Microsoft KB Archive/34611

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HOWTO: Allocate and Use Class and Window Extra Bytes


The information in this article applies to:

  • Microsoft Windows Software Development Kit (SDK)
  • Microsoft Win32 Software Development Kit (SDK)


The WNDCLASS structure contains two fields, cbClsExtra and cbWndExtra, which can be used to specify a number of additional bytes of memory to be allocated to the class structure itself or to each window created using that class.


Every application that uses class extra bytes and window extra bytes must specify the appropriate number of bytes before the window class is registered. If no bytes are specified, an attempt to store information in extra bytes will cause the application to write into some random portion of Windows memory, causing data corruption.

Windows version 3.1 will FatalExit if extra bytes are used improperly.

If an application does not use class extra bytes or window extra bytes, it is important that the cbClsExtra and cbWndExtra fields be set to zero.

Class and window extra bytes are a scarce resource. If more than 4 extra bytes are required, use the GlobalAlloc fucntion to allocate a block of memory and store the handle in class or window extra bytes.

Class Extra Bytes

For example, setting the value of the cbClsExtra field to 4 will cause 4 extra bytes to be added to the end of the class structure when the class is registered. This memory is accessible by all windows of that class. The number of additional bytes allocated to a window's class can be retrieved through the following call to the GetClassWord function:

nClassExtraBytes = GetClassWord(hWnd, GCW_CBCLSEXTRA);

The additional memory can be accessed one word at a time by specifying an offset, in BYTES (starting at 0), as the nIndex parameter in calls to the GetClassWord function. These values can be set using the SetClassWord function.

The GetClassLong and SetClassLong functions perform in a similar manner and get or set four bytes of memory respectively:

nClassExtraBytes = GetClassLong(hWnd, GCL_CBCLSEXTRA);

NOTE: A Win32-based application should use GetClassLong and SetClassLong, because the GCW_ indices are obsolete under Win32.

Window Extra Bytes

Assigning a value to cbWndExtra will cause additional memory to be allocated for each window of the class. If, for example, cbWndExtra is set to 4, every window created using that class will have 4 extra bytes allocated for it. This memory is accessible only by using the GetWindowWord and GetWindowLong functions, and specifying a handle to the window. These values can be set by calling the SetClassWord or SetClassLong functions. As with the class structures, the offset is always specified in bytes.

An example of using window extra bytes would be a text editor that has a variable number of files open at once. The file handle and other file-specific variables can be stored in the window extra bytes of the corresponding text window. This eliminates the requirement to always consume memory for the maximum number of handles or to search a data structure each time a window is opened or closed.

Additional query words:

Keywords : kbOSWinNT kbOSWin2000 kbSDKWin32 kbGrpDSUser kbOSWin kbWndw
Issue type : kbhowto
Technology : kbWin32SDKSearch kbAudDeveloper kbSDKSearch kbWin32sSearch kbWinSDKSearch

Last Reviewed: July 9, 2000
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