Re: VFS: C99 sparse format for struct vfsops

On Tue, 2003-06-03 at 23:09, M. Warner Losh wrote:
> In message: <1054590840.1641.12.camel_at_cf.freebsd-services.com>
>             Paul Richards <paul_at_freebsd-services.com> writes:
> : The possible methods available in an interface are fixed, they're
> : defined in the .m files.
> 
> No it isn't.  One can add additional interfaces at any time without
> breaking binary compatibility, so you don't know, a priori, the number
> of methods in a given interface when you build a client of that
> interface.

I don't think that's true.

The interface is defined in the .m file and it's created using makeobjs.
You can't do that at runtime because it creates the kobj_desc struct
that uniquely represents the existence of that method globally for the
whole of the kobj subsystem. The set of all kobj_desc defines all
possible methods that can be implemented by an object, and yes you can
extend that interface later and previously built modules will still work
because they only implement a subset of those possible methods, but the
set of all methods that can exist is determined at compile time from the
interface definitions

A class is then defined which specifies the set of methods that objects
instantiated into that class *can* implement (but are note required to).
The set of methods in a class is also fixed, since it's basically the
method table plus some class fields and the method table is created at
compile time. Again though, if you later extend the class older compiled
modules will still work because if the method doesn't exist in that
older module the default from the kobj_desc will be used or the
kobj_error_method will be called (which is safe).

So yes you can extend the interface and the class and keep backwards
compatibility but that all happens at compile time, and therefore at
runtime when the object is instantiated you know the maximum number of
methods that the object can possibly call.

-- 
Tis a wise thing to know what is wanted, wiser still to know when
it has been achieved and wisest of all to know when it is unachievable
for then striving is folly. [Magician]