On Tue, 21 Sep 2021, adriano <randomloo...@riseup.net> wrote: > Hi Olivier, > > thank you very much for your reply > > Il giorno dom, 19/09/2021 alle 14.11 -0400, Olivier Dion ha scritto: >> On Sun, 19 Sep 2021, adriano <randomloo...@riseup.net> wrote: >> > >> > > > > > >> > It'd be so nice to have an example >> >> (define (with-my-resource token proc) >> (let ((resource #f)) >> (dynamic-wind >> (lambda () >> (set! resource (open-my-resource% token))) >> >> (proc resource) >> >> (lambda () >> (when resource >> (close-my-resource% resource)))))) >> >> (with-my-resource "some-internal-token" (lambda ())) > > Oh my, thank you for this ! > > This should be included in the manual ! > > The example that's there currently is totally indequate, in my opinion > >> > >> >> Says you have `open_my_resource()` and `close_my_resource()` in C in >> library "libfoo.so" where open_my_resource takes a C string and returns >> an integer for the resource while close_my_resource takes the integer >> of >> the resource: >> >> (define open-my-resource% >> (eval-when (eval load compile) >> (let ((this-lib (load-foreign-library "libfoo"))) >> (foreign-library-function this-lib "open_my_resource" >> #:return-type int >> #:arg-types (list '*)))) >> >> (define open-my-resource% >> (eval-when (eval load compile) >> (let ((this-lib (load-foreign-library "libfoo"))) >> (foreign-library-function this-lib "open_my_resource" >> #:return-type int >> #:arg-types (list int))))) > > Uhmm... I see 2 versions of open-my-resource% > The only slight difference I see is in the #:arg-types > > The first one has > > (list '*) > > and the second one has > > (list int) > > > Maybe you you got confused while editing ?
You're right. I copy paste the form two times and forget to edit the second binding name. I meant `close-my-resource%` for the second define. > > Thank you again Just make sure to read what Maxime says about escape continuation. Dynamic wind are not bullet proof. As the manual says: If, any time during the execution of THUNK, the dynamic extent of the ‘dynamic-wind’ expression is escaped non-locally, OUT_GUARD is called. If the dynamic extent of the dynamic-wind is re-entered, IN_GUARD is called. Thus IN_GUARD and OUT_GUARD may be called any number of times. Thus, you could also do a `(set! resource #f)` after freeing it, so that the overall dynamic-wind is reentrant and you don't end up with double free of resource. -- Olivier Dion Polymtl