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From: gwyn@brl-smoke.ARPA (Doug Gwyn )
Newsgroups: comp.lang.c
Subject: Re: Yet another fseek() question
Message-ID: <6442@brl-smoke.ARPA>
Date: Sun, 20-Sep-87 20:47:49 EDT
Article-I.D.: brl-smok.6442
Posted: Sun Sep 20 20:47:49 1987
Date-Received: Mon, 21-Sep-87 03:39:42 EDT
References: <165600012@uiucdcsb>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 32

In article <165600012@uiucdcsb> kenny@uiucdcsb.cs.uiuc.edu writes:
>Fseek() as described for TEXT files can be made to work even on
>magnetic tapes with comparatively little effort, except for one minor
>problem:  tapes cannot reliably read data beyind the last data
>written.
>Yet I don't see anything in the standard forbidding forward read after
>write.  Did the implementors not consider tape I/O, figure that an
>implementation can just disallow "r+" and "w+" modes for tapes,
>consider tapes to be non-seekable files (but they CAN forward-space,
>backspace, and rewind), or have I overlooked something?

The following is NOT an official X3J11 response:

Magtapes have always been problematic for simple file models.
(Consider that some interfaces do not support writing records
containing an odd number of bytes, and that one cannot in
general write on a magtape file unless there are no files
following it.)  If an implementation cannot support the "+"
modes in a reasonable manner (block-replaceable magtape a la
DECtape), then it really ought to return an error indication
when an attempt is made to open a magtape device in such a
mode.  Alternatively, it could defer the error indication
until the I/O operation that won't work right is attempted,
although this is much less desirable.  I know that it is hard
to construct such an implementation on some operating systems
(including many UNIX variants), but there really isn't much
that can be done about this at the C standardization level.

Not all magtape implementations allow convenient access to
positioning operations.  In particular, some older UNIX
magtape drivers did not support such operations (other than
rewind, and occasionally file-skip on close).