Subject: RE: Prototype kernel continuation-passing for NetBSD
To: Bill Studenmund <wrstuden@NetBSD.org>
From: Gordon Waidhofer <email@example.com>
Date: 01/28/2004 15:47:59
> Do you have any recomended reading for those of us who aren't
> familiar with continuations? :-)
There are surely better answers, but here's an answer....
Continuations are hard to sum up in a nutshell. See the
Scheme procedure call-with-current-continuation, aka call/cc.
I tried to find a short, illuminating description. Short is
possible. Illuminating, well.....
Continuations proper can be thought of as setjmp/longjmp
on steroids. If foo() calls setjmp(jb), you can not pass
jb to longjmp() after foo() returns. That's setjmp()/longjmp().
But call/cc allows "unlimited extent", meaning jb remains
valid until you abandon it or kill it. You can even use jb
an unlimited number of times. It amounts to copying
the stack. That's continuations proper, in a nutshell. It is
a pale explanation compared to what's possible.
The kcont mechanism posted is a nice framework for
callbacks, as the poster (Johnathon Stone) said.
I can see how it took its inspiration from continuations.
All the fiddling to get spls() right for callbacks,
sequencing, off-level deferal, etc, etc, are kinda
bundled up here. It looks like one of those things
that, once you get used to it, you wonder how you did
without it. We all do the same sorts of things all
the time without the framework.