easy way to determine if a stream or fd is seekable

[Juergen Lock]

In article <D8DC6262-7C9C-4064-B2A6-AC29AC4DFC49 at kientzle.com> you write:
>On Nov 16, 2011, at 4:24 PM, Alexander Best wrote:
>
>> On Thu Nov 17 11, Joerg Sonnenberger wrote:
>>> On Wed, Nov 16, 2011 at 01:14:28PM +0000, Alexander Best wrote:
>>>> On Wed Nov 16 11, Joerg Sonnenberger wrote:
>>>>> On Tue, Nov 15, 2011 at 08:24:50PM +0000, Alexander Best wrote:
>>>>>> one of the things i'm missing is an easy way to determine, whether a stream or
>>>>>> fd is seekable. i checked the dd(1) and hd(1) sources and those tools are
>>>>>> performing so much stuff just to find out if this is the case, and they still
>>>>>> are doing a very poor job.
>>>>> 
>>>>> Isn't the primary issue that FreeBSD doesn't properly report errors for
>>>>> lseek(2)? I think you should start from that and not hack around the
>>>>> fallout...
>>>> 
>>>> what do you mean? lseek(2) returns -1, when the file descriptor is not
>>>> seekable. i fired lseek(2) at all sorts of file types (dir, sockets, ...)
>>>> and it always returned the correct result.
>>> 
>>> If that were the case, you wouldn't need your flag to detect seek
>>> support. But e.g. some devices silently ignore seek requests without
>>> reporting errors. At least that is what I remember from the last time
>>> this has brought up.
>> 
>> this is the first time i hear about problems with seek requests. would be nice
>> to see some examples cases. was this discussed on the mailinglists? or
>> submitted as a problem report?
>
>There was a version of bsdtar that made the mistake of assuming
>lseek() would return an error.
>
>lseek() on a tape drive does not return an error, nor does it
>actually do anything.
>
>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"... :)