Date: Tue, 22 Aug 2017 20:30:09 +0200 From: Nikolaus Rath <Nikolaus@rath.org> To: freebsd-fs@freebsd.org Subject: Re: Can telldir() == 0 value be made special? Message-ID: <87lgmbfob2.fsf@vostro.rath.org> In-Reply-To: <CAG6CVpVViqnbGuVn_JETBC5Q0NoYfC4VJK7iWjEfA7tFurQ2ag@mail.gmail.com> (Conrad Meyer's message of "Tue, 22 Aug 2017 07:42:36 -0700") References: <87bmn7kf3b.fsf@vostro.rath.org> <CAG6CVpVViqnbGuVn_JETBC5Q0NoYfC4VJK7iWjEfA7tFurQ2ag@mail.gmail.com>
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On Aug 22 2017, Conrad Meyer <cem@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 =3D=3D NULL) {
> + /* Create special zero telldir entry, similar to Linux */
> + if (dirp->dd_td->td_loccnt =3D=3D 0 && dirp->dd_loc !=3D =
0) {
> + lp =3D malloc(sizeof(struct ddloc));
> + if (lp =3D=3D NULL) {
> + if (__isthreaded)
> + _pthread_mutex_unlock(&dirp->dd_l=
ock);
> + return (-1);
> + }
> + lp->loc_index =3D dirp->dd_td->td_loccnt++;
> + lp->loc_seek =3D 0;
> + lp->loc_loc =3D 0;
> + LIST_INSERT_HEAD(&dirp->dd_td->td_locq, lp, loc_l=
qe);
> + }
> +
> lp =3D malloc(sizeof(struct ddloc));
> if (lp =3D=3D 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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