From owner-freebsd-hackers@FreeBSD.ORG  Sun Nov 20 17:51:00 2011
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Date: Sun, 20 Nov 2011 17:51:00 +0000
From: Alexander Best <arundel@freebsd.org>
To: Dieter BSD <dieterbsd@engineer.com>
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Cc: freebsd-hackers@freebsd.org
Subject: Re: easy way to determine if a stream or fd is seekable
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On Sun Nov 20 11, Dieter BSD wrote:
> > something like the following inside lseek() would take care of tape drives:
> >
> >        if (S_ISCHR(sb.st_mode) || S_ISBLK(sb.st_mode)) {
> >                if (ioctl(io->fd, FIODTYPE, &type) == -1)
> >                        err(1, "%s", io->name);
> >
> >                if (type & D_TAPE)
> >                        return(EBADF)
> >        }
> 
> I'd suggest ENODEV ("Operation not supported by device") rather than
> EBADF ("Bad file descriptor").
> 
> To do this correctly, we'd need some standard way to ask the
> device driver if the device can perform the seek or not.

this might be a bit of a problem, since those devices and drivers that don't
support seeking are mostly legacy tape drives. i don't think these drivers
are actively being maintained atm.

another issue are removable media. running the patched version of hexdump
(i posted a patch earlier today) shows that running fseeko() against /dev/cd0,
where no media is present, succeeds. with a media inserted however, /dev/cd0 is
very well capable of seeking.

cheers.
alex

>