easy way to determine if a stream or fd is seekable

[Tim Kientzle]

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.

Cheers,

Tim