easy way to determine if a stream or fd is seekable

[Juergen Lock]

On Fri, Nov 18, 2011 at 12:00:07PM -0800, Tim Kientzle wrote:
> 
> On Nov 17, 2011, at 12:55 PM, Juergen Lock wrote:
> 
> >> 
> >> After a few experiments, bsdtar stopped using lseek() on
> >> FreeBSD for anything other than regular files and block
> >> devices.   I believe there are other things that do support
> >> seeking, but I don't believe there is an accurate mechanism
> >> for determining whether lseek() is correctly supported.
> > 
> > Ah is that the reason why my patch never made it into FreeBSD 9?
> > I'm talking about this thread, where I also commented on seeking
> > on tape:
> > 
> > 	http://docs.freebsd.org/cgi/mid.cgi?20100220101724.GA26604
> > 	(Re: "tar tfv /dev/cd0" speedup patch)
> > 
> > 	entire thread here:
> > 	http://markmail.org/message/nfznipqik3tuhbqp
> > 
> > Cheers,
> > 	Juergen (who would still like to see a faster "tar tfv /dev/cd0"... :)
> 
> I would like to see that as well.
> 
> Take a look at 
> 
> http://libarchive.googlecode.com/svn/trunk/libarchive/archive_read_open_filename.c
> 
> Especially the comments about detecting "disk-like" devices.
> I rewrote a bunch of this code to introduce an explicit
> notion of "strategy" so that we could optimize access
> to a variety of different devices.
> 
> This code has a notion of "disk-like" file descriptors and
> some optimizations for such.  There are some comments
> in there outlining similar optimizations that could be made
> for "tape-like" or "socket-like" devices.

Ah so it's `just' a slow release cycle?

% grep DIOCGMEDIASIZE /home/ncvs/src/lib/libarchive/archive_read_open_filename.c,v 
%

 When will we see this code in FreeBSD?  10.0?  9.1?  8.3? :)

 Curious...
	Juergen