Re: [Chicken-users] Re: FFI to libgmp questions

[Peter Keller]

On Fri, Jun 28, 2002 at 12:40:36PM -0500, Peter Keller wrote:
> So, this is an attempt to do this:
> 
> ;; convert a pointer known to be a char* into a c-string so that the 
> ;; garbage collector can get it
> (define c-pointer->c-string!
>     (foreign-lambda* c-string ((c-pointer ptr))
>         "   C_word *arena = C_alloc(C_SIZEOF_STRING(strlen(ptr) + 1));
>                       C_word newstr;
>                       newstr = C_string2(&arena, ptr);
>             free(ptr); /* get rid of original memory */
>             return (newstr); /* copied into gc and newstr goes away */ "))
> 
> (define mpz_get_str
>     (let ((alloc (foreign-lambda c-pointer "mpz_get_str" c-string int mpz_t)))
>     (lambda (ptr base mpz)
>         (let ((c-strptr (alloc ptr base mpz)))
>             (if (equal? ptr #f)
>                 (c-pointer->c-string! c-strptr)
>                 ptr)))))
> 
> Unfortunately, a call to (mpz_get_str #f 10 var) returns junk and I'm
> not sure why. I'm sure I'm missing something simple in how I'm supposed
> to do this.

Ok, here is another attempt that finally *appears* to work:

;; convert a pointer known to be a char* into a c-string so I can free the
;; c pointer later and still have a garbage collected scheme string laying
;; around.
(define c-pointer->c-string
    (foreign-callback-lambda* c-string ((c-pointer ptr))
        "   char *arena = C_alloc(C_SIZEOF_STRING(strlen(ptr) + 1));
            strcpy(arena, ptr);
            return (arena); /* copied into gc-memory??? */ "))

;; if there is a return char *, then manually copy it into a c-string, and
;; free the real c-pointer returned by the function. This should make garbage
;; collection happy.
(define mpz_get_str
    (let (  (alloc (foreign-lambda c-pointer "mpz_get_str" c-string int mpz_t))
            (dealloc (foreign-lambda void "free" c-pointer)))
    (lambda (ptr base mpz)
        (let ((c-ptr (alloc ptr base mpz)))
            (if (equal? ptr #f)
                (let ((str (c-pointer->c-string c-ptr))) ;; copy the string...
                    (dealloc c-ptr) ;; remove the mem alloced by mpz_get_str
                    str)
                ptr)))))


Does that look correct? in c-pointer->c-string, is the arena memory 
going to get copied into the garbage collector correctly without leaking
any memory or using any dangling pointers?

Thanks.

-pete