Interfacing Prolog and C: Pl_Query_Next_Solution fails

Hi,
Working in natural langugage processing/speech recognition, I develop  
a program using C and Prolog.
In my C source, I need to process results of Prolog queries, but the  
Pl_Query_Next_Solution function fails: only the first solution  
obtained using Pl_Query_Call is returned.

So, to be sure that my program is not the problem, I tried the  
following example coming from the gnu prolog manual:
( see http://pauillac.inria.fr/~diaz/gnu-prolog/manual/ 
manual070.html#toc310 )

Prolog source:

parent(bob,mary).
parent(jane,mary).
parent(mary,peter).
parent(paul,peter).
parent(peter,john).

anc(X,Y):-parent(X,Y).
anc(X,Z):-parent(X,Y), anc(Y,Z).


C source:

#include <string.h>
#include "gprolog.h"

static int
Main_Wrapper(int argc, char *argv[])
{
   int func;
   WamWord arg[10];
   char str[100];
   char *sol[100];
   int i, nb_sol = 0;
   Bool res;

   Start_Prolog(argc, argv);

   func = Find_Atom("anc");
   for (;;)
     {
       printf("\nEnter a name (or 'end' to finish): ");
       scanf("%s", str);

       if (strcmp(str, "end") == 0)
         break;

       Pl_Query_Begin(TRUE);

       arg[0] = Mk_Variable();
       arg[1] = Mk_String(str);
       nb_sol = 0;
       res = Pl_Query_Call(func, 2, arg);
       while (res)
         {
           sol[nb_sol++] = Rd_String(arg[0]);
           res = Pl_Query_Next_Solution();
         }
       Pl_Query_End(PL_RECOVER);

       for (i = 0; i < nb_sol; i++)
         printf("  solution: %s\n", sol[i]);
       printf("%d solution(s)\n", nb_sol);
     }

   Stop_Prolog();
   return 0;
}

int
main(int argc, char *argv[])
{
   return Main_Wrapper(argc, argv);
}


As shown in the manual, the following behaviour should be observed:
Enter a name (or 'end' to finish): john
   solution: peter
   solution: bob
   solution: jane
   solution: mary
   solution: paul
5 solution(s)

But, this is what I obtain:
Enter a name (or 'end' to finish): john
   solution: peter
1 solution(s)


Can you help me?

Here some features about my workstation (MacOS X, Tiger)

OS: Darwin horus.univ-lemans.fr 8.1.0 Darwin Kernel Version 8.1.0:  
Tue May 10 18:16:08 PDT 2005; root:xnu-792.1.5.obj~4/RELEASE_PPC  
Power Macintosh powerpc
GNU Prolog version: Prolog top-Level (GNU Prolog) 1.2.16

GNU Prolog was installed using fink:
Package: gprolog
Version: 1.2.16
Revision: 1
Source: gnu
Source-MD5: d7fe87106cd3e7e770375f04dd0d14c2
DocFiles: ChangeLog COPYING INSTALL NEWS PROBLEMS README VERSION
ConfigureParams: --with-install-dir=%i --with-html-dir=%i/share/doc/% 
n/html --with-doc-dir=%i/share/doc/%n/doc --without-links-dir --with- 
examples-dir=%i/share/doc/%n/examples
CompileScript: (cd src; ./configure %c; make)
InstallScript: <<
(cd src; make install)
(cd src; rm %i/COPYING %i/VERSION %i/NEWS %i/ChangeLog)
<<
Description: Prolog (ISO) and constraint-solving language
License: GPL
Homepage: http://www.gnu.org/software/gprolog/
Maintainer: Lindsey Spratt <[email protected]>