ID Number: Q85285
3.10
WINDOWS
Summary:
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 reference words: 3.10