Microsoft KB Archive/131288

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PRB: RegisterClass/ClassEx Fails If cbWndExtra > 40 Bytes

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Q131288

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The information in this article applies to:


 * Microsoft Win32 Application Programming Interface (API), included with:
 * Microsoft Windows 95

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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.

Additional query words:

Keywords : kbGrpDSUser kbOSWin95 kbWndw kbWndwClass

Issue type : kbprb

Technology : kbAudDeveloper kbWin32sSearch kbWin32API