A Simpler 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 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 */);
     ...