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