AppInit_DLLs Registry Value and Windows 95

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To implement a system-wide hook, you must ensure that the hooked function (callback function) exists in a DLL. Then, when the this function is called, the operating system maps the hooked DLL into the target application's address space. The actual hooked function then operates as part of the target application's process.

There are essentially two steps involved in creating a system-wide hook:

1. Create a DLL with an exported function that is used as the hooking function. In the sample function that follows, the callback function is modeled after a callback function required to implement a WH_KEYBOARD system-wide hook:

         // Trap keyboard messages
         __declspec(dllexport) LRESULT CALLBACK HookFunction(
                          int code,
                          WPARAM wParam,
                          LPARAM lParam)
         {
            char szVCode[50];
   
            //display the virtual key code trapped
            sprintf(szVCode, "Virtual Key code: %lx", wParam);
            MessageBox(NULL, szVCode,"Key stroke", MB_OK);
                   :
                   :
         }
   
      The associated .def file for this DLL might resemble this:
   
         LIBRARY      HOOK
   
         EXPORTS
            HookFunction
   						

2. Install the system-wide hook. To install the hook, the DLL must be loaded, the hook function's address retrieved, and SetWindowsHookEx called with the function's address. Here's an example:

         // add system-wide hook
         hHookDll = LoadLibrary("hook");
         hHookProc = (HOOKPROC) GetProcAddress(hHookDll, "HookFunction");
   
         // Install keyboard hook to trap all keyboard messages
         hSystemHook =  SetWindowsHookEx(WH_KEYBOARD,hHookProc,hHookDll,0);
   						

   Once the application has finished with the system-wide hook, be sure to undo the hooking process as follows:

         // Remove the hook and unload the DLL used for the hooking process
         UnhookWindowsHookEx(hSystemHook);
         FreeLibrary(hHookDll);