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From: dave@micropen.UUCP
Newsgroups: comp.unix.wizards
Subject: Re: Question about Sys V shared memory
Message-ID: <167@micropen>
Date: Wed, 25-Mar-87 09:31:58 EST
Article-I.D.: micropen.167
Posted: Wed Mar 25 09:31:58 1987
Date-Received: Fri, 27-Mar-87 01:32:29 EST
References: <314@nbc1.UUCP>
Organization: Micropen Direct Writing Systems, Pittsford, NY
Lines: 22
Summary: relative addressing

In article <314@nbc1.UUCP>, abs@nbc1.UUCP (Andrew Siegel) writes:
> The question:
> 
> If process A attaches the shm segment and assigns an address within
> the segment's memory pool to one of these pointers, will this
> address have any meaning to other processes that attach the
> segment?
> 
At least in Sys V, I would have each process call the allocator with
a pointer to the beginning of the shm ( the pointer returned to the 
process at the attach).  Then, the allocator routine would allocate
the memory and pass back a pointer with the appropriate displacement
into the segment.  Thus, addressing within the shm region is relative
to the virtual address that each process holds on the segment.  And
K&R say that pointer/integer arithmetic is cool (although some machines
like Intel or pdp's will place a limit on the size of memory in any
one segment but your nice VAX won't care a bit.)
-- 
David F. Carlson, Micropen, Inc.
...!{seismo}!rochester!ur-valhalla!micropen!dave

"The faster I go, the behinder I get." --Lewis Carroll