[PATCH 1/2] Move offset to end of file when fdopen is in "a" mode (#16532)

[Rich Felker]

On Tue, Feb 11, 2014 at 11:22:31AM +0100, Ondřej Bílka wrote:
> On Tue, Feb 11, 2014 at 03:35:58PM +0530, Siddhesh Poyarekar wrote:
> > On Tue, Feb 11, 2014 at 04:54:55AM -0500, Rich Felker wrote:
> > > Are you sure this fixes the issue? I think there are still cases where
> > > the wrong offset might be seen. For instance it's legal to call lseek
> > > on the fd after calling fdopen (the FILE is not the "active handle"
> > > until you read/write with it) and then ftell could still report the
> > > wrong result.
> > 
> > It shouldn't make a difference.  The only reason to seek to the end is
> > to record the offset of the file end into the FILE object.  If an
> > lseek moves it elsewhere, we don't care.  Data will be appended to the
> > end of the file in any case since we add an O_APPEND to the fd.
> >
> How does that handle a+ mode? A offset should be at start but you need
> to write at end.

Indeed, I can't even find any text that makes it conforming for an
implementation to seek to the end when opening in "a" mode, but maybe
I just didn't look hard enough.

Rich