GNU bug report logs - #7362
dd strangeness

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From: Pádraig Brady <P <at> draigBrady.com>
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: 7362 <at> debbugs.gnu.org
Subject: bug#7362: dd strangeness
Date: Wed, 02 Mar 2011 12:53:21 +0000

[Message part 1 (text/plain, inline)]

On 01/03/11 21:40, Pádraig Brady wrote:
> On 01/03/11 17:45, Paul Eggert wrote:
>> On 03/01/2011 03:27 AM, Pádraig Brady wrote:
>>
>>> So the standard way to accumulate short reads to a full write,
>>> is to specify separate ibs and obs (we'd probably want to prompt about
>>> setting obs too for efficiency)
>>
>> Yes, good point, the diagnostic should suggest ibs=N obs=N
>> (instead of just ibs=N).
>>
>> By the way, the relationship between fullblock and ibs=N obs=N is
>> a curious one, one that I don't fully understand.  If you have
>> ibs=N obs=N, why would you need fullblock?  This should probably
>> be documented (preferably by someone who understands it :-).
> 
> Well as I understand it, it's to do with 'count'.
> count refers to the number of input reads,
> both partial and full.
> 
> So the advice to use iflag=fullblock is probably safer,
> especially when a count (or skip) is specified.

Thinking about it more, we should at least split up the patch.
So for the oflag=direct case the attached just enables fullblock
(as using C_TWOBUFS would require more mem, CPU, and also messes
up if the user specified a count).

I'm not sure we should try to be more clever than this,
and accept that dd is a low level tool that can be
used in a myriad of ways.

cheers,
Pádraig.

[dd-fullblock.diff (text/x-patch, attachment)]

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