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From: dudek@endor.harvard.edu (Glen Dudek)
Newsgroups: comp.bugs.4bsd
Subject: Re: tmscp tape drives give "hard error" on reading too small a record
Message-ID: <1696@husc6.UUCP>
Date: Mon, 20-Apr-87 07:19:02 EST
Article-I.D.: husc6.1696
Posted: Mon Apr 20 07:19:02 1987
Date-Received: Tue, 21-Apr-87 00:33:35 EST
References: <1681@husc6.UUCP> <2412@ulysses.homer.nj.att.com>
Sender: news@husc6.UUCP
Reply-To: dudek@harvard.harvard.edu (Glen Dudek)
Organization: Aiken Computation Lab Harvard, Cambridge, MA
Lines: 26
Keywords: tmscp, TU81, DEC

I realize the dangers of silently truncating data, but UNIX semantics for raw
tape devices require this behaviour (at least, all such raw tape drivers I've
ever seen behave this way).  Until the interface is changed to allow more
out-of-band information to be passed along with the data (e.g., data truncated,
physical end-of-tape, etc.), vendors need to stick with existing semantics.
I found this problem when a remote software installation broke because the
installation assumed 'dd if=/dev/rmt4 of=/dev/null' would successfully
skip a tape file independent of blocksize.  Since the default blocksize for
dd is 512, and the tape blocksize was larger, the device reported a hard error.

How can we change the interface to such devices to allow reporting these kinds
of errors?  Do we need to change the system call interface to do this?  I
suppose ioctl can be used for the application above, but that would require
two system calls for every device write.  How about another int (or preferably,
a more complex structure which hides the data better and allows some
extensibility) analagous to errno, but which is valid after all system calls
and can be used to pass such information?  The data can be kept in the user
structure and copied to some well-known address in the processes' virtual
address space (blech, kludge, any better ideas?).

Is anyone working on increasing the information passed through the system
call interface?

Glen Dudek
Computer Facilities Manager
Aiken Computation Lab