Can telldir() == 0 value be made special?

[Nikolaus Rath]

On Aug 22 2017, Conrad Meyer <cem at freebsd.org> wrote:
> Hi Nikolaus,
>
> As you have surmised, DIR* seekpoints are created dynamically whenever
> requested by user's telldir() call:

Good to know, thanks!

> I believe we could special case zero without breaking ABI
> compatibility of correct programs.  Something like this:
>
> --- a/lib/libc/gen/telldir.c
> +++ b/lib/libc/gen/telldir.c
> @@ -70,6 +70,20 @@ telldir(DIR *dirp)
>                 }
>         }
>         if (lp == NULL) {
> +               /* Create special zero telldir entry, similar to Linux */
> +               if (dirp->dd_td->td_loccnt == 0 && dirp->dd_loc != 0) {
> +                       lp = malloc(sizeof(struct ddloc));
> +                       if (lp == NULL) {
> +                               if (__isthreaded)
> +                                       _pthread_mutex_unlock(&dirp->dd_lock);
> +                               return (-1);
> +                       }
> +                       lp->loc_index = dirp->dd_td->td_loccnt++;
> +                       lp->loc_seek = 0;
> +                       lp->loc_loc = 0;
> +                       LIST_INSERT_HEAD(&dirp->dd_td->td_locq, lp, loc_lqe);
> +               }
> +
>                 lp = malloc(sizeof(struct ddloc));
>                 if (lp == NULL) {
>                         if (__isthreaded)
>
> I don't know if there are any downsides to special-casing zero like
> this, other than additional code complexity.

This seems a little more complex than I would have expected. Is this
maybe turning zero into a valid argument for seekdir() even if telldir()
has never been called?

That would be even better, but the minimal change that I need is just to
ensure that telldir() either (a) never returns zero, or (b) only returns
zero if called at the beginning of the stream.


Best,
-Nikolaus

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