linux-kernel - Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK

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Date:	Mon, 25 Apr 2011 06:37:37 -0600
From:	Eric Blake <eblake@...hat.com>
To:	Jamie Lokier <jamie@...reable.org>
CC:	Sunil Mushran <sunil.mushran@...cle.com>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Christoph Hellwig <hch@...radead.org>,
	Josef Bacik <josef@...icpanda.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK_DATA flags

On 04/24/2011 11:49 AM, Jamie Lokier wrote:
>> My problem with FIND_* is that we are messing with the well understood
>> semantics of lseek().
> 
> fcntl() looks a better fit for FIND_HOLE/DATA anyway.

With fcntl(), it would have to be something like:

off_t offset = start;
if (fcntl (fd, F_FIND_HOLE, &offset) != 0)
    ; // find failed
// offset is now set to the next location after start

That is, offset has to be passed as an input _and_ output parameter,
since we can't use the int return value of fcntl to convey off_t
information, and since the optional third argument of fcntl has
traditionally been no wider than a pointer which is not the case for
off_t on 32-bit platforms.

-- 
Eric Blake   eblake@...hat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org


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