关于滥用 Windows 窗口类额外字节的兼容性说明A compatibility note on the abuse of Windows window class extra bytes

June 30th, 2026

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During my discussion of the evolution of system-windows window and class extra bytes, I noted that even though IDs are typically small integers, people liked to stash pointers there, so we had to expand the ID field to a pointer-sized integer.

One thing I’ve learned is that anywhere it’s possible to hide a pointer, people will hide a pointer there. This is true even for small integers.

As I was digging up the history of the extra bytes, I saw a special note in the 16-bit code for Set­Class­Word: It says that there’s an app that expects to be able to modify the value of GWW_CB­CLS­EXTRA.

Now, modifying this value has no practical effect because the memory for the class was allocated when you called Register­Class. You can’t go back in time and change the allocation size.

But one program realized that it could use this value as a place to store some private data, so they did. Sure, that’s not the purpose of the GWW_CB­CLS­EXTRA, but that never stopped them.

For compatibility, Windows lets 16-bit programs modify GWW_CB­CLS­EXTRA. But at least it blocks it for 32-bit and 64-bit programs. One loophole closed. Countless more to go.

Author

Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.

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