Re: RFC: should lseek(SEEK_DATA/SEEK_HOLE) return ENOTTY?

On Sun, 2019-08-11 at 09:12 -0600, Alan Somers wrote:
> On Sun, Aug 11, 2019 at 8:57 AM Ian Lepore <ian_at_freebsd.org> wrote:
> > 
> > On Sun, 2019-08-11 at 09:04 +0200, Gary Jennejohn wrote:
> > > On Sun, 11 Aug 2019 02:03:10 +0000
> > > Rick Macklem <rmacklem_at_uoguelph.ca> wrote:
> > > 
> > > > Hi,
> > > > 
> > > > I've noticed that, if you do a lseek(SEEK_DATA/SEEK_HOLE) on a
> > > > file
> > > > that
> > > > resides in a file system that does not support holes, ENOTTY is
> > > > returned.
> > > > 
> > > > This error isn't listed for lseek() and seems a liitle weird.
> > > > 
> > > 
> > > ENOTTY is the standard error return for an unimplemented
> > > ioctl(2),
> > > and SEEK_HOLE ultimately becomes a call to fo_ioctl().
> > > 
> > > > I can see a couple of alternatives to this:
> > > > 1 - Return a different error. Maybe ENXIO?
> > > > or
> > > > 2 - Have lseek() do the trivial implementation when the
> > > > VOP_IOCTL()
> > > > fails.
> > > >    - For SEEK_DATA, just return the offset given as argument
> > > > and
> > > > for SEEK_HOLE
> > > >       return the file's size as the offset.
> > > > 
> > > > What do others think? rick
> > > > ps: The man page should be updated, whatever is done w.r.t.
> > > > this.
> > > > 
> > > 
> > > I also vote for option 2
> > > 
> > 
> > If SEEK_DATA and SEEK_HOLE don't return the standard "ioctl not
> > supported" error code and return a fake result, how are you
> > supposed to
> > determine at runtime whether SEEK_HOLE is supported or not?
> > 
> > -- Ian
> 
> pathconf(2) will tell you.
> 

Ahh, I wasn't aware of that.

For option 2, lseek() has to not just return the info, but must also
actually set the file position accordingly, and has to treat offset >=
filesize as an error.

-- Ian