Sample C foreign interface for SWI-Prolog called with my_function('6',N) loaded with :-use_foreign_library(foreign('test.so')). compile: swipl-ld -shared -o test test.c

gistfile1.c

#include <SWI-Prolog.h>
#include <stdio.h>
#include <stdlib.h>

typedef struct context              /* define a context structure */
{
 int max;
} context;

foreign_t
my_function(term_t in, term_t out, control_t handle)
{ struct context * ctxt;

  switch( PL_foreign_control(handle) )
  { case PL_FIRST_CALL:
        ctxt = malloc(sizeof(struct context));
        char *s_max;
        if(PL_get_atom_chars(in, &s_max)) {
		    ctxt->max = atoi(s_max) - 1;
            printf("PL_FIRST_CALL\n");
            if(PL_unify_integer(out,ctxt->max)) {
                PL_retry_address(ctxt);
            }
            else {
                printf("failed to unify\n");
                PL_fail;
            }
        }
        else {
            PL_fail;
        }
    case PL_REDO:
        ctxt = PL_foreign_context_address(handle);
        printf("PL_REDO\n");
        if(ctxt->max>0) {
		  ctxt->max = ctxt->max -1;
          PL_unify_integer(out,ctxt->max);
          PL_retry_address(ctxt); 
        }
        else {
          printf("0, abording\n");
          free(ctxt); 
          PL_fail; 
        }
    case PL_PRUNED:
        printf("PL_PRUNED\n");
        ctxt = PL_foreign_context_address(handle);
        free(ctxt);
        PL_succeed;
    default:
		PL_fail;
  }
}


install_t
install_test()
{ PL_register_foreign("my_function", 2, my_function, PL_FA_NONDETERMINISTIC);
}