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 <[email protected]> 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 <[email protected]> > 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 > > > > > > > ____________________ > > > Racket Users list: > > > http://lists.racket-lang.org/users > > > > ____________________ > > Racket Users list: > > http://lists.racket-lang.org/users > ____________________ > Racket Users list: > http://lists.racket-lang.org/users >
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