Re: [PATCH] Never cache offset when the stream handle is not active (BZ #16680)

[Rich Felker <dalias at aerifal dot cx>]

On Tue, Mar 11, 2014 at 08:24:26AM +0530, Siddhesh Poyarekar wrote:
> On Tuesday, 11 March 2014, Rich Felker <dalias@aerifal.cx> wrote:
> 
> > On Tue, Mar 11, 2014 at 06:09:35AM +0530, Siddhesh Poyarekar wrote:
> > > On Mon, Mar 10, 2014 at 02:34:59PM -0400, Rich Felker wrote:
> > > > Caching the offset is valid when writing in append mode. The one case
> > > > that's special for append mode is that, even once the handle is
> > > > active, you can't save the resulting offset at the time of seek/rewind
> > > > and use it for ftell during subsequent writing. This is because the
> > > > (first) write after seeking could change the current offset. One easy
> > > > way to deal with this is to treat seek/rewind as deactivating the
> > > > handle when the stream is in append mode, but more fine-grained
> > > > solutions are possible too.
> > >
> > > Treating seek/rewind as deactivating the handle won't help for append
> > > mode because we want to simulate the file position as being the end of
> > > file regardless of the file position when the handle is inactive, but
> >
> > Why do you want to do this? It's wrong. If the handle is inactive, the
> 
> only way you can get the position is by calling lseek(fd,0,SEEK_CUR).
> 
> This is because operations on another handle could have changed the
> > position without stdio's knowledge.
> >
> 
> Because that would give an inconsistent result. Initial position for append
> mode files is implementation defined and glibc has set it as end of file in
> the past. Changing that would be a breakage, wouldn't it?

Then fopen needs to physically seek to the end of the file with lseek
when append mode is requested. Faking the position is non-conforming
because it does not respect the rules for active handles.

Rich