> FreeBSD has a flag in the devsw structure to allow the close function
> to be called at each close call on the device.

> Couldn't we do that, too, either adding a field at the end of struct
> {c,b}devsw or using a special value for d_type?

Probably, but would it do what you want?  It's entirely possible to
have multiple closes for a single open; consider a process that forks.

Unless you call close once per open file table entry rather than once
per descriptor.  I would expect that be very hard to define the
semantics of, short of resorting to describing the kernel internals in
question.  It also amounts to cloning devices, as I understand them,
only more difficult for the driver to deal with.

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