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From: jab@lll-crg.arpa
Newsgroups: comp.unix.wizards
Subject: Shared memory
Message-ID: <5701@brl-adm.ARPA>
Date: Wed, 25-Mar-87 08:29:29 EST
Article-I.D.: brl-adm.5701
Posted: Wed Mar 25 08:29:29 1987
Date-Received: Fri, 27-Mar-87 01:22:18 EST
Sender: news@brl-adm.ARPA
Lines: 35

> From: Andrew Siegel <abs@nbc1.uucp>
> Subject: Question about Sys V shared memory
> Date: 23 Mar 87 22:28:19 GMT
> To:       unix-wizards@brl-sem.arpa
> 
> The situation:
> 
> I have a large shared memory segment.  I would like to be able to
> have a dynamic data structure within that segment, i.e. set aside
> some memory within the shm segment for allocation, and have one
> or more pointers within the shm segment pointing into this memory
> pool.
> 

Great, but unless the SHM segment is attached to the same virtual
address in each process, you can't use pointers to describe
the free addresses, since they'll be at different virtual addresses
in each process.

Choices:
	1) Store the info on what's free as "byte offset into SHM segment".
	   Then if the SHM segment lives at different places, the addresses
	   each process use would be relative to the beginning of the SHM
	   segment, and each process would know where *THAT* was.
	2) Always locate the SHM segment in the same virtual address for all
	   processes, period.
The first choice is a lot of things, including more portable and less
cumbersome. Unfortunately, it takes more CPU operations to evaluate
	*(base + offset)
than if you know the real address itself.

You can see the problems with the second approach.

	Jeff Bowles
	Summit, NJ