From: Mikael Djurfeldt <djurfeldt@nada.kth.se>
Cc: djurfeldt@nada.kth.se
Subject: Re: weak key hash versus display
Date: Thu, 30 Jan 2003 10:27:31 +0100 [thread overview]
Message-ID: <xy7fzrbgf7g.fsf@nada.kth.se> (raw)
In-Reply-To: <877kcn5yqc.fsf@zip.com.au> (Kevin Ryde's message of "Thu, 30 Jan 2003 09:21:15 +1000")
Kevin Ryde <user42@zip.com.au> writes:
> In guile 1.6.1 or the cvs on a recent i386 debian, I noticed that
> doing a display of a weak key hash table can seemingly prevent a key
> from being garbage collected. For instance
>
> (define h (make-weak-key-hash-table 7))
>
> (define k "mykey")
> (hash-set! h (string-copy k) 12345)
>
> (display (hash-ref h k)) (newline)
> (display h) (newline)
> (gc)
> (display (hash-ref h k)) (newline)
>
> run with "guile -s foo.scm" produces
>
> 12345
> #wh(() () () () () ((mykey . 12345)) ())
> 12345
>
> whereas I might have expected the gc to have collected the entry just
> set, making the second hash-ref give #f rather than 12345. This is
> what happens if the (display h) is not present.
And if you keep (display h) but add
(display <some deep list-structure>),
hash-ref will give #f again.
The reason is that in order to avoid infinite loops due to circular
data-structures print functions need to keep track of printed
references. These are stored in a "print-state" attached to the
port. After displaying h, the print-state contains the reference to
the pair (mykey . 12345) which protects it from GC.
We should probably replace this vector with a weak vector, or (perhaps
more efficient) clear the references from the vector.
> I don't really know if this is a bug, or ignorance on my part, but it
> seemed more than a little strange.
Well, it's not really a bug. The Guile GC doesn't make any guarantees
that objects will get GC:d. For example, if the C stack happens to
contain an integer which happens to coincide with a reference on the
heap, that object won't get GC:d. A conservative GC only behaves
nicely in a statisticial sense.
Thanks for your observation.
Best regards,
Mikael Djurfeldt
_______________________________________________
Bug-guile mailing list
Bug-guile@gnu.org
http://mail.gnu.org/mailman/listinfo/bug-guile
next prev parent reply other threads:[~2003-01-30 9:27 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-01-29 23:21 weak key hash versus display Kevin Ryde
2003-01-30 9:27 ` Mikael Djurfeldt [this message]
2003-01-31 21:56 ` Kevin Ryde
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
List information: https://www.gnu.org/software/guile/
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=xy7fzrbgf7g.fsf@nada.kth.se \
--to=djurfeldt@nada.kth.se \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).