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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: Sun, 22 Feb 2009 19:37:34 +0100

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.

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