dynamic-ffi: Responsibility for freeing returned pointers?

dynamic-ffi: Responsibility for freeing returned pointers?

[Tom Jakubowski]

Hi!

Say I have a C function like this:

char *tom_hello() {
  return strdup("hello!");
}

And, in Guile land, I make a procedure c-tom-hello out of it using
dynamic-ffi and wrap it in a function that returns a Scheme string:

(define exe (dynamic-link))
(define c-tom-hello (pointer->procedure '* (dynamic-func "tom_hello" exe)
'())
(define (tom-hello)
  (let ((str (pointer->string c-tom-hello)))
    ;; free pointer?
    str))

My assumption (supported by reading strings.c, but not in any docs I can
find) is that pointer->string makes a copy of the string it's passed. This
leaves open the question of freeing the original string returned by the
tom_hello() C function.

Is the usual technique here to use dynamic-ffi to make a procedure that's
bound to C's free(), and call that on the C string after making the Guile
string from it? It seems like the obvious choice, I'd just like to be sure
I'm not missing some higher level approach I should use instead.

Thanks!
Tom

Re: dynamic-ffi: Responsibility for freeing returned pointers?

[Ludovic Courtès]

Hi Tom,

Tom Jakubowski <tjakubowski@oblong.com> skribis:

> Say I have a C function like this:
>
> char *tom_hello() {
>   return strdup("hello!");
> }
>
> And, in Guile land, I make a procedure c-tom-hello out of it using
> dynamic-ffi and wrap it in a function that returns a Scheme string:
>
> (define exe (dynamic-link))
> (define c-tom-hello (pointer->procedure '* (dynamic-func "tom_hello" exe)
> '())
> (define (tom-hello)
>   (let ((str (pointer->string c-tom-hello)))
>     ;; free pointer?
>     str))
>
> My assumption (supported by reading strings.c, but not in any docs I can
> find) is that pointer->string makes a copy of the string it's passed.

Indeed.

> This leaves open the question of freeing the original string returned
> by the tom_hello() C function.

It’s up to the programmer, as in C.  In this case, you could add an
explicit call to ‘free’, or do:

  (set-pointer-finalizer! ptr (dynamic-func "free" (dynamic-link)))

so that ‘free’ gets called eventually, when ‘ptr’ becomes unreachable.

> Is the usual technique here to use dynamic-ffi to make a procedure that's
> bound to C's free(), and call that on the C string after making the Guile
> string from it? It seems like the obvious choice, I'd just like to be sure
> I'm not missing some higher level approach I should use instead.

I think it’s the right approach.

HTH,
Ludo’.