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From: ggs@ulysses.UUCP
Newsgroups: comp.bugs.4bsd
Subject: Re: tmscp tape drives give "hard error" on reading too small a record
Message-ID: <2412@ulysses.homer.nj.att.com>
Date: Fri, 17-Apr-87 21:27:55 EST
Article-I.D.: ulysses.2412
Posted: Fri Apr 17 21:27:55 1987
Date-Received: Sat, 18-Apr-87 08:53:22 EST
References: <1681@husc6.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 37
Keywords: tmscp, TU81, DEC
Summary: truncated reads ARE an error!

In article <1681@husc6.UUCP>, dudek@endor.UUCP writes:
> Index:	/usr/sys/vaxuba/tmscp.c 4.3BSD, Ultrix 1.2
> 
> Description:
> 	When reading a tape written with a blocksize larger than you are
> 	using to read, any tmscp tape drive (e.g., TU81, TK50) will die
> 	with a kernel printf "hard error: record data truncated" and an
> 	I/O error.  This should not be an error when using the raw device.

Please don't destroy what feeble progress has been made in improving UNIX
system tape drivers.  I agree with your objection to the printf, but
there should be some notification when data is lost.
...

> ! 			if (st != M_ST_TAPEM)
>   				{
>   				tprintf(tms->tms_ttyp,
>   				    "tms%d: hard error bn%d\n",

Yes, this is a nasty thing to do.  A simple arror return would have been
much more appropriate.  There should be an error code that signals "data
lost", but in lieu of that, ENOMEM is a crude substitute.

The problem with quietly returning the head of truncated blocks is that
people may do quite a bit of processing on the resulting trash before
they realize something is missing.  "But the C program won't compile"
or "tar will complain soon enough" you say?  Not in my building; we get
a lot of IBM tapes with simple data on them.  If you guess the wrong
block size but get the right logical record length, many logical
records are neatly snipped off and thrown into the bit bucket.  It may
take several days and a few thousand dollars before the mistake is
discovered.
-- 
Griff Smith	AT&T (Bell Laboratories), Murray Hill
Phone:		1-201-582-7736
UUCP:		{allegra|ihnp4}!ulysses!ggs
Internet:	ggs@ulysses.uucp