From owner-freebsd-hackers@FreeBSD.ORG  Thu Nov 17 21:48:05 2011
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Date: Thu, 17 Nov 2011 21:48:05 +0000
From: Alexander Best <arundel@freebsd.org>
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Subject: Re: easy way to determine if a stream or fd is seekable
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On Thu Nov 17 11, Dieter BSD wrote:
> > lseek() on a tape drive does not return an error, nor does it
> > actually do anything.
> 
> IIRC some tape drives can seek, while others cannot.
> Vague memories that it is supposed to be possible to put a
> filesystem on a DECtape and mount the filesystem.

or how about the following:

1) if the file argument we're seeking on is a tape drive, just do a regular
   seek operation.
2) afterwards use ftell() to verify that the seek REALLY happend. if it didn't,
   return -1 and set errno = EBADF.

cheers.
alex

> 
> It might be that FreeBSD doesn't currently support seeking
> on a tape, but we shouldn't paint ourselves into a corner
> by assuming that it is fundimentally impossible.