linux-kernel - Re: Man page doc for SEEK_DATA/SEEK

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Date:	Mon, 19 Sep 2011 12:44:07 -0600
From:	Eric Blake <eblake@...hat.com>
To:	Sunil Mushran <sunil.mushran@...cle.com>
CC:	mtk.manpages@...il.com, Josef Bacik <josef@...hat.com>,
	linux-man@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Man page doc for SEEK_DATA/SEEK_HOLE

On 09/19/2011 12:27 PM, Sunil Mushran wrote:
> On 09/19/2011 10:57 AM, Eric Blake wrote:
>> Also, it seems a shame that the kernel can fail with EINVAL instead of
>> properly emulating SEEK_HOLE and SEEK_DATA even on file systems with
>> no underlying support for reporting holes.
>>
>
> Why do you say that? If I am reading generic_file_llseek_unlocked()
> correctly, the default behavior is treat offset < i_size as data.

The proposed wording states:

>  .B EINVAL
>  .I whence
> -is not one of
> -.BR SEEK_SET ,
> -.BR SEEK_CUR ,
> -.BR SEEK_END ;
> -or the resulting file offset would be negative,
> +is not valid (this error may be returned if
> +.I whence
> +is
> +.BR SEEK_DATA
> +or
> +.BR SEEK_HOLE
> +and the underlying file system does not support the operation).

I guess it should instead read:

EINVAL whence is not valid (this error may be returned if whence is 
SEEK_DATA or SEEK_HOLE but the kernel does not support the operation).

Given your argument that new enough kernels understand SEEK_DATA and 
SEEK_HOLE for all file systems.

I agree that EINVAL will occur if you compile against new enough glibc 
that exposes the constants, but then run against an older kernel that 
does not yet understand them.  But I want the text to be clarified to be 
bullet-proof that if I am running against kernel 3.1 or newer, the only 
way I will ever get EINVAL for these two constants is if I do something 
else invalid, like a negative offset.

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