Passing pointers between Prolog and Foreign Code

Suppose we have a FORTRAN subroutine that multiplies a 3-element vector by a 3-by-3 matrix, returning a 3-element vector. This situation is then represented by the following code:

                                     
C code
     
     
     typedef double vec_3[3];
     typedef double mat_3_3[3][3];
     
     vec_3 *make_vec(a, b, c)
         double a, b, c;
         {
             register vec_3 *x;
     
             x = (vec_3*)malloc(sizeof(vec_3));
             (*x)[0] = a, (*x)[1] = b, (*x)[2] = c;
             return x;
         }
     
     mat_3_3 *make_mat(a0, a1, a2, b0, b1, b2, c0, c1, c2)
         double a0, a1, a2, b0, b1, b2, c0, c1, c2;
         {
             register mat_3_3 *x;
     
             x = (mat_3_3*)malloc(sizeof(mat_3_3));
             (*x)[0][0] = a0, (*x)[0][1] = a1, (*x)[0][2] = a2,
             (*x)[1][0] = b0, (*x)[1][1] = b1, (*x)[1][2] = b2,
             (*x)[2][0] = c0, (*x)[2][1] = c1, (*x)[2][2] = c2;
             return x;
         }
     

                               
FORTRAN code
     
     
           subroutine matvec(mat, vec, ans)
               real mat(3,3), vec(3), ans(3)
     
               ans(1) = mat(1,1)*vec(1)+mat(2,1)*vec(2)+mat(3,1)*vec(3)
               ans(2) = mat(1,2)*vec(2)+mat(2,2)*vec(2)+mat(3,2)*vec(3)
               ans(3) = mat(1,3)*vec(3)+mat(2,3)*vec(2)+mat(3,3)*vec(3)
               return
           end
     

                               
Prolog Code:
     
     
     foreign(make_vec, c, make_vec(+float,+float,+float,
                                   [-address(vec_3)])).
     foreign(make_mat, c, make_mat(+float,+float,+float,
                                   +float,+float,+float,
                                   +float,+float,+float,
                                   [-address(mat_3_3)])).
     foreign(matvec_, fortran, matvec(+address(float),+address(float),
                                     +address(float))).      % note all +!
     
     make_vec([A,B,C], X) :-
             make_vec(A, B, C, X).
     
     make_mat([[A0,A1,A2],[B0,B1,B2],[C0,C1,C2]], X) :-
             make_mat(A0,A1,A2, B0,B1,B2, C0,C1,C2, X).
     
     do_matvec(Vec, Mat, AnsObj) :-
             make_vec(Vec, VecObj),
             make_mat(Mat, MatObj),
             make_vec(0.0, 0.0, 0.0, AnsObj),
             matvec(VecObj, MatObj, AnsObj).