Question

I have a Visual Studio solution which consists of a DLL and an EXE. My program sets a global WH_CALLWNDPROC hook. The hook procedure is defined by the DLL. I have verified that the DLL gets properly injected into all the processes I am interested in. The DLL exports a few procedures, which are defined in a header file, not in a DEF file. The EXE automatically loads the DLL and calls a method in the DLL to set the hook. When the DLL is loaded, DllMain sets an internal HMODULE variable which contains the DLL’s module handle. When the EXE calls the installHook procedure, the DLL sets the hook. All this works fine.

When my hook procedure receives a WM_SIZING message, it executes another internal procedure, which is supposed to use GDI+ to draw something on the window’s client DC. Using standard GDI works. However, GDI+ (which I need to use) does not work: the Graphics::Graphics(HDC) constructor causes the any program to crash as soon as I try to resize the window. Here is a snippet of the code that causes the crash:

void myFaultyProcedure(HWND hWnd) {
    RECT wndRect;
    GetWindowRect(hWnd,&wndRect);
    unsigned int wndWidth=wndRect.right-wndRect.left;
    unsigned int wndHeight=wndRect.bottom-wndRect.top;
    HDC hDc;
    PAINTSTRUCT ps;
    ULONG_PTR gdiplusToken;
    GdiplusStartupInput gdiplusStartupInput;
    GdiplusStartup(&gdiplusToken,&gdiplusStartupInput,NULL);
    hDc=BeginPaint(hWnd,&ps);
    Graphics graphics(hDc); // I think that this causes the program to crash
    delete &graphics;
    EndPaint(hWnd,&ps);
    ReleaseDC(hWnd,hDc);
    GdiplusShutdown(gdiplusToken);
}

The code calculates the width and height of a given window, gets a DC, starts GDI+, creates a Graphics object, deletes the Graphics object, releases the DC, and shuts down GDI+. I cannot image why programs would crash because of these lines. Notepad and Windows Explorer both crash (The Windows Explorer window is in a separate process from the Windows Explorer Shell).

Thanks!

Answer 1

Pretty sure it is the next line

 delete &graphics;

that is making your code blow up. delete should only be used if the pointer was obtained by new, here you’re giving it something on the stack. Calling delete on a stack allocated variable makes no sense.

To ensure that the Graphics instance is destroyed before GdiplusShutdown is called you can introduce a new scope:

{
   Graphics g(...);
   g.DoStuff();
   ...
} // g is destroyed here
GdiplusShutdown(...)