BUG: GP Fault When Drawing on a Memory-Based DC

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1

SYMPTOMS

A general protection (GP) fault can occur on a memory-based device context (DC) when a bitmap previously selected in the DC is selected out and another is selected in. This is a common problem when using the SaveDC() and RestoreDC() functions with a memory DC.

CAUSE

In a regular (nonmemory-based) DC, when the SaveDC() and RestoreDC() functions are used, the size of the window client area of the DC does not change. The size of the drawable area in a memory-based DC is dependent on the size of the current bitmap selected in the DC.

For example, suppose the default 1 x 1 monochrome bitmap is selected in a memory DC and you call SaveDC(). Now, you select a 200 x 200 color bitmap into the DC and you perform some drawing commands. Then, you call RestoreDC() to restore the DC. This action will select the old 1 x 1 bitmap back into the DC. Unfortunately, GDI does not update the clipping region when this happens, and when you draw outside the 1 x 1 region, you can receive a GP fault.

RESOLUTION

After you call RestoreDC() on a memory-based DC, you need to call SelectObject() to select the bitmap that was selected in the DC before RestoreDC() was called.

STATUS

Microsoft has confirmed this to be a bug in Windows version 3.1. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

The following code exhibits the problem outlined above. When CreateCompatibleDC() is called, a default 1 x 1 monochrome bitmap is selected into the DC returned from the function. By restoring the initial 1 x 1 bitmap after getting a visible region of 200 x 200, GDI thinks the 1 x 1 surface has a clipping rectangle of 200 x 200. This is what causes the overwrite.

Sample Code

   HBITMAP hbm1,hbm2;
   HDC     hdcBits,hdcScreen;

   hdcScreen = GetDC(NULL);
   hdcBits = CreateCompatibleDC(hdcScreen);
   hbm1 = CreateCompatibleBitmap(hdcScreen,200,200);
   hbm2 = CreateCompatibleBitmap(hdcScreen,200,200);

   SaveDC(hdcBits);
   SelectObject(hdcBits,hbm1);
   PatBlt(hdcBits,0,0,200,200,WHITENESS);
   RestoreDC(hdcBits,-1);
   SaveDC(hdcBits);
   SelectObject(hdcBits,hbm2);
   PatBlt(hdcBits,0,0,200,200,WHITENESS);
   Ellipse(hdcBits,0,0,200,200);
   RestoreDC(hdcBits,-1);
   Rectangle(hdcBits,0,0,200,200);