Using foreign values in CL expressions

[Mirko Vukovic]

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
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