Re: GLOBALSZ leak issue?

[Paulo Moura]

> On 06/08/2015, at 18:18, emacstheviking <address@hidden> wrote:
> 
> I downloaded and built a fresh binary from here:
> 
> git://git.code.sf.net/p/gprolog/code <http://git.code.sf.net/p/gprolog/code> 
> master
> 
> I then loaded this code:
> 
> gtest :-
>     statistics,
>     gtest.
> 
> It looks like it would run forever as the values didn't change.
> 
> Then I ran this code:
> 
> gtest :-
>     statistics(global_stack, GS),
>     gtest.
> 
> Fatal Error: trail stack overflow (size: 16384 Kb, reached: 16384 Kb, 
> environment variable used: TRAILSZ)
> 
> TRAILSZ is used for, AIUI, "variable bindings to undo", so that's 
> understandable;  I am creating GS on each pass and then calling myself so 
> it's stashing the current value of GS jut in case we ever came back.
> 
> gtest :-
>     statistics(global_stack, _),
>     gtest.
> 
> This still causes TRAILSZ to exhaust, which I expected because it is still a 
> binding that has to be stashed I guess.
> 
> Continuing... 
> 
> gtest :-
>         statistics(global_stack, _),
>         !,
>       gtest.
> 
> Same again, TRAILSZ exhausted. From my obviously incorrect understanding of 
> prolog, only fail/0 would cause any memory allocation to be released at this 
> point?
> 
> SO... how does one use tail-recursion to keep an application running without 
> running out of stack space?

How about:

gtest :-
    statistics(global_stack, GS),
    fail.
gtest :-
    gtest.

> This really feels like "worrying about consing" when you first learn Lisp. 
> Ultimately, unless you are out for performance the runtime will do it for you 
> etc. BUT for Prolog, which is still new to me, I am now completely at a loss 
> as to how to proceed.
> 
> Paulo, we've spoken briefly in the past and I keep meaning to use LogTalk but 
> would that allow me to use my custom "gprolog" binary with my SDL extensions? 
> It ought to I guess.

Logtalk, not LogTalk :-) You can easily embed Logtalk and a Logtalk application 
in a GNU Prolog executable. See:

https://github.com/LogtalkDotOrg/logtalk3/wiki/Embedding-Logtalk

Just make sure you use the latest Logtalk git or stable version (as I added a 
workaround a few days ago for a gplc bug triggered when embedding Logtalk).

> Firstly, I will try Lindsey-s advice and better my understanding of DCG 
> productions as state transformers but I can't help feeling a little bemused 
> and stuck at the moment as I have failed so far to understand the nature of 
> the problem i.e. why is it apparently so  hard to be able to use gprolog for 
> a long-running application?
> 
> Many thanks chaps for your time so far!

You're welcome. Cheers,

Paulo

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Paulo Moura
Logtalk developer

Email: <mailto:address@hidden>
Web:   <http://logtalk.org/>
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