extension allocating shared memory

What is the best way for an extension to allocate shared memory and to
access it from every backend?  Or, if there is no support existing for
that, what advice do people have if I want to make that happen?  I don't
need a lot (probably 1KB would do).
If this just "can't be done" I guess I could start a little daemon to
run alongside PostgreSQL and talk to it through a pipe or TCP on
localhost, but it seems cleaner to have it run within PostgreSQL if
feasible.
-Kevin