Re: ocaml sefault in bytecode: unanswered questions

[Sylvain Le Gall <sylvain@le-gall.net>]

On 08-08-2009, ivan chollet <ivan.chollet@free.fr> wrote:
>
> Yes it was a freebsd 6.4 with ocaml 3.10.2
>

OCaml version is quite old, but should be ok.

> I'll run the program on linux later and see how it goes.
>
>  
>
> Thanks for your advices regarding debugging. I pretty much tried all of
> these though. the thing is my error is not an ocaml error at runtime but an
> error of the ocaml runtime. And to analyze a core dump of ocamlrun, I just
> thought my best bet was gdb. Whatever.
>
>  
>
> OK I'll try to provide you with a minimal ocaml code that produce an
> ocamlrun error. Might take a little while as I'm not free.
>
> In the meantime, I've got a newbie question regarding ocaml garbage
> collector and the same List.iter stuff:
>
> Say you do a "List.iter myfun !myref", where !myref is a list (hehe.), and
> where myfun is a function that does reallocations of myref (that is
> affectations like myref := [some new or old objects]). The pointers myref
> that are generated through this process are destroyed each time a new
> reallocation of myref is done. Of course the underlying linked lists that
> are not referenced anymore shouldn't be collected by the GC before the end
> of the main "List.iter", otherwise it's iterating through a linked list that
> has been garbage collected.
>
> My question is: does the GC know that it cannot collect the unreferenced
> myref pointers before the end of the List.iter? 
>


Why "the underlying linked lists that not referenced anymore shouldn't
be collected by the GC before the end the main "List.iter"" ?

OCaml list are single-linked list, whenever you cannot access the head
of the list everything until the current element can be GCed.

Here is the implementation of List.iter

let rec iter f = function
    [] -> ()
  | a::l -> f a; iter f l

As soon as you apply "myfun" to "a", the reference to a is not used so
it can be collected by the GC if there is no more reference to it.

I think the GC CAN collect the unreferenced myref pointers before the
end of the List.iter...

Regards
Sylvain Le Gall