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From: rcw@qetzal.UUCP (sysop)
Newsgroups: comp.unix.wizards
Subject: read() applied to mag tapes
Message-ID: <153@qetzal.UUCP>
Date: Wed, 1-Jul-87 01:37:28 EDT
Article-I.D.: qetzal.153
Posted: Wed Jul  1 01:37:28 1987
Date-Received: Sat, 4-Jul-87 07:43:29 EDT
Organization: Graphics Information, Inc., Denver, CO.
Lines: 25
Keywords: what is your experience

Hello Gurus,

I have been trying to write some routines to read from mag tape
using the read() system call, and have discovered an interesting
thing. I guess it makes sense, but I am not sure how to get
around it. If I have a tape where logical record size = 80 bytes, and
physical record size = 8000 bytes, and I do two discrete read() calls,
from /dev/rmt0, I get the first 80 bytes from each PHYSICAL record,
not 160 bytes in succession.  (Wicat 160 w/Uniplus V.2 + Cipher 
1600bpi drive)

I can understand how this might be a characteristic of the tape drive
hardware. It looked like setvbuf might provide a way around this
problem, but ours doesn't seem to work. I'd be interested in
hearing how others have solved this problem in general terms.
THe manual makes no mention of this sort of behavior in the
description for read(). Please send mail, and if this turns
out to be a general problem, I'll post a summary. If this turns
out to be a site specific problem, I'll flame the Unix vendor. Gracias.

-- 
Robert C. White, Jr. Graphics Information, Inc.   ****************
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