Re: [PATCH] Don't do unneccesary memory copies in dd.

[Pádraig Brady]

Pádraig Brady wrote:
> Paul Eggert wrote:
>> $ (echo 'x'; sleep 10; echo y) | dd ibs=3 obs=3
>>
>> POSIX says that in this case the output data must be reblocked into
>> blocks of 3 bytes.  With the working 'dd', you'll see a pause of 10
>> seconds (because the first 'echo' outputs only 2 bytes), then the x and
>> y right away (because we now have all 4 input bytes, and can issue a
>> write of 3 bytes and a write of 1 byte).  With a buggy 'dd', you'll see
>> the 'x' right away (because we output a 2-byte block), then a pause of
>> 10 seconds, then a 'y' (the other 2-byte block).
>>
>> The simplest fix I see is to revert the patch.  Of course one could up
>> with something fancier....
>>
> 
> Thanks for that Paul.
> That certainly isn't obvious, and deserves a comment in the code.
> 
> A question so on the default behaviour of dd.
> Should it behave like bs=512 or ibs=512 obs=512,
> i.e. should it memcpy/reblock by default?

So I read the actual POSIX spec and it's quite clear
that a single buffer should only be used if bs= is specified.
Note it also states that bs= supersedes [io]bs=
I.E. if you specify [io]bs= and bs= then the code with
revert applied will erroneously use 2 buffers?

Not a major issue I know, but this little bit
of logic has been very confusing for me.
I've replied to another email in this thread with
a cleanup patch which hopefully is less confusing
and addresses the issue above.

cheers,
Pádraig.