🚀 go-pugleaf

On Tuesday 16 April 2002 18:09, Lionel Ains wrote:
> Hello,
>
> I would like to know how to pass an exception from one C
> function to another one in foreign code.
> The idea behind this is to handle the following thing:
>
> foreign_t pl_do_sth() {
>
> int i;
>
>  for (i=0; i<10; i++) {
>  compute_sth_from(i);
>  }
>  PL_succeed;
> }
>
> Let's say that pl_do_sth is the C-implementation associated
> with the Prolog predicate do_sth/0
> I would like to allow the C function "compute_sth_from()" to
> raise exception, that will not be caught by pl_do_sth() and
> should thus be thrown outside of pl_do_sth(), thus outside
> of the foreign code, to handle it at the Prolog level.
> Do I have to declare compute_sth_from() as returning a
> foreign_t type?
> How do I test whether compute_sth_from() raised an
> exception? Do I specifically have to test it, and to
> manually pass the exception by getting the exception object
> thrown by compute_sth_from() and re-raise the same in
> pl_do_sth()?
>
> The workaround I am using right now is to make
> compute_sth_from() return an integer that will be evaluated
> by pl_do_sth(), which will, in turn, raise an exception
> according to the value returned. This forces me to repeat
> the code that raises the exception in all the function that
> will call compute_sth_from() though... so it's not a very
> clean way of doing it.

Lionel,

Its not really clear what you want.  Is the (possible) exception
of compute_sth_from(i) raised in C or from a call-back to Prolog?

Maybe it helps if I explain how it works.  On calling PL_raise_exception(),
the term is bound to a global term-reference, called the `exception-term'.
If control returns from C to Prolog with failure and there is a term in
this exception-term the system will raise a Prolog exception.  Thats
one part.

If you call Prolog using PL_open_query(), ..., PL_next_solution(), and
Prolog raises an exception, this exception is associated with the query
and accessible through PL_exception(qid).  You can (and maybe thats the
trick for you) add PL_Q_PASS_EXCEPTION to the flags of PL_open_query().
If you do that, the exception is placed (actually left) in the same
`exception-term' and therefore returning with failure from the foreign
code will re-raise the exception in the calling Prolog context.

No one says you need to return immediately after a PL_raise_exception().
PL_throw() is an entirely different matter: it long_jmp()s back into
the system and should be used with care (as always required with
long_jmp()).

There are various examples in the system sources as well as in the
C++ interface (if I recall correctly).

	Cheers --- Jan











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