Date: Mon, 1 Feb 2016 12:14:30 -0800 From: Maxim Sobolev <sobomax@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: freebsd-fs@freebsd.org, Kirk McKusick <mckusick@mckusick.com> Subject: Re: Inconsistency between lseek(SEEK_HOLE) and lseek(SEEK_DATA) Message-ID: <CAH7qZfufYKYD%2BmY8d2wcx3k1kG9CTRXBqXabtJ2p-RKU7uCPfw@mail.gmail.com> In-Reply-To: <20160201194014.GQ91220@kib.kiev.ua> References: <CAH7qZfuZNZ%2BJDPC4D1sjXj2tFxZKBiYVyTp-Y3UUUoq9er%2BWYQ@mail.gmail.com> <20160201165648.GM91220@kib.kiev.ua> <CAH7qZfvcpBo%2BvDho4GeNYWh6N83sebUi-DSG9--T%2BnxQiLhJ1A@mail.gmail.com> <20160201182257.GN91220@kib.kiev.ua> <CAH7qZftsv_0ersqexJ0fTnSQexe4WvpMLnF6X9bj_wX6q9Ewfw@mail.gmail.com> <20160201194014.GQ91220@kib.kiev.ua>
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Yeah, I've noticed that text now. It looks a lot like the sentence has been copied around and some part of it had lost in transition. In any case here is a small manpage patch to make a "vurtual hole" more pronounced and also explain how it affects return value of the syscall. https://reviews.freebsd.org/D5162 On Mon, Feb 1, 2016 at 11:40 AM, Konstantin Belousov <kostikbel@gmail.com> wrote: > On Mon, Feb 01, 2016 at 11:22:18AM -0800, Maxim Sobolev wrote: > > Well, it's still seems to be quite obscure. At the very least, the > lseek(2) > > manual page needs to reflect that. Right now it says: > > > > ERRORS > > [...] > > [ENXIO] For SEEK_DATA, there are no more data regions > past > > the > > supplied offset. For SEEK_HOLE, there are no > more > > holes past the supplied offset. > > > > Which is not true, the SEEK_HOLE would return st_size when there are no > > more holes past the supplied offset, not ENXIO. It is also interesting > that > > somehow empty file is a special case as well. Both SEEK_HOLE and > SEEK_DATA > > return -1 on those. Anybody who programs to that document would probably > > get as confused as myself. > > > > However, having said that, our cousin Linux behaves the same - i.e. > returns > > EOF+1 on SEEK_HOLE and -1 on SEEK_DATA, and does the same for empty > files, > > so at least we are consistent with that. > > Actually, since you referred to the man page for lseek(2), which seems to > be copied from the Solaris man page: > ... > The existence of a hole at the end of every data region allows for easy > programming and implies that a virtual hole exists at the end of the > file. > ... > > And, the text you quoted, does not imply that the call must return ENXIO > at the EOF for hole. It only allows the call to do it, but other language > makes this unreasonable. > > Note that it is Solaris, not Linux, which implementation of the SEEK_HOLE > and SEEK_DATA is the arbitration sample for the behavior. We got it with > the ZFS import. Our UFS implementation, and whatever Linux does, are only > reimplementation without clean documentation, and were done by observing > ZFS behaviour. > >
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