Date: Sun, 13 Apr 2008 13:16:59 -1000 (HST) From: Jeff Roberson <jroberson@jroberson.net> To: David Schultz <das@FreeBSD.ORG> Cc: arch@FreeBSD.ORG Subject: Re: f_offset Message-ID: <20080413131422.V959@desktop> In-Reply-To: <20080413160829.GA42972@zim.MIT.EDU> References: <20080412132457.W43186@desktop> <20080413160829.GA42972@zim.MIT.EDU>
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On Sun, 13 Apr 2008, David Schultz wrote: > On Sat, Apr 12, 2008, Jeff Roberson wrote: >> It's worth discussing what posix actually guarantees for f_offset as well >> as what other operating systems do. POSIX actually does not guarantee any >> behavior with simultaneous access. Multiple readers may read the same >> position in the file concurrently and update the position to different >> offsets. Multiple writers may write to the same file location, although >> the io should be serialized by some other means. Posix allows for and >> Solaris, Linux, and historic implementations of f_offset work in the >> following way: > > This is not entirely true. In particular, files opened with > O_APPEND have stronger guarantees, and this behavior can be > useful. For example, I imagine that a database that opens its log > file with O_APPEND can depend on being able to write log entries > concurrently without losing any data. (There are also stronger > requirements for pipes, FIFOs, etc.) As alfred mentioned append is handled in a different way. I'm not suggesting we break posix semantics for append. Also, pipes and fifos don't have an f_offset and you can't call seek on them. > > As I recall, empiricial evidence shows that SunOS 5.10 and FreeBSD > both make stronger guarantees than Linux in the presence of > multiple concurrent writers. I haven't tested readers or looked > at the fdesc code for any of these. > Yes I slightly misspoke about solaris. They use the exclusive vnode lock to protect f_offset for writers. However, f_offset is fetched and set with a shared vnode lock for readers. These are the same semantics that I'm proposing. Thanks, Jeff
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