"libffi closures"?

[Ranjit Mathew]

> I get the same.  The failing test here was written since SPARC closures
> were added to libffi.

This might be a very dumb question to ask but I guess asking
a dumb question is better than nodding in apparent comprehension
and completely missing the significance: what *are* libffi
closures and why is it such a big deal when a target gets
them?

I *vaguely* remember closures from my undergrad days and
this page seems to agree with my memories:

http://research.microsoft.com/vault/learn/tutorial/closures.htm

but how exactly are they significant for GCJ? I mean,
without closures what does a target lack?

Is there much voodoo involved in implementing closures 
for a target that most initial ports of GCJ for a target
come without it?

Many thanks in advance to anyone to clarifies this
for me.

Ranjit.