PRB: RegisterClass()/ClassEx() Fails If cbWndExtra > 40 Bytes (131288)
The information in this article applies to:
- Microsoft Win32 Application Programming Interface (API), when used with:
This article was previously published under Q131288 SYMPTOMS
Under Windows 95, a call to RegisterClass() or RegisterClassEx() returns
NULL if a value greater than 40 is specified for the cbWndExtra or
cbClsExtra members of the WNDCLASS or WNDCLASSEX structure.
CAUSE
Windows 95 checks to see if cbWndExtra or cbClsExtra is greater than 40. If
so, it outputs these debug messages and returns NULL to indicate failure:
RegisterClassEx: Unusually large cbClsExtra (>40)
RegisterClassEx: Unusually large cbWndExtra (>40)
RESOLUTION
If more than 40 bytes are needed to store window-specific or class-specific
information, an application should allocate memory. Then set the cbWndExtra
or cbClsExtra to 4 bytes, and pass the pointer to the allocated memory by
using SetClassLong() as follows:
SetClassLong (hWnd, GCL_CBCLSEXTRA, lpMemoryAllocated);
SetClassLong (hWnd, GCL_CBWNDEXTRA, lpMemoryAllocated);
The pointer can then be retrieved when needed by using GetClassLong() as
follows:
lpMemoryAllocated = GetClassLong (hWnd, GCL_CBCLSEXTRA);
lpMemoryAllocated = GetClassLong (hWnd, GCL_CBWNDEXTRA);
STATUS
This behavior is by design. However, as of version 3.51, Windows NT does
not have this 40-byte limitation.
| Modification Type: | Minor | Last Reviewed: | 5/1/2004 |
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| Keywords: | kbprb kbWndw kbWndwClass KB131288 |
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