DOCUMENT:Q111864  06-NOV-1999  [win16sdk]
TITLE   :BUG: GP Fault When Drawing on a Memory-Based DC
PRODUCT :Microsoft Windows Software Development Kit
PROD/VER:WINDOWS:3.1
OPER/SYS:
KEYWORDS:

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The information in this article applies to:

 - Microsoft Windows Software Development Kit (SDK) 3.1 
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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);

Additional query words: buglist3.10 3.10 gpf gp-fault 1x1 200x200

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Keywords          :  
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK310
Version           : WINDOWS:3.1

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