with-foreign-pointer ((var size &optional size-var) &body body)

var --
size-var -- A symbol.
size -- An integer.
body -- A list of forms to be executed.

The with-foreign-pointer macro, binds var to size bytes of foreign memory during body. The pointer in var is invalid beyond the dynamic extend of body and may be stack-allocated if supported by the implementation.

If size-var is supplied, it will be bound to size during body.

  CFFI> (with-foreign-pointer (string 4 size)
          (setf (mem-ref string :char (1- size)) 0)
          (lisp-string-to-foreign "Popcorns" string size)
          (loop for i from 0 below size
                collect (code-char (mem-ref string :char i))))
  => (#P #o #p #Null)