[racket] FFI, string, malloc/free

That explains a lot, thanks. Just found the relevant doc part:
http://docs.racket-lang.org/foreign/intro.html?q=_string#%28part._.Pointers_and_.Manual_.Allocation%29
(and next subsection)
Maybe just adding a link to this paragraph under the doc for `_string'
could be useful then.

Laurent


On Sat, Jun 15, 2013 at 5:15 PM, Matthew Flatt <mflatt at cs.utah.edu> wrote:

> With
>
>  (fun _string -> _void)
>
> keep in mind that `_string' means that a Racket-level Unicode string is
> converted (by UTF-8 encoding) to a newly allocated array of bytes.
> Nothing that you do to with Racket string affects the memory management
> of the allocated bytes.
>
> The byte string is allocated as GCable, which means that the bytes can
> move or be collected. That works fine as long as the foreign function
> is effectively atomic (i.e., no GCs happen between the call and return
> of the foreign function).
>
> If the foreign function retains the pointer you give it, then you'll
> need to take much more control. Either use `(malloc 'raw ...)' and
> `free', or use `(malloc 'atomic-interior ...)' and ensure that the
> result remains reachable for as long as its referenced by the foreign
> function. Also, use the type
>
>   (fun _pointer -> _void)
>
> for the foreign function, so that the pointer you allocate is passed
> through.
>
>
> At Sat, 15 Jun 2013 08:26:39 -0400, Hendrik Boom wrote:
> > On Sat, Jun 15, 2013 at 02:15:04PM +0200, Laurent wrote:
> > > On Sat, Jun 15, 2013 at 1:55 PM, Roman Klochkov <kalimehtar at mail.ru>
> wrote:
> > >
> > > > What about
> > > >
> > > > (define-lib foo (fun _string -> _void))
> > > >
> > > > (foo "Test")
> > > >
> > > > ?
> > > >
> > > > Will be the pointer from string "Test" freed?
> > > >
> > >
> > > I *think* Racket will garbage collect the "Test" value when it is not
> used
> > > anymore on the Racket side, but it won't know what happens on the ffi C
> > > side (hence the "ffi/unsafe").
> > > So I think the lib may point to a freed value if you're not cautious.
> > >
> > >
> > > > And how to make it retain, if I need to save it on the foreign side?
> > > >
> > >
> > > You can probably at least simply store it in a global variable. There
> may
> > > be a better way to do it though.
> > > For example (not tested) :
> > > #lang racket
> > > (define dontfreethem '())
> > > (define (dontfreeme x)
> > >   (set! dontfreethem (cons x dontfreethem))
> > >   x)
> > >
> > > (foo (dontfreeme "Test"))
> > >
> > > Laurent
> >
> > But the garbage collector can still move the string when collecting, so
> > protecting it from being freed isn't enough.
> >
> > Isn't there some way to declare an object immovable?
> >
> > -- hendrik
> >
> >
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