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Date: Fri, 29 Apr 2016 13:04:29 +0200
From: =?UTF-8?Q?Markus_Wei=C3=9Fmann?= <markus.weissmann@in.tum.de>
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Subject: [Caml-list] Obj.out_of_heap_tag, out of minor heap or memory
 corruption?

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?
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/