Re: [Caml-list] Obj.out_of_heap_tag, out of minor heap or memory corruption?

[Gerd Stolpmann <info@gerd-stolpmann.de>]

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Am Freitag, den 29.04.2016, 13:04 +0200 schrieb Markus Weißmann:
> Hello,
> 
> I have a server program that crashes after some time with a very 
> strange error:
> The (=) comparison of two values returns false, even though they are 
> pretty identical.
> They are of type { a : int; b : int } and the comparison always fails 
> on the second integer.
> When printing the compared integers, they are always 0 (as expected) -- 
> both of them; but are not equal (=).
> I started inspecting the values with the "Obj.tag" and found them to be 
> always of type Obj.int_tag -- until the non-equalness strikes:
> One of the compared integers then always has Obj.out_of_heap_tag. This 
> value surprisingly behaved like an integer unless compared to another:
> 
> let (x : int) =
>    print_int x; (* "0" *)
>    assert (x = 0) (* fails! *)
> 
> Can someone explain what this tag means exactly and if this 
> works-as-intended or if my heap must have gotten hit by some faulty C 
> bindings?

Obj.tag is meaningless for ints.

What could have happened is that a C lib did not set the LSB of the word
(which is required for ints in order to make them distinguishable from
pointers). The C lib MUST use Val_int or Val_long to create the values
(and these macros set the LSB).

Check whether Obj.is_int returns true. If not, something is wrong.

Actually, I wonder what Obj.int_tag=1000 is for. It's not something that
can really occur, because tags are 8 bits wide. Maybe it's a replacement
when Obj.tag is called for non-blocks?

Gerd


> Is this some out-of-memory handling of the minor heap? If I set 
> Gc.minor_heap_size to 4 MByte I don't get the Obj.out_of_heap_tag -- or 
> at least haven't yet.
> 
> This is ocaml 4.02.3 on amd64.
> 
> Regards
> Markus
> 
> -- 
> Markus Weißmann, M.Sc.
> Technische Universität München
> Institut für Informatik
> Boltzmannstr. 3
> D-85748 Garching
> Germany
> http://wwwknoll.in.tum.de/
> 
> 

-- 
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Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
My OCaml site:          http://www.camlcity.org
Contact details:        http://www.camlcity.org/contact.html
Company homepage:       http://www.gerd-stolpmann.de
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