Node:An Alternative Way to Define C Predicates, Previous:Creating the Linked Foreign Resource, Up:Calling C from Prolog

An Alternative Way to Define C Predicates

SP_define_c_predicate() defines a Prolog predicate such that when the Prolog predicate is called it will call a C function with a term corresponding to the Prolog goal. The arguments to the predicate can then be examined using the usual term access functions, e.g. SP_get_arg() (see Accessing Prolog Terms).

typedef int SP_CPredFun(SP_term_ref goal, void *stash);
int SP_define_c_predicate(char *name, int arity, char *module,
                          SP_CPredFun *proc, void *stash)

The Prolog predicate module:name/arity will be defined (the module module must already exist). The stash argument can be anything and is simply passed as the second argument to the C function proc.

The C function should return SP_SUCCESS for success and SP_FAILURE for failure. The C function may also call SP_fail() or SP_raise_exception() in which case the return value will be ignored.

Here is an end-to-end example of the above:

                                
% square.pl

foreign_resource(square, [init(square_init)]).

:- load_foreign_resource(square).

                             
/* square.c */

#include <sicstus/sicstus.h>

static int square_it(SP_term_ref goal, void *stash)
{
  long arg1;
  SP_term_ref tmp = SP_new_term_ref();
  SP_term_ref square_term = SP_new_term_ref();
  long the_square;

  /* goal will be a term like square(42,X) */
  SP_get_arg(1,goal,tmp); /* extract first arg */
  if (!SP_get_integer(tmp,&arg1))
    return SP_FAILURE;   /* type check first arg */

  SP_put_integer(square_term, arg1*arg1);
  SP_get_arg(2,goal,tmp); /* extract second arg */

  /* Unify output argument.
     SP_put_integer(tmp,...) would *not* work!  */
  return (SP_unify(tmp, square_term) ? SP_SUCCESS : SP_FAILURE);
}

void square_init(int when)
{
  (void)when;                   /* unused */
  /* Install square_it as user:square/2 */
  SP_define_c_predicate("square", 2, "user", square_it, NULL /* unused */);
}

                                 
# terminal

% splfr square.pl square.c
% sicstus -f -l square
% compiling /home/matsc/tmp/square.pl...
%  loading foreign resource /home/matsc/tmp/square.so in module user
% compiled /home/matsc/tmp/square.pl in module user, 0 msec 816 bytes
SICStus 3.12.1 ...
Licensed to SICS
| ?- square(4711, X).
X = 22193521 ?
yes
| ?- square(not_an_int, X).
no