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Re: bug: dd copies a random number of bytes

From: Jim Meyering
Subject: Re: bug: dd copies a random number of bytes
Date: Mon, 23 Feb 2009 19:46:39 +0100

Robb Matzke wrote:
> I'm actually not concerned with the SIGPIPE and expect it. What concerns me
> is that the final counts reported by *dd* are wrong. It should report 1024
> records copied, but instead reports a seemingly random and variable amount.
> Why is dd closing its stdin earlier than I asked it to? Why is dd transfering
> a random amount of data?

You chose a block size of 1MB.
That is way larger than will ever be filled via a pipe,
so every read gets a partial "block".
dd stops after 1024 of those.

The output you see,

    0+1024 records in
    0+1024 records out
    4259840 bytes (4.3 MB) copied, 0.00729445 s, 584 MB/s

tells you that there were 0 full blocks read and 1024 partials.

>> >    cat /dev/zero |dd bs=1MB count=1024 of=/dev/null
>> >    cat /dev/null |dd bs=1MB count=1024 >/dev/null
>> >
>> > An strace on dd shows that the last read/write pair both succeed and then 
>> > dd
>> > inexplicably closes both files.  None of the reads return zero or failure.
>> > An strace on cat shows that its last write to stdout gets a sigpipe, so it
>> > really is trying to send the data to dd and it's dd that's closing the 
>> > pipe.
>> That's expected and in fact required.
>> You're telling dd to exit after reading 1024*1MB.
>> Once dd exits, it closes its side of the pipe, but cat is still writing
>> to the other end.  Writing to a closed pipe provokes a SIGPIPE, by default.
>> How much cat actually writes before it's killed by the
>> SIGPIPE depends on kernel buffering, hence the variability.
>> > Replacing /dev/null with a real file exhibits the behavior one would 
>> > expect.
>> > Substituting some other I/O-counting command for dd works fine.

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