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.
