ID Number: Q65257
2.03 2.10 3.00
WINDOWS
Summary:
The RegisterClass() and CreateWindow() functions fail when the system
resources are used up. The percentage of free system resources
reflects the amount of available space in the USER and GDI heaps
within Windows. The smaller amount of free space is reported in the
Program Manager's About box because if either heap fills up, functions
fail.
If the amount of free system resources remains low after the
application is exited, it is more likely that the GDI heap is filling.
The main reason for the GDI heap filling is that GDI objects that are
created by the application are not deleted or destroyed when they are
no longer needed, or when the program terminates. Windows does not
delete GDI objects (pens, brushes, fonts, regions, and bitmaps) when
the program exits. Objects must be properly deleted or destroyed.
The following are two situations that can cause the USER heap to get
full:
1. Memory is allocated for "extra bytes" associated with window
classes and windows themselves. Make sure that the cbClsExtra and
cbWndExtra fields in the WNDCLASS structure are set to 0 (zero),
unless they really are being used.
2. Menus are stored in the USER heap. If menus are added but are not
destroyed when they are no longer needed, or when the application
terminates, system resources will go down.
CreateWindow() will also fail under the following conditions:
1. Windows cannot find the window procedure listed in the
CreateWindow() call. Avoid this by ensuring that each window
procedure is listed in the EXPORTS section of the program's DEF
file.
2. CreateWindow() cannot find the specified window class.
3. The hwndparent is incorrect (make sure to use debug Windows to see
the RIPs).
4. CreateWindow() cannot allocate memory for internal structures in
USER heap.
5. The application returns 0 (zero) to the WM_NCCREATE message.
6. The application returns -1 to the WM_CREATE message.