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From: jeff@voder.UUCP (Jeff Gilliam)
Newsgroups: comp.bugs.4bsd
Subject: Re: tmscp tape drives give "hard error" on reading too small a record
Message-ID: <2901@voder.UUCP>
Date: Tue, 21-Apr-87 00:50:00 EST
Article-I.D.: voder.2901
Posted: Tue Apr 21 00:50:00 1987
Date-Received: Wed, 22-Apr-87 01:38:21 EST
References: <1681@husc6.UUCP> <2412@ulysses.homer.nj.att.com> <1696@husc6.UUCP> <2426@ulysses.homer.nj.att.com>
Followup-To: comp.unix.wizards
Organization: National Semiconductor, Santa Clara
Lines: 34
Keywords: tmscp, TU81, DEC

Some facts for consideration:

1) Is is *extremely* dangerous to silently discard data.  That's why tape
   controllers have a special error bit to indicate that the record length
   was larger than number of bytes read.

2) All the 4.3 BSD tape drivers silently ignore a "record too long" error
   if the raw device is being accessed.  They even have a comment to the
   effect that this behavior is deliberate, though it doesn't explain why.

3) The man pages (at least since 7th Edition) indicate that a long record
   returns an error indication.

Conclusions?  This situation ought to return an error.  A unique error
code would be ideal (I kinda like ERLE: Record Length Error) to enable
user code to distinguish between a problem reading the tape and a
simple record length error.  This error should *not* provoke a printf
to the system console; it's not indicative of either a hardware or
kernel software problem.

If somebody really feels it necessary (examples, please) an ioctl()
could be added to cause the drive to ignore rle's (analagous to 4.3's
ioctl() to ignore EOT).

The end result is that "naive" code won't lose data without receiving
some indication.  (I'm glossing over "stupid" code that ignores the
return value from read().  But then, such code is likely to have worse
problems than this ... like ignoring EOF.)  And, "sophisticated" code
can control how it deals with record length errors.  Sounds like the
best of all possible worlds.

-- 

Jeff Gilliam	{ucbvax,pyramid,nsc}!voder!jeff