Path: utzoo!mnetor!uunet!husc6!psuvax1!vu-vlsi!dsinc!syd
From: syd@dsinc.UUCP (Syd Weinstein)
Newsgroups: comp.unix.questions
Subject: Re: tar or cpio?
Message-ID: <325@dsinc.UUCP>
Date: 18 Feb 88 00:42:01 GMT
References: <246@mancol.UUCP> <1629@cuuxb.ATT.COM> <2506@mibte.UUCP> <41499@sun.uucp> <2071@bsu-cs.UUCP> <699@mcdsun.UUCP>
Reply-To: syd@dsinc.UUCP (Syd Weinstein)
Organization: Datacomp Systems, Inc., Huntingdon Valley, PA 19006
Lines: 40

In article <699@mcdsun.UUCP> fnf@mcdsun.UUCP (Fred Fish) writes:
:In article <2071@bsu-cs.UUCP> dhesi@bsu-cs.UUCP (Rahul Dhesi) writes:
:>An even more correct thing to do is for cpio to always write archive headers
:>in a canonical format that is not dependent on the byte-ordering of the
:>hardware.  E.g., all header data written least significant byte first.
:>
[ stuff deleted for brevity ]
:
:Unfortunately this will not work for one not-so-obvious reason, and that
:is because there are systems that when reading the exact same media,
:will return bytes ordered differently.
[again deleted]
:Several years ago after I got my first Unix system, I was so disgusted
:with tar and cpio that I wrote my own backup/archiver type program (bru)
:which has always handled this problem completely transparently to the
:user by writing ASCII formatted archives and doing whatever byte swapping
:was necessary.  The basic algorithm is:
:
:	1.	Examine block's magic number, if correct, no swapping.
:	2.	Swap all bytes and try again.  If correct, note byte swap
:		needed for all blocks.
:	3.	Swap all shorts and try again.  If correct, note short swap
:		also needed for all blocks.
:	4.	Swap all bytes and try again.  If correct, reset byte swapping
:		flag (swap shorts only).
:
:I have never encountered a machine where one of these four combinations of
:byte/short swapping didn't result in a readable archive, but I've seen
:each combination needed at least once for at least one machine.

I had to write a backup system also a while back, and I can add another
step to your algorithm that will force it to 8 steps.  There are some
machines out there that invert the bits on tapes.  IE, you need to
NOT(~) it to make it work.   Adding this makes one repeat all four steps
again for bit inverted tapes.  Boy was that a pain to figure out.
-- 
=====================================================================
Sydney S. Weinstein, CDP, CCP
Datacomp Systems, Inc.				Voice: (215) 947-9900
{allegra,bellcore,bpa,vu-vlsi}!dsinc!syd	FAX:   (215) 938-0235