From owner-freebsd-hackers@FreeBSD.ORG Wed Nov 16 10:14:23 2011 Return-Path: Delivered-To: freebsd-hackers@freebsd.org Received: by hub.freebsd.org (Postfix, from userid 1233) id 021A11065670; Wed, 16 Nov 2011 10:14:23 +0000 (UTC) Date: Wed, 16 Nov 2011 10:14:22 +0000 From: Alexander Best To: Brandon Gooch Message-ID: <20111116101422.GA20453@freebsd.org> References: <20111115202450.GA73512@freebsd.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Cc: freebsd-hackers@freebsd.org Subject: Re: easy way to determine if a stream or fd is seekable X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 16 Nov 2011 10:14:23 -0000 On Tue Nov 15 11, Brandon Gooch wrote: > On Nov 15, 2011 2:25 PM, "Alexander Best" wrote: > > > > hi there, > > > > 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. > > > > dd(1) e.g. identifies /dev/zero as non-seekable, even though it is. the > result: > > > > `dd if=/dev/zero bs=1m count=1 skip=100000`: > > > > on freebsd: > > 57,41 real 0,05 user 43,21 sys > > > > on openbsd: > > 0,88 real 0,00 user 0,07 sys > > > > on freebsd dd(1) is very easy fixable (1 line diff). the point however is: > > > > there doesn't seem to exist a unified way of checking seekable == TRUE. so > > every userspace application seems to do it differently and most of them > (dd(1) > > and hd(1) e.g) aren't doing it right. hd(1) e.g. believes that only > regular > > files are seekable. this means that hd(1) fired at /dev/ada* with a high > skip > > value takes ages! dd(1) is a bit smarter in this case, but still not > correct. > > > > idealy there would be something like stat.st_mode (see stat(2)) where one > > could simply do a S_ISSEEK(m). so far the best and easiest solution i've > seen > > is: > > > > fd = open(argv[1], O_RDONLY); > > if (fd == -1) > > exit(1); > > if (lseek(fd, 0, SEEK_CUR) != -1) > > printf ("%d is seekable.\n", fd); > > else > > printf ("%d is not seekable.\n", fd); > > > > seeing an application iterate through a stream or fd via getchar(), > although > > a simple seek() would work is very frustrating. i think what would be > needed > > is a simple function or macro that userspace applications can make use of. > > maybe such a thing already exists in linux, openbsd, netbsd, dragonflybsd, > > solaris or plan9? > > > > cheers. > > alex > > > > references: > > [1] > http://www.mavetju.org/mail/view_thread.php?list=freebsd-hackers&id=3290708&thread=yes > > [2] http://www.freebsd.org/cgi/query-pr.cgi?pr=152485 > > [3] http://www.freebsd.org/cgi/query-pr.cgi?pr=86485 > > So, according to APUE 2nd Edition, seek should be supported on anything > that's not a pipe, FIFO, or socket. In fact, the "test for seekability" > you've provided above closely resembles the example given in that text. if this really is the case, things could even be easier in a freebsd specific manor than the code above: !S_ISFIFO(m) && !S_ISSOCK(m) this means it would be trivial to write a new macro S_ISSEEK which would test stat.st_mode against the according bits. cheers. alex > > Need to think about this more before commenting further... > > -Brandon