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From: kend@data.UUCP (Ken Dickey)
Newsgroups: comp.lang.scheme
Subject: Re: Ports & GC
Message-ID: <480@data.UUCP>
Date: 26 Apr 91 15:40:30 GMT
References: <6260006@otter.hpl.hp.com>
Organization: Microcosm, Beaverton, OR
Lines: 40

sfk@otter.hpl.hp.com (Steve Knight) writes:

>I've been reading the most recent draft standard I've got (Nov89) and can't
>determine the answer to the following question.  When a port is garbage
>collected, is it automatically closed on your behalf?  ...

This is unspecified in the standard.


>If this isn't a question answered by the draft standard then I'd appreciate
>knowing the general implementation strategies.

Here are a couple:

  Maintain a table of open ports.  At collection time, scan the table and
close any ports which have been collected.

 Implement finalization objects.  I did this in a local copy of the
Gambit runtime (v1.5) by implementing weak-pairs (the car goes to nil
when collected).  The finalization procedure registers each object to
be finalized with a thunk which does the cleanup.  The object is in
the car of a weak pair, the thunk in the cdr.  After a collection, the
weak-pairs are scanned and any who's car is nil is thrown away after
the finalization thunk is invoked.  {The rules are that such a thunk
does not cons and does not invoke random continuations.}

==========

A finalization service is also helpful for freeing storage for dumb
allocators like C's malloc.  Weaks/populations, etc. are helpful in a
variety of ways.

Weak-pairs are easy to implement in a stop-and-copy collector, by the
way.  There is a very clear description of this in Jim Millers thesis:
"MultiScheme", MIT/LCS/TR-402, September 1987.

I can send you code if that is interesting.


-Ken Dickey				kend@data.uucp