Date: Sun, 13 Apr 2008 12:08:29 -0400 From: David Schultz <das@FreeBSD.ORG> To: Jeff Roberson <jroberson@jroberson.net> Cc: arch@FreeBSD.ORG Subject: Re: f_offset Message-ID: <20080413160829.GA42972@zim.MIT.EDU> In-Reply-To: <20080412132457.W43186@desktop> References: <20080412132457.W43186@desktop>
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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.
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