[Up] [Contents] [Index] [Summary]
The Prolog data created and term-references needed to setup the call
and/or analyse the result can in most cases be discarded right after the
call. PL_close_query() allows for destructing the data, while leaving
the term-references. The calls below may be used to destroy
term-references and data. See figure
9 for an example.
- fid_t PL_open_foreign_frame()
-
Created a foreign frame, holding a mark that allows the system to undo
bindings and destroy data created after it as well as providing the
environment for creating term-references. This function is called by the
kernel before calling a foreign predicate.
- void PL_close_foreign_frame(fid_t
id)
-
Discard all term-references created after the frame was opened. All
other Prolog data is retained. This function is called by the kernel
whenever a foreign function returns control back to Prolog.
- void PL_discard_foreign_frame(fid_t
id)
-
Same as PL_close_foreign_frame(), but also undo all bindings made since
the open and destroy all Prolog data.
It is obligatory to call either of the two closing functions to
discard a foreign frame. Foreign frames may be nested.
<![CDATA[
int
count_atoms()
{ fid_t fid = PL_open_foreign_frame();
term_t goal = PL_new_term_ref();
term_t a1 = PL_new_term_ref();
term_t a2 = PL_new_term_ref();
functor_t s2 = PL_new_functor(PL_new_atom("statistics"), 2);
int atoms;
PL_put_atom_chars(a1, "atoms");
PL_cons_functor(goal, s2, a1, a2);
PL_call(t, NULL); /* call it in current module */
PL_get_integer(a2, &atoms);
PL_discard_foreign_frame(fid);
return atoms;
}
]]>
Figure 9 : Calling Prolog