Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cmcl2!phri!marob!samperi
From: samperi@marob.masa.com (Dominick Samperi)
Newsgroups: comp.unix.wizards
Subject: Re: Reading past end-of-tape?
Keywords: tar, EOT
Message-ID: <722@marob.masa.com>
Date: 3 Jun 89 02:45:29 GMT
References: <8413@pyr.gatech.EDU>
Reply-To: samperi@marob.masa.com (Dominick Samperi)
Organization: ESCC  New York City
Lines: 29

In article <8413@pyr.gatech.EDU> david@pyr.gatech.edu (David Brown) writes:
>A short while ago, someone put some very important drawings
>onto a 1/4" tape using tar.  Subsequently, and end-of-tape
>marker was written at the beginning of the tape.  I think
>that trashed the first drawing. but the others should still
>be around, just not readily accessable (correct me if I'm
>wrong).  Is there a PD or standard UN*X utility that will 
>read the info that's past the end-of-tape marker?

No EOT marker is placed after the last file on streaming 1/4" mag tape.
Each file (or segment) is terminated with a file-mark, and there is an
end-of-media hole at the physical end of tape. The end of readable data
is located by reading file-marks until a "No Data Detected" error is
returned from the controller. Data can be appended after this state is
reached, but I do not think these drives/controllers can resync and start
reading blocks after the "No Data Detected" point. One reason for this is
the fact that these "intelligent" drives do error detection by keeping
track of the block sequence number, and they refuse to pass data to the
host if blocks are not read in the correct order. For this reason, all
software that is written for these drives MUST detect when the user
is attempting to overwrite existing data, and issue appropriate
warnings before continuing.


readable data on the tape by reading file-marks until a "No Data Detected"
-- 
Dominick Samperi -- ESCC
samperi@marob.masa.com
uunet!hombre!samperi