Date: Tue, 22 Aug 2017 12:05:29 -0700 From: Conrad Meyer <cem@freebsd.org> To: "freebsd-fs@freebsd.org" <freebsd-fs@freebsd.org> Subject: Re: Can telldir() == 0 value be made special? Message-ID: <CAG6CVpWf33CxbVJLtr-2-NESdoiPUWQHfcL_qaYrCUR2kQFS4g@mail.gmail.com> In-Reply-To: <CAF6rxg=J7TYQQtG4%2B9TzEPPAb7LTReAm6U3A19Aq24PWrnK4vw@mail.gmail.com> References: <87bmn7kf3b.fsf@vostro.rath.org> <CAG6CVpVViqnbGuVn_JETBC5Q0NoYfC4VJK7iWjEfA7tFurQ2ag@mail.gmail.com> <87lgmbfob2.fsf@vostro.rath.org> <CAF6rxg=J7TYQQtG4%2B9TzEPPAb7LTReAm6U3A19Aq24PWrnK4vw@mail.gmail.com>
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One could also make it reserve the zero slot for the loc == 0 entry, but not create it dynamically. Just set loccnt=1 at initialization and then special-case dd_loc==0. Like this: --- a/lib/libc/gen/opendir.c +++ b/lib/libc/gen/opendir.c @@ -295,7 +295,7 @@ __opendir_common(int fd, int flags, bool use_current_pos) dirp->dd_lock = NULL; dirp->dd_td = (struct _telldir *)((char *)dirp + sizeof(DIR)); LIST_INIT(&dirp->dd_td->td_locq); - dirp->dd_td->td_loccnt = 0; + dirp->dd_td->td_loccnt = 1; dirp->dd_compat_de = NULL; /* --- a/lib/libc/gen/telldir.c +++ b/lib/libc/gen/telldir.c @@ -76,7 +76,10 @@ telldir(DIR *dirp) _pthread_mutex_unlock(&dirp->dd_lock); return (-1); } - lp->loc_index = dirp->dd_td->td_loccnt++; + if (dirp->dd_loc == 0) + lp->loc_index = 0; + else + lp->loc_index = dirp->dd_td->td_loccnt++; lp->loc_seek = dirp->dd_seek; lp->loc_loc = dirp->dd_loc; if (flp != NULL) Best, Conrad On Tue, Aug 22, 2017 at 11:35 AM, Eitan Adler <lists@eitanadler.com> wrote: > On 22 August 2017 at 14:30, Nikolaus Rath <Nikolaus@rath.org> wrote: >> 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 == 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? > > No, this code is called inside of telldir. I'd be against adding that > behavior by contract since it complicates the contract of seekdir. No > objection to this behavior by implementation though if it is simpler > than modifying telldir (doubtful). > > What this is doing is ensuring that there will always be a 0 entry > *after* telldir is called, and that seekdir(0) will return to the > start of the directory if called with an argument of 0. > > >> 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.
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