Example:

Consider the Prolog predicate:

      :-mode p(+,?).
      p(a,f(1)).
      p(b,[1]).
      p(c,1.2).

where the first argument is given and the second is unknown. The following steps show how to define this predicate in C and make it callable from Prolog.

Step 1

. Write a C function to implement the predicate. The following shows a sample:

#include "bprolog.h"

p(){
  TERM a1,a2,a,b,c,f1,l1,f12;
  char *name_ptr;
  
  /*   prepare Prolog terms */
  a1 = bp_get_call_arg(1,2);  /* first argument */
  a2 =  bp_get_call_arg(2,2); /* second argument */
  a = bp_build_atom("a");
  b = bp_build_atom("b");
  c = bp_build_atom("c");
  f1 = bp_build_structure("f",1);  /* f(1) */
  bp_unify(bp_get_arg(1,f1),bp_build_integer(1));
  l1 = bp_build_list();            /* [1] */ 
  bp_unify(bp_get_car(l1),bp_build_integer(1));
  bp_unify(bp_get_cdr(l1),bp_build_nil());
  f12 = bp_build_float(1.2);       /* 1.2 */
  
  /* code for the clauses */
  if (!bp_is_atom(a1)) return BP_FALSE;
  name_ptr = bp_get_name(a1);
  switch (*name_ptr){
  case 'a': 
    return (bp_unify(a1,a) ? bp_unify(a2,f1) : BP_FALSE);
  case 'b': 
    return (bp_unify(a1,b) ? bp_unify(a2,l1) : BP_FALSE);
  case 'c': 
    return (bp_unify(a1,c) ? bp_unify(a2,f12) : BP_FALSE);
  default: return BP_FALSE;
  }
}

Step 2

Insert the folloiwng two lines into Cboot() in cpreds.c:

      extern int p();
      insert_cpred("p",2,p);

Step 3

Recompile the system. Now, p/2 is in the group of built-ins in B-Prolog.