SetWindowLongPtr not seem to work

Question

I tried to subclass another window (in another process) so I injected a dll, which calls SetWindowLongPtr, but it fails and GetLastError returns 5.

BOOL APIENTRY DllMain (HANDLE hDll, DWORD dwReason, LPVOID lpReserved)
{
    switch (dwReason)
    {
        case DLL_PROCESS_ATTACH:
        {
            HWND hwnd = GetHwndProc();

            if (!(orgWndProc = (WNDPROC)SetWindowLongPtr(hwnd, GWLP_WNDPROC, (LONG_PTR)SubclassProc)))
            {
                char buf[40];
                sprintf(buf, "Error code: %d", GetLastError());
                MessageBox(hwnd, buf, "Error", MB_OK);
            }   
            break;
        }
    }
    return TRUE;
}

EDIT: Its defiantly the right PID.

EDIT 2: I was getting the wrong HWND but that is fixed now (edited the code as well) I'm no longer getting the error 5 (from GetLastError)

    HWND GetHwndProc()
{
    HWND hwnd = GetTopWindow(NULL);
    DWORD currentPID = GetCurrentProcessId();
    do
    {
        char title[256];
        if ((GetWindowText(hwnd, title, 256) > 0) && (IsWindowVisible(hwnd)))
        {
            DWORD procId;
            GetWindowThreadProcessId(hwnd, &procId);

            if (procId == currentPID)   
            {
                MessageBox(hwnd, title, "", MB_OK);
                return hwnd;
            }
        }

        hwnd = GetNextWindow(hwnd, GW_HWNDNEXT);
    } while (hwnd);
}

WNDPROC orgWndProc;
LRESULT APIENTRY SubclassProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    switch (msg)
    {
        case WM_LBUTTONDOWN:
            MessageBox(0, "Subclass", "", 0);
            return TRUE;

        default:
            return CallWindowProc(orgWndProc, hwnd, msg, wParam, lParam);
    }
}

Thank you for reading!

Sounds to me you are just injecting the wrong process. Make sure it isn't your own. Display the value of GetCurrentProcessId() to verify. — Hans Passant, Jan 01 '13 at 14:51
@HansPassant Yes its the right PID, maybe its not the right hwnd then? (its the hwnd with the title, so I thought that should be the WndProc. I'll post my GetHwndProc()) — joell, Jan 02 '13 at 10:53
@DavidHeffernan what should I link? I included Commctrl.h and linked with Commctl32 but still no success. — joell, Jan 02 '13 at 12:07

K-ballo · Answer 1 · 2013-01-01T13:14:36.853

2

You need to call SetWindowSubclass from the thread where the window was created, at which the message queue associated with it runs. From SetWindowSubclass reference:

Warning You cannot use the subclassing helper functions to subclass a window across threads.

In turn SetWindowLongPtr must be called from the process where the window was created. From SetWindowLongPtr reference:

Windows XP/2000: The SetWindowLongPtr function fails if the window specified by the hWnd parameter does not belong to the same process as the calling thread.

There is also the User Interface Privilege Isolation which restricts access even further.

edited Jan 01 '13 at 13:14

answered Jan 01 '13 at 13:08

K-ballo

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Ah well, thank you but its not really helpful as mingw doesn't include/support that function: Undefined reference to SetWindowSubclass (also for DefSubclassProc) – joell Jan 01 '13 at 13:13
SetWindowSubclass can't work across threads because it maintains a per-thread chain of subclasses internally, but sub-classing using SetWindowLongPtr should work. I suggest you try the steps Hans suggested above. – Jonathan Potter Jan 01 '13 at 20:01

SetWindowLongPtr not seem to work

1 Answers1