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From: koerberm@nixsin.UUCP (Mathias Koerber)
Newsgroups: comp.unix.questions
Subject: atomic check and update of shm flag
Message-ID: <1053@nixsin.UUCP>
Date: 4 Jun 90 10:43:15 GMT
Organization: Nixdorf Regional HQ Pte Ltd, Singapore
Lines: 23

Can I safely assume, that a contruct like

	while (shm_addr->flag == 0)
		;
	shm_addr->flag = getpid();

ensures that only one of several equal processes at a time
have access to a resource? I fear that one process finds the
flag set to zero, but is running out of time-slice, before it can set the
flag, and then another process also finds the flag set to zero and
uses the resource. If the above example doesn't work, what works?

I'd like to have a solution with shm, since I don't want to add even
another IPC-system (semaphores etc) only for this to work.

Please e-mail, address as in my (.sig).

Thx, Mathias
-- 
Mathias Koerber           |Tel:   +65 / 7473828 ext 1852|Fax: +65 / 7474331
Nixdorf Computer Singapore|EUnet: koerber.sin@nixpbe    |nerv:  koerber.sin
2 Kallang Sector          |uunet: uunet!linus!nixbur!koerber.sin
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