f_offset
David Schultz
das at FreeBSD.ORG
Sun Apr 13 16:05:20 UTC 2008
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.
More information about the freebsd-arch
mailing list