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Re: bug: dd copies a random number of bytes
From: |
Robb Matzke |
Subject: |
Re: bug: dd copies a random number of bytes |
Date: |
Mon, 23 Feb 2009 13:34:31 -0500 |
Hi Jim,
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?
--Robb
On Sun, 22 Feb 2009 19:37:34 +0100
Jim Meyering <jim@meyering.net> wrote:
> Robb Matzke wrote:
> > I'm seeing some strange behavior with dd. The first two commands work and
> > report the expected number of records transferred (1024):
> >
> > dd if=/dev/zero bs=1MB count=1024 of=/dev/null
> > dd bs=1MB count=1024 </dev/zero >/dev/null
> >
> > But these report a random number of records, usually somewhere around 200,
> > but as few as 0 or as many as 1005:
> >
> > 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.
--
Robb Matzke L-159 | Ph: 812-967-6325 | I/O, I/O, it's off to disk we go,
Lawrence Livermore | Fax: 812-967-6326 | A petabyte to read or write,
National Laboratory | Hrs: 3am-11am PDT | I/O, I/O.