Create 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. Each call to a foreign
predicate is wrapped in a PL_open_foreign_frame()
and
PL_close_foreign_frame()
pair. This ensures that term_t handles created during the
execution of a foreign predicate are scoped to this execution. Note that
if the foreign predicate is
non-deterministic, term_t handles are scoped to
each activation of the foreign function.
The user may create explicit foreign frames to undo (backtrack)
changes to Prolog terms. See PL_unify()
for an example. An explicit foreign frame must also be used for creating
a callback from C to Prolog (see PL_open_query())
to ensure the existence of such a frame and to scope the term_t
handles needed to setup the call to Prolog.
On success, the stack has room for at least 10 term_t
handles. This implies that foreign predicates as well as code inside an
explicitly created foreign frame may use PL_new_term_ref()
to create up to 10 term_t handles without checking the
return status.
Returns (fid_t)0 on failure. Failure is either lack of
space on the stacks, in which case a resource exception is scheduled or
atom-gc being in progress in the current thread, in which case no
exception is scheduled. The latter is an exceptional case that prevents
doing a callback on Prolog from blob release handlers.230Such
a callback would deadlock if the callback creates new atoms or
requires stack shifts or garbage collection.
