Date: Sun, 13 Apr 2008 09:39:29 -0700 From: Alfred Perlstein <alfred@freebsd.org> To: Jeff Roberson <jroberson@jroberson.net>, arch@FreeBSD.ORG Subject: Re: f_offset Message-ID: <20080413163929.GE95731@elvis.mu.org> In-Reply-To: <20080413160829.GA42972@zim.MIT.EDU> References: <20080412132457.W43186@desktop> <20080413160829.GA42972@zim.MIT.EDU>
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* David Schultz <das@FreeBSD.ORG> [080413 09:05] 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 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. O_APPEND is kept inside of f_flags and passed down into the VOP layer so that the filesystem can "do the right thing", basically always append and get rid of the f_offset problem. Sort of. -- - Alfred Perlstein
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