another problem interfacing prolog and C

[Erick Alphonse]

Hello,

Still playing with the interface, I would like to call C from prolog and 
then from this C function call prolog back. Unfortunately, I can't get 
it working.

% examp.pl
:- initialization(top).

:- foreign(call_c(+term,-term)).

top :-
        call_c(a,_L).

% examp_c.c
#include <string.h>
#include "gprolog.h"

Bool
call_c(PlTerm In,PlTerm* Out)
{
  Pl_Query_Begin(TRUE);
  Pl_Query_Call(Find_Atom("write"),1,&In);
  Pl_Query_End(PL_RECOVER);

  *Out = Mk_Atom(atom_nil);

  return PL_SUCCESS;
}

And I get :
874# gplc examp.pl examp_c.c
875# ./examp
warning: /home/alphonse/tmp/bug-prolog/examp.pl:1: user directive failed
a
GNU Prolog 1.2.18
By Daniel Diaz
Copyright (C) 1999-2004 Daniel Diaz
| ?-

That is, the variable _L in top/0 cannot be unified at the exit of 
call_c, like if the inner query was messing with the stack. If I comment 
the inner query, it works of course. Could someone help me out and tell 
me if, first, what I try to do is possible with the gprolog interface 
and how to do it properly? I can't figure it out myself as I guess I 
exhausted all the examples in the doc.

Thanks in advance for your help, hopefully I could get back to work.

Erick.