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 use
SP_fail() or SP_raise_exception() in which case the return
value will be ignored.
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) */
if (!SP_get_arg(1,goal,tmp)) /* extract first arg */
return SP_FAILURE; /* should not happen */
if (!SP_get_integer(tmp,&arg1))
return SP_FAILURE; /* arg 1 not an integer */
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! */
if (SP_unify(tmp, square_term))
return SP_SUCCESS;
else return SP_FAILURE;
}
...
/* Install square_it as user:square/2 */
SP_define_c_predicate("square", 2, "user", square_it, NULL /* unused */);
...