From: ilmari@ilmari.org (Dagfinn Ilmari =?utf-8?Q?Manns=C3=A5ker?=)
To: Magnus Hagander <magnus@hagander.net>
Cc: Victor Wagner <vitus@wagner.pp.ru>,
  PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Use pread and pwrite instead of lseek + write and read
References: <a86bd200-ebbe-d829-e3ca-0c4474b2fcb7@ohmu.fi>
	<20160817123437.44f3f60a@fafnir.local.vm>
	<CABUevEzZ=CGdmwSZwW9oNuf4pQZMExk33jcNO7rseqrAgKzj5Q@mail.gmail.com>
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Date: Wed, 17 Aug 2016 11:41:40 +0100
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 <CABUevEzZ=CGdmwSZwW9oNuf4pQZMExk33jcNO7rseqrAgKzj5Q@mail.gmail.com>
	(Magnus Hagander's message of "Wed, 17 Aug 2016 12:17:35 +0200")
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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.
>
> 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.

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.

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