Comments:	Authenticated sender is <alaric@[127.0.0.1]>
From:	"Alaric B. Williams" <alaric AT abwillms DOT demon DOT co DOT uk>
To:	Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
Date:	Sun, 22 Dec 1996 21:58:16 +0000
MIME-Version:	1.0
Subject:	Re: windows <-> cwsdpmi
Reply-to:	alaric AT abwillms DOT demon DOT co DOT uk
CC:	djgpp AT delorie DOT com
Message-ID:	<851364688.618879.0@abwillms.demon.co.uk>

On 22 Dec 96 at 9:33, Eli Zaretskii wrote:
 
> On Thu, 19 Dec 1996, Alaric B. Williams wrote:
> 
> > an implementation of garbage collection that can be turned on at 
> compile time would be nice, too...
> 
> I don't see how a garbage collecting could be built into a 
> general-purpose memory allocator of a C library.  Since in C, every 
> pointer is just an address of a memory chunk, how would you relocate data 
> (to compact used memory) without breaking C code?  IMHO, there's no 
> simple way to even know which pointers are unused and can be freed.  
> Maybe in C++, but not in C.  Do I miss something?

Yup, but sadly I don't have the URL for it anymore :-)

Conservative GC can be used in C. It works like so:

The search for accessed blocks starts with the registers, the stack, 
and the data segment. These are always considered "live"; to start 
off with, all malloced data is not considered live.

For every block being searched, consider each 32 bit word. If the 
value is an address within a malloced block - found by walking the 
heap - then mark that block live and search it, unless it was live 
already, in which case we bottom out.

Then free all non-live memory blocks.

Tada! It's called conservative GC because if anything - maybe not 
even a pointer - seems to refer to a block, it stays. Therefore, some 
blocks hang around longer than they need to. But we've got to be real 
sure in a non-typesafe C program.

ABW
--
Governments are merely protection rackets with good images.

Alaric B. Williams Internet : alaric AT abwillms DOT demon DOT co DOT uk
http://www.abwillms.demon.co.uk/

- Raw text -