I am figurirng out my way through cffi, and I cannot figure out the following (this is in clisp):

Suppose I have a C functions that returns a result via a pointer. Its definition is:

foo a, b, &c

and then I want to do 3 * c.

What do I have to give for c? I do have its address. Should I use mem-ref, or convert-from-foreign, or something else?

Here is a scetch (where I already did defcfun ("foo" foo) ...)):
(let ((*c (foreign-alloc :float))

(foo a b *c)

(* 3 (??? *c))

Thank you,

Mirko