From: Bruce Harvey (BruceH@ROUTESCAPE.COM)
Date: Thu Nov 21 2002 - 15:22:03 EST
Here is a link to IPC
http://www.ibiblio.org/mdw/LDP/lpg/node21.html
and a link to shared memory
http://www.ibiblio.org/mdw/LDP/lpg/node65.html#SECTION00744000000000000000
or
http://www.ecst.csuchico.edu/~beej/guide/ipc/shmem.html
In any event, shared memory is vital for certain multi-process programs to
get their work done. The problem with corrupted shared memory arrives when
they don't have a GOOD method for managing who's allowed to write to the
memory when. Semaphores are sometimes used (another IPC), as are message
queues, but most often, the shared memory itself contains areas where
processes will place their process ids, "locking" a memory region for their
use. However, if two processes write their ids at about the same time, they
each may think they own it, so there needs to be a reliable 'retry'
'recheck' and so forth method to absolutely guarantee no one else is writing
when you are. For example, you write your id, wait a second, then read it.
If it's changed, you wait a given time and so forth. Or you pass around a
key like token ring does. Or you Message or semaphore.
Ever wonder why UUCP just about never has any conflicts? See if you can
find code outlining exactly what it does to restrict access to a port that's
in use (lock files that really work).
---------------------------------------
Bruce T. Harvey, Special Projects Developer
bruceh@routescape.com, www.routescape.com
Insight Distribution Systems --- Hunt Valley, MD
-----Original Message-----
From: Webb, Eric [mailto:EWebb@COOPERLIGHTING.COM]
Sent: Thursday, November 21, 2002 3:06 PM
To: aix-l@Princeton.EDU
Subject: [aix-l] SHM primer?
Do we have any programmer-types out there that could point me in the
direction of a shared memory primer from a programmer's perspective? I'd
like to know what it is, how it's used, etc. Something from a C/C++
background would be good.
One of our applications here seems to use it rather extensively, and most of
the quirks we see with it seem to be rooted in shared memory corruption.
> Eric C. Webb
> Sr. Systems Analyst / AIX System Administrator
>
> Cooper Lighting Division IT
> (770) 486-4623 (770) 486-4677 FAX
>
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