Re: [Qemu-devel] [PATCH v2 3/4] raw-posix: Fix try_seek_hole()'s handling of SEEK_DATA failure

[Eric Blake]

On 11/13/2014 08:47 AM, Eric Blake wrote:

> The two calls are both necessary, in order to learn which extant type
> offset belongs to, and to tell where that extant ends; and the behaviors
> are distinguishable (if both lseek() succeed, we have both numbers we
> want; if both fail with ENXIO, we know the offset is at or beyond EOF;
> and if only SEEK_HOLE fails with ENXIO, we know we have a trailing
              ^
I meant SEEK_DATA here.

> hole); and we can tell at runtime what to do about a trailing hole (if
> the return value is offset, we need one more lseek(fd, 0, SEEK_END) to
> find EOF; if the return value is larger than offset, we have EOF for
> free).  You can optimize by calling SEEK_HOLE first (if it fails with
> ENXIO, there is no need to try SEEK_DATA); but SEEK_HOLE in isolation is
> insufficient to give you all the information you need.
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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