Create a native Windows window in JNA and some GetWindowLong with GWL_WNDPROC

Question

Good day,

I have been using JNA for a while to interact with the Windows API and now I am stuck when creating a window. As far as I have done the following: 1. Have created a child window of an existing window and obtained a valid handler to it. 2. Understood that every window in Windows has a non-stop message-dispatch loop. 3. Understood that the best way to include my window in the message-dispatch loop is to use something like the following code (not mine, but that is what I would do as well):

final LONG_PTR prevWndProc = new LONG_PTR(User32.INSTANCE.GetWindowLong(hwnd, User32.GWL_WNDPROC));       //this is to obtain a pointer to the WNDPROC of the parent window, which we are going to need later
  wndProcCallbackListener = new WndProcCallbackListener()
   {
      public LRESULT callback(HWND hWnd, int uMsg, WPARAM uParam, LPARAM lParam)
      {
         if (uMsg == WTSAPI.WM_POWERBROADCAST)
         {
           System.out.println("WM_POWERBROADCAST Event: hWnd="+hwnd+", uMsg="+uMsg+", uParam="+uParam+", lParam="+lParam);
         }
         else if (uMsg == WTSAPI.WTS_SESSION_CHANGE)
         {
           System.out.println("WTS_SESSION_CHANGE Event: hWnd="+hwnd+", uMsg="+uMsg+", uParam="+uParam+", lParam="+lParam);
         }

        //Call the window's actual WndProc so the events get processed.
        return User32.INSTANCE.CallWindowProc(prevWndProc, hWnd, uMsg, uParam, lParam);
      }
   };
      //Set the WndProc function to use our callback listener instead of the window's one. 
   int result = User32.INSTANCE.SetWindowLong(hwnd, User32.GWL_WNDPROC, wndProcCallbackListener);

However, my problem is when I call the GetWindowLong() for the parent window (my first line of code) I get a 0 for the pointer which indicated the function did not complete successfully. A subsequent call to GetLastError() and a quick check in the error codes give me an 'Access is denied' error. This, of course, is logical, since I am trying from my own thread to access the address of the WNDPROC of another, but I was wondering if there is any way (there should be, of course) to circumvent that.

Any pointers? (pun intended)

Answer 1

Do not use GetLastError() after a JNA call. JNA & JNI may call other APIs that may change the last error. Declare SetWindowLong with the clause throws LastErrorException, like this:

int SetWindowLongA(int hWnd, int nIndex, WndProcCallbackListener dwNewLong)
    throws LastErrorException;

Notice the 'A' after the name. It makes explicit use of ANSI version. You could use SetWindowLongW as well.

Make sure your callback implements both Callback and StdCall. I prefer using primitive types as much as possible, because this makes mapping fast and obvious to JNA:

public interface WndProcCallbackListener extends Callback, StdCall {

    int callback(int hWnd, int Msg, int wParam, int lParam);

}