Date: Fri, 18 Nov 2011 12:32:13 -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: <34A2877A-977A-4668-8E37-642C6062AAC1@kientzle.com> In-Reply-To: <20111117214805.GA96937@freebsd.org> References: <20111117175514.274040@gmx.com> <20111117214805.GA96937@freebsd.org>
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On Nov 17, 2011, at 1:48 PM, Alexander Best wrote: > On Thu Nov 17 11, Dieter BSD wrote: >>> lseek() on a tape drive does not return an error, nor does it >>> actually do anything. >>=20 >> IIRC some tape drives can seek, while others cannot. >> Vague memories that it is supposed to be possible to put a >> filesystem on a DECtape and mount the filesystem. >=20 > or how about the following: >=20 > 1) if the file argument we're seeking on is a tape drive, just do a = regular > seek operation. > 2) afterwards use ftell() to verify that the seek REALLY happend. if = it didn't, > return -1 and set errno =3D EBADF. ftell() can't tell whether the seek really happened or not. All it can do is ask the kernel, and the kernel doesn't know. Here is my current understanding: When you call lseek(), the kernel just stores the requested offset in the file descriptor. lseek() always succeeds because storing the requested offset in the file descriptor always succeeds. (Except that the kernel specially checks if the file descriptor is a pipe.) ftell() just obtains the data from the file descriptor. So it always succeeds and always returns the data from the previous seek request, regardless of whether that seek actually did anything. With the next read or write request, the device driver may inspect the offset information in the file descriptor. Or it can ignore it. Almost all tape device drivers ignore it. Filesystems uniformly support it. Disk drivers uniformly support it. Other drivers vary considerably. In short: No, there is no "easy way" to determine if an arbitrary stream or fd is seekable. If you try to create a function that determines this, be certain to include "Don't Know" as a possible return value. Tim
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