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Re: Extending C with GC?

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From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c
Subject Re: Extending C with GC?
Date Wed, 03 Aug 2016 15:26:42 -0700
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Keith Thompson <kst-u@mib.org> writes:
> luser droog <luser.droog@gmail.com> writes:
>> In a recent thread that touched upon closures, it was made apparent
>> that automatic storage reclamation would be a practical necessity
>> to support closures. 
>>
>> So how would C work with a garbage collector? How do existing 
>> extensions work? Is there a new allocator function to allow a
>> "mixed" style, where you manually free the things that are easy
>> and rely on the GC for the things that are hard? Or does it 
>> take-over malloc(), if so what does free() do?
>>
>> Since I use C primarily as the implementation language for other
>> programming languages, I'd love to stop writing gc code.
>
> There is a narrow technical point that implies that garbage
> collection is incompatible with conformance to the C standard.
>
> GC depends on the ability to detect that a chunk of memory is no
> longer in use, and can be recovered.  There are various methods
> to do this, but basically if there's currently nothing pointing to
> a chunk of memory, it's considered safe to assume that that chunk
> can be reclaimed.

[snip discussion of ways a pointer value can be "hidden" from a GC
system]

There's also an inverse problem, though it's not as severe.

A GC system that depends on scanning memory and registers looking
for in-use addresses can also get false positives.

A GC-aware program might build, say, a large binary tree in memory.
Let's say the root node is allocated at (void*)0xdeadbeef, and
the rest of the tree is accessible only via the root.  The program
might then set its only copy of the root pointer to NULL with the
expectation that the whole tree will then be garbage-collected.
But if something that looks like the address of the root node happens
to be stored elsewhere, it might never be collected, resulting in
a memory leak.

I believe that GC systems for some languages other than C can
deal with this, but I don't know how a C garbage collector could
avoid this problem in all cases.  Perhaps a compiler could provide
information that the GC can access at run time that tells it which
objects are actually pointers -- but I don't know (a) whether
compilers actually do this, or (b) whether it's even possible
in general.

I'm fairly sure this is a well known issue.  Perhaps there are
solutions I don't know about.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

Extending C with GC? luser droog <luser.droog@gmail.com> - 2016-07-31 15:31 -0700
  Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-07-31 15:52 -0700
    Re: Extending C with GC? Tim Rentsch <txr@alumni.caltech.edu> - 2016-07-31 17:21 -0700
    Re: Extending C with GC? Jean-Marc Bourguet <jm@bourguet.org> - 2016-08-01 20:34 +0200
      Re: Extending C with GC? supercat@casperkitty.com - 2016-08-01 12:14 -0700
      Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-01 12:46 -0700
        Re: Extending C with GC? "James R. Kuyper" <jameskuyper@verizon.net> - 2016-08-01 15:56 -0400
          Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-01 14:28 -0700
        Re: Extending C with GC? Jean-Marc Bourguet <jm@bourguet.org> - 2016-08-01 23:37 +0200
          Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-01 15:03 -0700
          Re: Extending C with GC? jacobnavia <jacob@jacob.remcomp.fr> - 2016-08-02 00:11 +0200
            Re: Extending C with GC? "James R. Kuyper" <jameskuyper@verizon.net> - 2016-08-01 18:19 -0400
              Re: Extending C with GC? BGB <cr88192@hotmail.com> - 2016-08-02 12:46 -0500
    Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-03 15:26 -0700
      Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-03 15:49 -0700
        Re: Extending C with GC? Ian Collins <ian-news@hotmail.com> - 2016-08-04 14:07 +1200
          Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-03 19:26 -0700
            Re: Extending C with GC? Ian Collins <ian-news@hotmail.com> - 2016-08-04 14:55 +1200
          Re: Extending C with GC? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-08-03 20:38 -0700
            Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-04 05:10 -0700
              Re: Extending C with GC? Siri Cruise <chine.bleu@yahoo.com> - 2016-08-04 05:57 -0700
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-04 06:13 -0700
              Re: Extending C with GC? Ian Collins <ian-news@hotmail.com> - 2016-08-05 07:24 +1200
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-04 12:47 -0700
                Re: Extending C with GC? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-08-04 23:32 -0700
                Re: Extending C with GC? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-08-05 10:09 +0100
                Re: Extending C with GC? Ian Collins <ian-news@hotmail.com> - 2016-08-05 20:32 +1200
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-05 06:14 -0700
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-06 04:14 -0700
                Re: Extending C with GC? Melzzzzz <mel@zzzzz.com> - 2016-08-06 13:23 +0200
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-06 04:38 -0700
                Re: Extending C with GC? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-08-06 04:34 -0700
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-06 05:28 -0700
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-06 06:05 -0700
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-06 06:12 -0700
                Re: Extending C with GC? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-08-06 14:02 +0100
                Re: Extending C with GC? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-08-06 06:11 -0700
                Re: Extending C with GC? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-08-06 14:50 +0100
                Re: Extending C with GC? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-08-06 07:57 -0700
                Re: Extending C with GC? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-08-06 17:02 +0100
                Re: Extending C with GC? jacobnavia <jacob@jacob.remcomp.fr> - 2016-08-06 19:20 +0200
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-06 14:27 +0100
                Re: Extending C with GC? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-08-06 14:45 +0100
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-06 09:59 -0700
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-06 19:38 +0100
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-06 17:03 -0700
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-07 09:54 +0100
                Re: Extending C with GC? Ian Collins <ian-news@hotmail.com> - 2016-08-07 21:53 +1200
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-07 11:34 +0100
                Re: Extending C with GC? anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-08-07 12:06 +0000
                Re: Extending C with GC? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-08-07 14:10 +0100
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-07 15:38 +0100
                Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-07 14:32 -0700
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-07 23:44 +0100
                Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-07 16:12 -0700
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-08 01:42 +0100
                Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-07 18:56 -0700
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-08 10:20 +0100
                Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-08 09:39 -0700
                Re: Extending C with GC? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-08-08 02:30 +0100
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-07 20:15 +0100
                Re: Extending C with GC? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-08-08 01:24 +0100
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-08 02:08 +0100
                Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-07 18:49 -0700
                Re: Extending C with GC? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-08-08 03:28 +0100
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-08 12:14 +0100
                Re: Extending C with GC? Ian Collins <ian-news@hotmail.com> - 2016-08-08 07:37 +1200
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-07 20:56 +0100
                Re: Extending C with GC? Ian Collins <ian-news@hotmail.com> - 2016-08-08 08:11 +1200
                Re: Extending C with GC? Melzzzzz <mel@zzzzz.com> - 2016-08-07 22:19 +0200
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-07 22:23 +0100
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-07 14:29 -0700
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-07 22:36 +0100
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-07 15:04 -0700
                Re: Extending C with GC? jameskuyper@verizon.net - 2016-08-07 15:19 -0700
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-08 00:04 +0100
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-07 16:20 -0700
                Re: Extending C with GC? Ian Collins <ian-news@hotmail.com> - 2016-08-08 15:21 +1200
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-08 10:35 +0100
                Re: Extending C with GC? Gareth Owen <gwowen@gmail.com> - 2016-08-08 18:13 +0100
                Re: Extending C with GC? supercat@casperkitty.com - 2016-08-08 11:53 -0700
                Re: Extending C with GC? raltbos@xs4all.nl (Richard Bos) - 2016-08-10 12:58 +0000
                Re: Extending C with GC? supercat@casperkitty.com - 2016-08-10 07:05 -0700
                Re: Extending C with GC? Ken Brody <kenbrody@spamcop.net> - 2016-08-10 12:29 -0400
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-08 21:15 +0100
                Re: Extending C with GC? supercat@casperkitty.com - 2016-08-08 08:57 -0700
                Re: Extending C with GC? luser droog <luser.droog@gmail.com> - 2016-08-07 15:37 -0700
                Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-07 14:58 -0700
                Re: Extending C with GC? Ian Collins <ian-news@hotmail.com> - 2016-08-08 15:26 +1200
                Re: Extending C with GC? jacobnavia <jacob@jacob.remcomp.fr> - 2016-08-08 10:26 +0200
                Re: Extending C with GC? Ian Collins <ian-news@hotmail.com> - 2016-08-08 21:13 +1200
                Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-08 10:41 +0100
                Re: Extending C with GC? jacobnavia <jacob@jacob.remcomp.fr> - 2016-08-08 12:00 +0200
                Re: Extending C with GC? Ian Collins <ian-news@hotmail.com> - 2016-08-08 22:07 +1200
                Re: Extending C with GC? David Brown <david.brown@hesbynett.no> - 2016-08-08 12:57 +0200
                Re: Extending C with GC? Gareth Owen <gwowen@gmail.com> - 2016-08-08 18:12 +0100
                Re: Extending C with GC? Ian Collins <ian-news@hotmail.com> - 2016-08-08 21:25 +1200
                Re: Extending C with GC? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-08-08 02:57 -0700
                Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-07 13:28 -0700
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-07 05:42 -0700
                Re: Extending C with GC? Florian Weimer <fw@deneb.enyo.de> - 2016-08-07 11:49 +0200
                Re: Extending C with GC? jacobnavia <jacob@jacob.remcomp.fr> - 2016-08-06 19:18 +0200
                Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-06 12:35 -0700
                Re: Extending C with GC? jacobnavia <jacob@jacob.remcomp.fr> - 2016-08-06 19:27 +0200
                Re: Extending C with GC? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-08-06 10:55 -0700
                Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-06 12:31 -0700
      Re: Extending C with GC? James Kuyper <jameskuyper@verizon.net> - 2016-08-03 22:36 -0400
        Re: Extending C with GC? David Brown <david.brown@hesbynett.no> - 2016-08-04 10:57 +0200
          Re: Extending C with GC? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-08-04 04:09 -0700
            Re: Extending C with GC? David Brown <david.brown@hesbynett.no> - 2016-08-04 14:01 +0200
        Re: Extending C with GC? supercat@casperkitty.com - 2016-08-04 07:52 -0700
      Re: Extending C with GC? Tim Rentsch <txr@alumni.caltech.edu> - 2016-08-04 08:57 -0700
        Re: Extending C with GC? supercat@casperkitty.com - 2016-08-04 09:19 -0700
          Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-04 12:30 -0700
            Re: Extending C with GC? supercat@casperkitty.com - 2016-08-04 15:21 -0700
              Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-04 16:50 -0700
                Re: Extending C with GC? supercat@casperkitty.com - 2016-08-05 07:31 -0700
          Re: Extending C with GC? Tim Rentsch <txr@alumni.caltech.edu> - 2016-08-05 16:01 -0700
        Re: Extending C with GC? jacobnavia <jacob@jacob.remcomp.fr> - 2016-08-04 21:00 +0200
          Re: Extending C with GC? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-08-04 20:26 +0100
            Re: Extending C with GC? jacobnavia <jacob@jacob.remcomp.fr> - 2016-08-05 10:37 +0200
              Re: Extending C with GC? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-08-05 15:52 +0100
              Re: Extending C with GC? antispam@math.uni.wroc.pl - 2016-08-21 00:09 +0000
                Re: Extending C with GC? Gareth Owen <gwowen@gmail.com> - 2016-08-21 08:58 +0100
                Re: Extending C with GC? jacobnavia <jacob@jacob.remcomp.fr> - 2016-08-21 10:48 +0200
                Re: Extending C with GC? "Pascal J. Bourguignon" <pjb@informatimago.com> - 2016-08-21 19:58 +0200
                Re: Extending C with GC? supercat@casperkitty.com - 2016-08-22 08:46 -0700
                Re: Extending C with GC? Öö Tiib <ootiib@hot.ee> - 2016-08-26 02:24 -0700
                Re: Extending C with GC? supercat@casperkitty.com - 2016-08-26 13:37 -0700
                Re: Extending C with GC? Melzzzzz <mel@zzzzz.com> - 2016-08-27 00:41 +0200
                Re: Extending C with GC? Öö Tiib <ootiib@hot.ee> - 2016-08-28 06:12 -0700
            Re: Extending C with GC? BartC <bc@freeuk.com> - 2016-08-05 10:06 +0100
              Re: Extending C with GC? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-08-05 15:26 +0100
          Re: Extending C with GC? Tim Rentsch <txr@alumni.caltech.edu> - 2016-08-05 08:11 -0700
            Re: Extending C with GC? jacobnavia <jacob@jacob.remcomp.fr> - 2016-08-05 17:43 +0200
              Re: Extending C with GC? Tim Rentsch <txr@alumni.caltech.edu> - 2016-08-05 15:28 -0700
        Re: Extending C with GC? BGB <cr88192@hotmail.com> - 2016-08-04 17:11 -0500
  Re: Extending C with GC? Tim Rentsch <txr@alumni.caltech.edu> - 2016-07-31 17:35 -0700
  Re: Extending C with GC? jacobnavia <jacob@jacob.remcomp.fr> - 2016-08-01 07:45 +0200
    Re: Extending C with GC? Keith Thompson <kst-u@mib.org> - 2016-08-01 08:49 -0700
    Re: Extending C with GC? supercat@casperkitty.com - 2016-08-01 09:06 -0700
      Re: Extending C with GC? Aleksandar Kuktin <akuktin@gmail.com> - 2016-08-05 18:11 +0000
        Re: Extending C with GC? luser droog <luser.droog@gmail.com> - 2016-08-05 11:24 -0700
  Re: Extending C with GC? "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-08-08 22:12 -0700

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