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

[Siddhesh Poyarekar <siddhesh at redhat dot com>]

On Mon, Mar 10, 2014 at 11:37:13AM -0400, Rich Felker wrote:
> On Mon, Mar 10, 2014 at 05:53:06PM +0530, Siddhesh Poyarekar wrote:
> > Hi,
> > 
> > The ftell implementation was made conservative to ensure that
> > incorrectly cached offsets never affect it.  However, this causes
> > problems for append mode when a file stream is rewound.
> > 
> > For a and a+ mode files in read mode, rewinding the stream should
> > result in ftell returning 0 as the offset, but without caching, it
> > just assumes that the file offset is the end of file (as opposed to
> > SEEK_CUR, since rewind correctly sets it).  Now I couldn't find
> > anything in POSIX that specifies the stream position after rewind()
> > for a file opened in 'a' mode, but for 'a+' mode it should be set to
> > 0.  For 'a' mode too, it probably makes sense to keep it set to 0 in
> > the interest of retaining old behavior.
> > 
> > The best way to fix this would be to avoid caching the offset before
> > the file handle is active.  With this change, the only time the offset
> > cache is not reliable is when the file is writing in any of the append
> > modes.
> 
> I'm confused. I thought we discussed before that caching the offset is
> _always_ invalid when the file handle is not active. This is not
> specific to append mode.

Sorry, I should have been specific - the only time the offset cache is
not reliable in an _active_ file handle is when the file is writing in
any of the append modes.  The cache is always invalid when the handle
is not active, which is why no function should cache the offset till
the handle is activated.  Does that sound correct now?

Thanks,
Siddhesh

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