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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 <address@hidden> 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.




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