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Re: Use pread and pwrite instead of lseek + write and read - Mailing list pgsql-hackers

From Magnus Hagander
Subject Re: Use pread and pwrite instead of lseek + write and read
Date
Msg-id CABUevEzF1eL02hkFbqhGmgguaq9PEKS49rWJwTU9kjHA0Hh6wA@mail.gmail.com
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In response to Re: Use pread and pwrite instead of lseek + write and read  (ilmari@ilmari.org (Dagfinn Ilmari Mannsåker))
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On Wed, Aug 17, 2016 at 12:41 PM, Dagfinn Ilmari Mannsåker <ilmari@ilmari.org> wrote:
Magnus Hagander <magnus@hagander.net> writes:

[pread/pwrite]

> Yeah, Windows does not have those API calls, but it shouldn't be rocket
> science to write a wrapper for it. The standard windows APIs can do the
> same thing -- but they'll need access to the HANDLE for the file and not
> the posix file descriptor.

Just to be clear, I'm referring to the standard ReadFile() and WriteFile() APIs here.

 
>
> It also has things like ReadFileScatter() (
> https://msdn.microsoft.com/en-us/library/windows/desktop/aa365469(v=vs.85).aspx)
> which is not the same, but might also be interesting as a future
> improvement.

That looks a lot like POSIX readv()
(http://pubs.opengroup.org/onlinepubs/9699919799/functions/readv.html),
and as far as I can tell it has the same issue as it in that it doesn't
take an offset argument, but requires you to seek first.

Ah yeah, for some reason I keep getting readv() and pread(). Which solve a different problem (see below about that function not having the same issues on windows -- but it's still not the problem we're trying to solve here)

 
Linux and modern BSDs however have preadv()
(http://manpages.ubuntu.com/manpages/xenial/en/man2/preadv.2.html),
which takes an offset and an iovec array.  I don't know if Windows and
other platforms have anything similar.

ReadFileScatter() can take the offset from OVERLAPPED (same as regular ReadFile) and an array of FILE_SEGMENT_ELEMENT, same as preadv(). But the APIs start looking more different the further down the rabbithole you go, I think. But the capability is definitely there (and has been for ages so is in all supported version).
 
--
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/

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