ASM Community

Hello,
I'm trying to store extra data on a window.
I have search the entire forum, with the GWL_USERDATA keyword, and found like 7 topics about that.
But still some windows crashes, and I dont know why.
I'm trying to subclass a window and store its old pointer with SetWindowLong and GWL_USERDATA, it works fine with some windows, but some crashes.
what could be the problem ?
please I need your help!

WinProc proc hwnd:DWORD, uMsg:DWORD, wParam:DWORD, lParam:DWORD

    invoke  GetWindowLong, hwnd, GWL_USERDATA

    invoke  CallWindowProc, eax, hwnd, uMsg, wParam, lParam

    ret

WinProc endp

  

invoke  GetWindowLong, hwnd, GWL_WNDPROC

invoke  SetWindowLong, hwnd, GWL_USERDATA, eax

invoke  SetWindowLong, hwnd, GWL_WNDPROC, ADDR WinProc

once again it works with some windows :(
Thank you very much

Use SetWindowLong with an index in the nIndex parameter instead of one of the pre-defined values. When you register the window class specify 4 bytes of cbClsExtra memory. Then when you use the SetWindowLong you can do this

invoke GetWindowLong, hwnd, GWL_WNDPROC
invoke SetWindowLong, hwnd, 0, eax
invoke SetWindowLong, hwnd, GWL_WNDPROC, ADDR WinProc

This will avoid any problems you might encounter using GWL_USERDATA and also allows you to store multiple procedures by registering the class with sufficient extra memory

Remember that the GetWindowLong and SetWindowLong functions use negative numbers to specify a predefined window parameter 0 to Number of extra bytes for user data. This would be the safer way to store data that you want to follow the window. Note that this does not work if you are using dialogs and not windows.

Donkey

Your Welcome. I wouldn't count on stability if this isn't your window. The cbClsExtra element of a class structure is defaulted to zero when the class is registered. If you can actually use it you may be writing to a different element of the class structure that just seems to have no effect or the original software would have had to expressly allocated that memory in order to use it. I think that hInstance is the next element in a window class so I doubt that you are overwriting that. That would probably mean that the programmer allocated that memory for a reason and may use it when some combination of events occur. Test your software thoroughly and keep your fingers crossed. The best way is like Sephiroth3 said, to store the old window proc in your program if you have no control over the creation of the window.

Donkey

Then you're better off using window properties. That's the third way to attach window specific information. With properties, you won't mess with someone else's use of GWL_USERDATA or cbClsExtra memory. Unlike the other two methods, properties are accessed by name (not position). Look up GetProp and SetProp.