problem interfacing prolog and C

Hello,

I would like to call prolog from C, but I can't even get it working on 
this very simple example:

% test.pl
:- initialization(top).

:- foreign(write_c(+term),[return(none)]).

top :-
         write_c(a).

% test_c.c
#include "gprolog.h"

void write_c(PlTerm In) {
   Create_Atom("writeq");
}

I compile with (under 1.2.18 or 1.2.16) : gplc test_c.c test.pl -o test
and I get a Fatal Error: Segmentation Violation.

Could someone explain to me what is wrong please?

Thanks a lot in advance for helping me out,
Regards,
Erick.