lines:
%macro API 2
import %1 %2
extern %1
%endmacro
API GetModuleHandleA, kernel32.dll
API LoadIconA,user32.dll
API LoadCursorA,user32.dll
API RegisterClassExA, user32.dll
API CreateWindowExA, user32.dll
API MessageBoxA, user32.dll
API SendMessageA, user32.dll
API DefWindowProcA, user32.dll
API ExitProcess, kernel32.dll
API GetMessageA, user32.dll
API DispatchMessageA, user32.dll
API TranslateMessage,user32.dll
API ShowWindow,user32.dll
API UpdateWindow,user32.dll
API GetCommandLineA,kernel32.dll
API PostQuitMessage,user32.dll
segment .data USE32
windowName db "Hello world!", 0
cmdLine dd 0
hWnd dd 0
hInst dd 0
hCursor dd 0
className db "moje_okno",0
blad db "Blad!!!",0
segment .bss
struc WNDCLASSEX
.sSize resb 4
.style resb 4
.wndProc resb 4
.clsExtra resb 4
.wndExtra resb 4
.hInstance resb 4
.hIcon resb 4
.hCursor resb 4
.background resb 4
.sMenuName resb 4
.sClassName resb 4
.hIconSm resb 4
endstruc
wndClass istruc WNDCLASSEX
iend
global ..start
segment .text USE32
..start:
push 0
call [GetModuleHandleA]
mov dword [hInst], eax ; application handle
push dword 0x00007f00 ; MAKEINTRESOURCE(32512)
push dword 0
call [LoadCursorA]
mov dword [hCursor], eax ; cursor handle
mov dword [wndClass + WNDCLASSEX.sSize], dword 48 ; struct size
mov dword [wndClass + WNDCLASSEX.style], dword 0 ; style
mov dword [wndClass + WNDCLASSEX.wndProc], wndproc ; window proc
mov dword [wndClass + WNDCLASSEX.clsExtra], dword 0
mov dword [wndClass + WNDCLASSEX.wndExtra], dword 0
mov eax, dword [hInst]
mov dword [wndClass + WNDCLASSEX.hInstance], eax ; handle
mov dword [wndClass + WNDCLASSEX.hIcon], dword 0
mov eax, dword [hCursor]
mov dword [wndClass + WNDCLASSEX.hCursor], eax
mov dword [wndClass + WNDCLASSEX.background], dword 0
mov dword [wndClass + WNDCLASSEX.sMenuName], dword 0
mov dword [wndClass + WNDCLASSEX.sClassName], className ; class name
mov dword [wndClass + WNDCLASSEX.hIconSm], dword 0
push wndClass
call [RegisterClassExA]
call near sprawdz_blad ; check return value of RegisterClassExA
push 0 ; param
push dword [hInst] ; handle
push 0 ;hMenu
push 0 ;parent
push 200 ;height
push 200 ;width
push 200 ;y
push 200 ;x
push 0 ;style
push className ;window name
push className ;window class
push 0 ;extended style
call [CreateWindowExA]
push eax
call near sprawdz_blad ;check return value of CreateWindowExA. RETURNS 0
push 0
call [ExitProcess]
wndproc:
; HERE I NEED ACCESS TO WINDOW PROC PARAMETERS: HWND, MSG, WPARAM, LPARAM
; I TRIED:
pop eax
pop ebx
pop ecx
pop edx
; BUT IT DOESN'T WORK
; THERE ARE NOT RIGHT VALUES IN THESE REGISTRIES
ret
box:
push 0
push blad
push blad
push 0
call [MessageBoxA]
ret
sprawdz_blad:
pop eax
cmp eax, 0
jne ok ; if function returns 0 everything is allright
push 0
push blad
push blad
push 0
call [MessageBoxA]
push 1
call [ExitProcess]
ok:
ret
I try to make it work for hours but I'm out of ideas.
Please help.
Greetings, Michal.

Answer 1

A called subroutine, wheether you call it yourself or it's called by Windows (like wndproc) has its return address as the first thing on the stack. You do NOT want to pop this! To access the parameters, you need to look farther up the stack. Try something like...
wndproc:
mov eax, [esp + 4]
mov ebx, [esp + 8]
; etc...
See if that helps...
Best,
Frank