How does CFFI handle returned string?

Imagine I have this function:

(cffi:defcfun ("linkage_print_diagram" linkage_print_diagram) :string
  (linkage :pointer)
  (display_walls :boolean)
  (screen_width :int))

Which wraps a function which returns an allocated memory, which it requests the callers of this function to free. Should I:

- return a :pointer rather than :string and free the memory pointed by the pointer when it's not needed, or
- do nothing, CFFI already did it. I.e. the meaning of returning a :string is that it was copied into Lisp string, and the original was deallocated.?

I don't have CFFI handy to try this out, but it appears that the :STRING type takes a :FREE-FROM-FOREIGN option that specifies whether to free the foreign string after it's been translated to a Lisp string. The default is NIL, so:

(cffi:defcfun ("linkage_print_diagram" linkage_print_diagram)
    (:string :free-from-foreign t)
  (linkage :pointer)
  (display_walls :boolean)
  (screen_width :int))

Oh, that's great. Even better, than I expected! Thanks.