Inconsistency between lseek(SEEK_HOLE) and lseek(SEEK_DATA)

[Maxim Sobolev]

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 at 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.
>
>