Date: Thu, 17 Nov 2011 21:55:35 +0100 (CET) From: Juergen Lock <nox@jelal.kn-bremen.de> To: tim@kientzle.com Cc: Alexander Best <arundel@freebsd.org>, freebsd-hackers@freebsd.org Subject: Re: easy way to determine if a stream or fd is seekable Message-ID: <201111172055.pAHKtZso061118@triton8.kn-bremen.de> In-Reply-To: <D8DC6262-7C9C-4064-B2A6-AC29AC4DFC49@kientzle.com> References: <20111115202450.GA73512@freebsd.org> <20111116102239.GA2687@britannica.bec.de> <20111116131428.GA40723@freebsd.org> <20111116232152.GC21793@britannica.bec.de> <20111117002438.GA55931@freebsd.org>
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In article <D8DC6262-7C9C-4064-B2A6-AC29AC4DFC49@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"... :)
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