Date: Wed, 16 Nov 2011 23:14:29 -0800 From: Tim Kientzle <tim@kientzle.com> To: Alexander Best <arundel@freebsd.org> Cc: freebsd-hackers@freebsd.org Subject: Re: easy way to determine if a stream or fd is seekable Message-ID: <D8DC6262-7C9C-4064-B2A6-AC29AC4DFC49@kientzle.com> In-Reply-To: <20111117002438.GA55931@freebsd.org> 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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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. >>>>=20 >>>> 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... >>>=20 >>> 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. >>=20 >> 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. >=20 > 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. Tim
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