Re: [Caml-list] memory corruption with Weak.t in Gc.finalise

["François Bobot" <francois.bobot@cea.fr>]

On 08/03/2016 20:04, Goswin von Brederlow wrote:
> On Tue, Mar 08, 2016 at 11:00:33AM -0500, Gabriel Scherer wrote:
>> It's great that you have a minimal reproduction example. Could you
>> create an issue on the issue tracker?
>>    http://caml.inria.fr/mantis/
>>
>> Issues on the bugtracker are easier to keep track of than mails on the
>> caml-list.
>
> If it is a bug in the compiler and not just my code then sure.
>

Perhaps a bug in your code, I haven't ran your code; just read it.

>>>
>>> class obj = object(self)
>>>    val weak = Weak.create 1
>>>    initializer Weak.set weak 0 (Some self)
>>>    initializer setup weak
>>>    initializer Gc.finalise cleanup self
>>> end
>>>
>>> let () =
>>>    Printf.printf "Creating object\n%!";
>>>    ignore (new obj);
>>>    test ()
>>>

>>> void test(void) {
>>>      CAMLparam0();
>>>      CAMLlocal1(obj);
>>>      printf("%s\n", __PRETTY_FUNCTION__);
>>>      printf("  weak = 0x%lx\n", weak);
>>>      if (weak != 0) {
>>>          obj = Field(weak, 1);
>>>          printf("  obj = 0x%lx\n", obj);
>>>          if (obj != 0) {
>>>              printf("  tag = %d\n", Tag_val(obj));
>>>          }

If the weak pointer is unset, obj is not equal to 0 but to caml_weak_none (in 4.03/trunk 
caml_ephe_none). And I think you are in this case since `(new obj)` can be immediately garbage 
collected. caml_ephe_none should not be dereferenced:
```
static value ephe_dummy = 0;
value caml_ephe_none = (value) &ephe_dummy;
```

Thank you for your mantis ticket http://caml.inria.fr/mantis/view.php?id=7173 that asks for a C API 
for weak pointers/ephemerons.

Best,

-- 
François