; So I realized I needed to do foreign-primitive and use C_alloc (according to sjamaan and the docs).
; But my struct is still not stored in a safe place:

; my c-code looks like this:
;struct line foo2 () {
;  struct line l;
;  l.origin.tag = 11;
;  l.origin.x = 12;
;  l.origin.y = 13;
;
;  l.direction.tag = 14;
;  l.direction.x = 15;
;  l.direction.y = 16;
;  return l;
;}


(define make-line
  (foreign-primitive
   (c-pointer (struct "line"))
   ()
   "struct line *_tmp=(struct line*)C_alloc(sizeof(struct line));
*_tmp=foo2();
return(_tmp);"))

; I tried C_return with same results.
; C_malloc seems to work, however!