GNU bug report logs - #7362
dd strangeness

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Message #10 received at 7362-done <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: Lucia Rotger <lucia <at> aircomp.aero>
Cc: 7362-done <at> debbugs.gnu.org
Subject: Re: bug#7362: dd strangeness
Date: Wed, 10 Nov 2010 15:29:41 +0000

On 10/11/10 10:22, Lucia Rotger wrote:
> I see this behavior in Solaris, Linux and BSD dd: if I send a big enough
> file they all read it short at the end of the stream.
> 
> This works as expected:
> 
> # cat /dev/zero | dd bs=512 count=293601280 | wc
> 
> I get the expected results, dd reads exactly 293601280 blocks and wc
> sees 150323855360 characters, 140 GB
> 
> Whereas substituting cat for zfs send doesn't:
> 
> # zfs send <backup> | dd bs=512 count=293601280 | wc

different write sizes to the pipe mean
in the later case, dd will get short reads.
IMHO dd is doing the wrong/most surprising thing here,
but it can't be changed for compatibility reasons.
You can get coreutils dd to do what you want with:

dd iflag=fullblock

cheers,
Pádraig.

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