Date: Thu, 8 Jun 2017 17:15:15 +0800 From: Christopher Hall <christopherhall.hsw@gmail.com> To: Dewayne Geraghty <dewaynegeraghty@gmail.com> Cc: freebsd-stable stable <freebsd-stable@freebsd.org> Subject: Re: dd is returning different content using same inputs Message-ID: <20170608171515.72fa7844@arria.bitmark.lan> In-Reply-To: <CAGnMC6ryoG7tDLthS8=6AC3vtXifaocnrBDxOO1kFjyToVPqgA@mail.gmail.com> References: <CAGnMC6ryoG7tDLthS8=6AC3vtXifaocnrBDxOO1kFjyToVPqgA@mail.gmail.com>
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Hello Dewayne, On Thu, 8 Jun 2017 18:10:11 +1000, Dewayne Geraghty <dewaynegeraghty@gmail.com> wrote: > I'm attempting to skip the first three bytes of a file and then pull > the header out of message. Please note the file size varies between > 105 and 54 bytes, even though the input file and everything else > remains constant, on > > FreeBSD hathor 11.1-PRERELEASE FreeBSD 11.1-PRERELEASE #0 r319562M: > Mon Jun 5 03:13:08 AEST 2017 > root@hathor:/110007/D/K8/hqdev-amd64-smp-vga amd64 1100514 1100514 > > # rm encheader;dd if=/tmp/tLbdoJ/encmessage bs=1 skip=3 | dd bs=384 > count=1 of=encheader ; ls -lrth encheader > 0+1 records in > 0+1 records out > 105 bytes transferred in 0.000026 secs (4071661 bytes/sec) > -rw-r--r-- 1 root wheel 105B Jun 8 16:35 encheader > # rm encheader;dd if=/tmp/tLbdoJ/encmessage bs=1 skip=3 | dd bs=384 > count=1 of=encheader ; ls -lrth encheader > 0+1 records in > 0+1 records out > 67 bytes transferred in 0.000026 secs (2554912 bytes/sec) > -rw-r--r-- 1 root wheel 67B Jun 8 16:35 encheader > # rm encheader;dd if=/tmp/tLbdoJ/encmessage bs=1 skip=3 | dd bs=384 > count=1 of=encheader ; ls -lrth encheader > 0+1 records in > 0+1 records out > 68 bytes transferred in 0.000024 secs (2867263 bytes/sec) > -rw-r--r-- 1 root wheel 68B Jun 8 16:35 encheader > # rm encheader;dd if=/tmp/tLbdoJ/encmessage bs=1 skip=3 | dd bs=384 > count=1 of=encheader ; ls -lrth encheader > 0+1 records in > 0+1 records out > 54 bytes transferred in 0.000025 secs (2167717 bytes/sec) > -rw-r--r-- 1 root wheel 54B Jun 8 16:35 encheader > # rm encheader;dd if=/tmp/tLbdoJ/encmessage bs=1 skip=3 | dd bs=384 > count=1 of=encheader ; ls -lrth encheader > 0+1 records in > 0+1 records out > 91 bytes transferred in 0.000024 secs (3751494 bytes/sec) > -rw-r--r-- 1 root wheel 91B Jun 8 16:35 encheader > > I used this around May 8th (from a OS and ports rebuild on 4th May) > and was performing some final testing on an 11.1 Prerelease built on > Jun 5. > It works similarly on TrueOS (FreeBSD 12) I suspect it is because you block sizes are different on the input and output dd commands. I would think that the output block size (obs) on 1st dd needs to be specified the same input block size on receiving dd to make it work correctly. > This works correctly so perhaps I'm not using dd correctly? > dd if=/tmp/tLbdoJ/encmessage skip=1 bs=3 | dd bs=1 count=384 > of=encheader ; hd encheader|head -n1; ls -l encheader > _______________________________________________ > freebsd-stable@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-stable > To unsubscribe, send any mail to > "freebsd-stable-unsubscribe@freebsd.org" -- Best Regards. Christopher Hall.
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