SWI-Prolog -- API-function PL_unify

Availability:C-language interface function

int PL_unify_list(term_t ?l, term_t -h, term_t -t)

Unify l with a list-cell (./2). If successful, write a reference to the head of the list into h and a reference to the tail of the list into t. This reference may be used for subsequent calls to this function. Suppose we want to return a list of atoms from a char **. We could use the example described by PL_put_list(), followed by a call to PL_unify(), or we can use the code below. If the predicate argument is unbound, the difference is minimal (the code based on PL_put_list() is probably slightly faster). If the argument is bound, the code below may fail before reaching the end of the word list, but even if the unification succeeds, this code avoids a duplicate (garbage) list and a deep unification.

foreign_t
pl_get_environ(term_t env)
{ term_t l = PL_copy_term_ref(env);
  term_t a = PL_new_term_ref();
  extern char **environ;
  char **e;

  for(e = environ; *e; e++)
  { if ( !PL_unify_list(l, a, l) ||
         !PL_unify_atom_chars(a, *e) )
      PL_fail;
  }

  return PL_unify_nil(l);
}