Path: utzoo!attcan!uunet!bywater!arnor!arnor!marc
From: marc@arnor.uucp
Newsgroups: comp.unix.aix
Subject: Re: cs system call on RS/6000
Message-ID: <MARC.90Dec10083337@marc.watson.ibm.com>
Date: 10 Dec 90 13:33:37 GMT
References: <1990Dec6.183659.1712@progress.com>
Sender: news@arnor.uucp (NNTP News Poster)
Organization: IBM T.J. Watson Research Center, Hawthorne, New York
Lines: 23
In-Reply-To: tma@progress.COM's message of 6 Dec 90 18:36:59 GMT

"cs" follows the "compare and swap" strategy used in the IBM S/370.
It is described in the on line documentation, which includes an
example.  The call is:

cs(int *dest, int comp, int value);

and the idea is that it will store value in destination only if
destination's old value was comp.  This is of course done atomically.
cs returns 0 if it succeeds, 1 if the old value of destination did not
match comp.

For examle, to set a simple spin lock in 

int lock

where the lock is 0 when free and 1 when held:

while (cs(&lock,0,1));

compare and swap can also be used to do many linked list manipulations
directly without ever setting a lock at all.

Marc Auslander