_afxWinState global - how can I use it?

In the CPrintingDialog class there is a variable _afxWinState.bUserAbort that notifies the underlying print loop that the user has clicked the 'cancel' button.

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In the VC6 MFC Source code I can see how _afxWinState is defined and used but I cannot find where it is initialized.

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How does one make _afxWinState... useable?

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I have sucessfully replaced _afxWinState.bUserAbort with my own global variable bUserAbort that is defined : extern "C" BOOL bUserAbort. It seems to work fine. Why does one need to use the _afx.. stuff?