>> How about just a "void *modecookie" or something that points to an
>> object that is opaque to wscons itself?
> Even if modecookie is used as opaque type from wscons related
> routines, "struct video_modetiming" is better than "void *", because
> compiler can check its type with the former.

This may not be good.  A given OS may support multiple cards with very
different parameters; using a single type for that requires either
constant casting (which defeats the typechecking you cite anyway) or a
single huge struct supporting all the parameters any such card wants.

> I think it is better to use this opaque technique for other opaque
> types (e.g. cookie for interrupt handler), too.

In some cases, when there really is only one possible type.  But if it
can be run-time polymorphic, there's little choice but void *.

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