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Date: Wed, 10 Dec 2008 18:29:50 +0100
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] Replace posix-aio with custom thread pool
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Andrea Arcangeli <aarcange@redhat.com>, qemu-devel@nongnu.org, kvm-devel <kvm@vger.kernel.org>

>> To solve this in userland without kernel aio we'd need to open (not
>> just dup)
> 
> Why not just dup?  I've implemented this and it seems to work.

unix keeps the file pointer in the (global) file table.  The
(per-process) file descriptor table references the file table.

opening twice gives you two file descriptor table entries referencing
two file table entries.  duping gives you two file descriptors
referencing the *same* file table entry.  Thus the two fds share the
file pointer.

HTH,
  Gerd