9.9 Example of Using the Foreign Interface

Below is an example showing all stages of the declaration of a foreign predicate that transforms atoms possibly holding uppercase letters into an atom only holding lower case letters. Figure 12 shows the C-source file, figure 13 illustrates compiling and loading of foreign code.

/*  Include file depends on local installation */
#include <SWI-Prolog.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

pl_lowercase(term_t u, term_t l)
{ char *copy;
  char *s, *q;
  int rval;

  if ( !PL_get_atom_chars(u, &s) )
    return PL_warning("lowercase/2: instantiation fault");
  copy = malloc(strlen(s)+1);

  for( q=copy; *s; q++, s++)
    *q = (isupper(*s) ? tolower(*s) : *s);
  *q = '\0';

  rval = PL_unify_atom_chars(l, copy);

  return rval;

{ PL_register_foreign("lowercase", 2, pl_lowercase, 0);
Figure 12 : Lowercase source file
% gcc -I/usr/local/lib/pl-\plversion/include -fpic -c lowercase.c
% gcc -shared -o lowercase.so lowercase.o
% pl
Welcome to SWI-Prolog (Version \plversion)
Copyright (c) 1993-1996 University of Amsterdam.  All rights reserved.

For help, use ?- help(Topic). or ?- apropos(Word).

1 ?- load_foreign_library(lowercase).

2 ?- lowercase('Hello World!', L).

L = 'hello world!' 

Figure 13 : Compiling the C-source and loading the object file