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

From: Ludovic Courtès
Subject: Re: dynamic-ffi: Responsibility for freeing returned pointers?
Date: Mon, 13 Nov 2017 14:51:39 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux)

Hi Tom,

Tom Jakubowski <address@hidden> 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.


> 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.


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