> IO_ALTSEMANTICS, as Jason mentioned, has an fs-defined meaning. Exactly
> what an fs considers "alternate" semantics is up to it.
>
> The reason for defining the concept of "alternate" semantics in an MI file
> is that otherwise each fs which wants to do something a little different
> has to define its own hooks for it.

Which is exactly *correct*.  Having a single flag that may do
completely different things depending on the file system is completely
wrong.

> And a specially-trapped ioctl domain isn't a special-case interface? :-)

No.  It's already done for other things (such as sockets), and the
interface is specifically designed to support it.

> We're going to have to add an extra VOP (so that these operations on
> device and fifo nodes go to the fs). Why add something which looks easy to
> out grow?

Why add something which is a *flagrant* bastardization of the fcntl(2)
API when you don't need to?