From: "Davis Herring" <herring@lanl.gov>
To: "Alin Soare" <as1789@gmail.com>
Cc: Emacs Dev <emacs-devel@gnu.org>
Subject: Re: please help concerning specpdl
Date: Mon, 6 Dec 2010 08:22:30 -0800 (PST) [thread overview]
Message-ID: <35942.130.55.118.19.1291652550.squirrel@webmail.lanl.gov> (raw)
In-Reply-To: <AANLkTim-O08+aton9kyOjf-EBfPUhL494rra5CaB7BzU@mail.gmail.com>
> count = SPECPDL_INDEX ();
> record_unwind_protect
> val = Fprogn (args);
> return unbind_to (count, val);
> Can you explain me the logic of specpdl please ?
The specpdl contains information needed to remove variable bindings and
provide other services that are guaranteed even in the case of abnormal
exit. Abnormal exits operate by calling longjmp(3), so there's no
guaranteed chance to perform cleanup in the normal flow of execution. But
the function that calls longjmp(3) (`unwind_to_catch') first peruses the
specpdl and handles its elements appropriately, so the guaranteed
operations happen. The actual unwinding is done by `unbind_to', so if the
body exits normally, we just call that to perform the unwinding operations
without longjmp(3).
So: the count identifies how far to unwind in case of normal exit (in case
of abnormal exit, we'll unwind farther anyway, so we don't need `count').
The `record_unwind_protect' registers something to do at unwind-time, and
then `unbind_to' performs it unless `unwind_to_catch' does. `val' is
passed to `unbind_to' for GC reasons, I believe.
Davis
--
This product is sold by volume, not by mass. If it appears too dense or
too sparse, it is because mass-energy conversion has occurred during
shipping.
next prev parent reply other threads:[~2010-12-06 16:22 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-12-04 5:33 please help concerning specpdl Alin Soare
2010-12-06 16:22 ` Davis Herring [this message]
2010-12-14 21:25 ` Alin Soare
2010-12-15 0:14 ` Davis Herring
2010-12-15 0:53 ` Ken Raeburn
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