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Date: Sun, 13 Apr 2008 12:08:29 -0400
From: David Schultz <das@FreeBSD.ORG>
To: Jeff Roberson <jroberson@jroberson.net>
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Cc: arch@FreeBSD.ORG
Subject: Re: f_offset
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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.