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From: Mathijs Romans <mathijs@romansland.nl>
To: Damien Doligez <damien.doligez@inria.fr>
Subject: Re: [Caml-list] What does "pc" mean in ocamldebug
Date: Thu, 7 Feb 2008 12:39:35 +0100
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References: <200801310045.38815.mathijs@romansland.nl> <08F4E8BB-F4BE-41E6-B437-437E6597435F@inria.fr>
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X-Spam: no; 0.00; ocamldebug:01 damien:01 parser:01 byte-code:01 bug:01 recursive:01 damien:01 3.08:01 recursive:01 equality:01 equality:01 doligez:01 wrote:01 wrote:01 overflows:01 

On Thursday 07 February 2008 11:46:47 Damien Doligez wrote:
> Hello,
>
> On 2008-01-31, at 00:45, Mathijs Romans wrote:
> > Hi!
> >
> > I am trying to make a Caml-application run that I have not written
> > myself. I
> > know very little about the language itself. Sadly, I get this error:
> >
> > Fatal error: exception Out_of_memory
> >
> > Using ocamldebugger I am trying to figure out what happened. The
> > last line
> > before failure is:
> > Time : 36381 - pc : 65532 - module Parser
> > 157     <|b|>if !current.son = !current then
> >
> > What caught my attention is that the number after "pc" is almost
> > 2^16, which
> > is probably the cause of my problem. I cannot find anywhere in the
> > documentation what "pc" means, nor how I can increase its maximum
> > value. Can
> > somebody help me?
>
> pc is the value of the program counter, i.e. the index of the current
> byte-code instruction.  It is absolutely not limited to 2^16, and
> I am quite sure its value has nothing to do with the bug you are
> looking for.
>
> Apparently, you're getting an Out_of_memory exception in the execution
> of
> the equality test.  Since equality is recursive, it has to go through
> both data structures and if they are cyclic it may not terminate.
> If they are cyclic and have enough complexity, it will recursively
> call itself until its stack overflows, which is reported as an
> Out_of_memory exception.
>
> The workaround is to avoid comparing cyclic data structures.
>
> -- Damien

Thanks for the information. You're right, the program counter has nothing to 
do with the issue. I learned that between versions 3.06 and 3.08 the rules 
for a recursive equality test have changed, and I now need to update my 
program.

Mathijs