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 10 shows the C-source file, figure 11 illustrates compiling and loading of foreign code.
/* Include file depends on local installation */
#include <SWI-Prolog.h>
#include <stdlib.h>
#include <ctype.h>
foreign_t
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);
free(copy);
return rval;
}
install_t
install()
{ PL_register_foreign("lowercase", 2, pl_lowercase, 0);
}
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% 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).
Yes
2 ?- lowercase('Hello World!', L).
L = 'hello world!'
Yes
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