Date: Fri, 12 May 2000 00:58:06 -0700 From: Alfred Perlstein <bright@wintelcom.net> To: Ville-Pertti Keinonen <will@iki.fi> Cc: FengYue <fengyue@bluerose.windmoon.nu>, hackers@FreeBSD.ORG Subject: Re: Why this works? Message-ID: <20000512005806.A29302@fw.wintelcom.net> In-Reply-To: <864s84cq35.fsf@not.demophon.com>; from will@iki.fi on Fri, May 12, 2000 at 08:09:02AM %2B0300 References: <20000511210915.A38341@Hamilton-ppp44812.sympatico.ca> <Pine.BSF.4.10.10005111955220.29094-100000@bluerose.windmoon.nu> <864s84cq35.fsf@not.demophon.com>
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* Ville-Pertti Keinonen <will@iki.fi> [000511 22:49] wrote: > > fengyue@bluerose.windmoon.nu (FengYue) writes: > > > I've 3 small programs. First one writes 4K of data contains 'A's into a > > file /tmp/pagetest and then lseek() to the begin of the file. > > Second one writes 4K of 'Z' into the same file /tmp/pagetest and > > then lseek() to the begin of the file. They both do that in a tight > > loop. Now, the third program reads 4K of data from /tmp/pagetest > > and exit if the 4K data does not contain all 'A's nor 'Z's. 3 programs > > run concurrently on the same machine (3.4). No lock in the code whatsoever, > > and all 3 programs use pure write() and read(). I thought the third > > program would exit pretty quickly since the data in the file may contain > > mixed of 'A's and 'Z's, but it has been running for hours and nothing > > happened. Could someone kindly explain this? I was told that this is > > because the pagesize is 4096 in the kernel, so that read()/write() 4K of > > data will not get context switched until the call is compeleted. > > Is that right? > > Not quite. If FreeBSD didn't perform locking, operations affecting > single filesystem blocks would probably be atomic (as long as the > userland buffer is in memory). > > However, FreeBSD does perform locking in read(2) and write(2) for > local files, so your third program should never fail and exit. > > Note that the system call interface does not guarantee reads or writes > to be atomic, this just happens to be how it is implemented at the > moment. Afaik several Unix standards mandate this behavior, Linux doesn't follow this standard though. -- -Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org] "I have the heart of a child; I keep it in a jar on my desk." To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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