Posted on 1997-04-21
After reading a bit about hooks (Richter, etc.), I am trying to implement
a program that will subclass the procedure belonging to a dialog of another
application (on a Win95 or NT system).
Unfortunately, I cannot make it work.
Can you please help me find what's wrong?
What I'm trying to do is:
In my EXE file (console app):
1) Start the target application using CreateProcess()
2) Load a DLL with the hook using LoadLibrary() and
get the hooking procedure address using GetProcAddress().
3) Call the hooking procedure with the target app's thread ID as a parameter.
4) Wait for the target app to exit (using WaitForSingleObject())
5) Remove the hook, free the DLL, cleanup and exit.
No message loop or anything fancy...
In my DLL:
1) In the DllMain() - Save the DLL hInstance.
2) In the hooking proc - call:
hHook = SetWindowsHookEx(WH_GETMESSAGE, GetMsgProc, hinstDll, dwThreadId);
passing the saved DLL hInstance and the target app's thread ID
(from the EXE step #3).
3) In the Hook - do something like:
if (code == HC_ACTION && wParam == PM_REMOVE &&
((MSG*)lParam)->message == WM_INITDIALOG &&
oldDlgProc = SubclassDialog(((MSG*)lParam)->hwnd, MyDlgProc);
return CallNextHookEx(hHook, code, wParam, lParam);
4) In MyCompareDialog() - get the dialog's title using GetWindowText()
and compare it to the title of the dialog I wish to subclass.
5) In MyDlgProc() - do something like:
switch GET_WM_COMMAND_ID(wParam, lParam)
case 57670: // A button identifier, found with Spy++
MessageBox(NULL, "Gotcha!", "Info", MB_ICONINFORMATION);
return TRUE; // Message was processed
// Call previous dialog procedure
return CallWindowProc((WNDPROC)oldDlgProc, hWnd, message, wParam, lParam);
Now the problems:
The call to SetWindowsHookEx() sometimes fails with error 87
(invalid parameter), but if I put a breakpoint on the line and
then continue, it seems to work OK. Huh?
The hook does not seem to catch WM_INITDIALOG.
In fact, the first message it gets is WM_MOUSEMOVE (0x200).