Q85285: PRB: Petzold’s COLORS1 Sample Application Consumes Resources

Article: Q85285
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 05-NOV-1999

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) 3.1 
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SYMPTOMS
========

When the COLORS1 sample application on pages 227-232 of the book "Programming
Windows 3" by Charles Petzold (Microsoft Press) is run under Windows 3.1, the
amount of free system resources available after the program is complete is less
than the amount available before running the program.

CAUSE
=====

The program allocates system resources and does not free them.

RESOLUTION
==========

Make the following modifications to the WndProc function in the COLORS1 sample:

1. In the code that processes the WM_VSCROLL message, find the following:

        DeleteObject (GetClassWord (hwnd, GCW_HBRBACKGROUND));
        SetClassWord (hwnd, GCW_HBRBACKGROUND,
             CreateSolidBrush (RGB (color[0], color[1], color[2])));

  Insert the following code in its place:

        DeleteObject (SetClassWord (hwnd, GCW_HBRBACKGROUND,
             CreateSolidBrush (RGB (color[0], color[1], color[2]))));

2. In the code that processes the WM_DESTROY message, find the following:

        DeleteObject (GetClassWord (hwnd, GCW_HBRBACKGROUND));

  Insert the following code in its place:

        DeleteObject (SetClassWord (hwnd, GCW_HBRBACKGROUND,
             GetStockObject (WHITE_BRUSH)));

MORE INFORMATION
================

In both cases above, the call to DeleteObject fails because the brush is
selected into a device context (DC). Each time the user moves one of the scroll
bars, the program creates a new brush from the GDI module's heap space. Once the
GDI heap reaches its limit of 64 kilobytes, none of the applications running in
the system can create GDI objects.

Under the debugging version of Windows 3.1, the following error message is
displayed on the debugging terminal each time the erroneous code is executed:

  err COLORS1 GDI: GDI:Attempt to delete object owned by system

The resolution to this situation is to select a different brush into the DC, and
then to delete the old brush. Because the SetClassWord function returns the
previous value for the specified class word, it is very straightforward to pass
the return value from SetClassWord to DeleteObject.

Additional query words: 3.10

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK310
Version           : WINDOWS:3.1

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