I think it makes sense indeed.
On Thu, Jun 10, 2010 at 07:04:20AM +0200, Michael Kerrisk wrote:
> Hi Pierre,
>
> Thanks for following up on this. I still think the current man page
> text is correct. See below.
>
> On Wed, Jun 9, 2010 at 12:46 PM, Pierre Habouzit <madcoder@xxxxxxxxxx> wrote:
> > On Thu, Jun 03, 2010 at 09:18:22AM +0200, Michael Kerrisk wrote:
> >> Hi Pierre,
> >>
> >> On Wed, Jun 2, 2010 at 11:42 PM, Pierre Habouzit <madcoder@xxxxxxxxxx> wrote:
> >> > On Mon, May 24, 2010 at 07:41:49PM +0200, Michael Kerrisk wrote:
> >> >> Hello Pierre,
> >> >>
> >> >> On Tue, Apr 6, 2010 at 11:03 AM, Pierre Habouzit <madcoder@xxxxxxxxxx> wrote:
> >> >> > See below a bug reported against the glibc. Since the glibc maintainer
> >> >> > dodged that one, I assume the bug indeed is in the documentation of
> >> >> > ftw(3). My manpages are the 3.24-1 Debian package.
> >> >>
> >> >> Yes. The man page is clearly incorrect. Thanks for reporting this.
> >> >>
> >> >> > IMHO the patch is:
> >> >> >
> >> >> > -fpath is the pathname of the entry relative to dirpath.
> >> >> > +fpath is the pathname of the entry relative to the current working directory.
> >> >> >
> >> >> > POSIX is very vague about what "fpath" should be btw.
> >> >>
> >> >> (Agreed. It could be more precise.)
> >> >>
> >> >> I believe the correct text should be this:
> >> >>
> >> >> fpath is the pathname of the entry, and is
> >> >> expressed either as a pathname relative to the
> >> >> calling process's current working directory at the
> >> >> time of the call to ftw(), if dirpath was expressed
> >> >> as a relative pathname, or as an absolute pathname,
> >> >> if dirpath was expressed as an absolute pathname..
> >> >>
> >> >> I have updated the man page accordingly, but would welcome
> >> >> review/checking of this text.
> >> >
> >> > Afaict, it's not correct: ftw may perform chdir() calls, so the pathname
> >> > is relative to the current working directory at the time `fn` is called.
> >> >
> >> > I'd rather phrase it that way (minus probable english mistakes):
> >> >
> >> > fpath is the pathname of the entry, and is either a relative
> >> > pathname to the current working directory of the application when
> >> > `fn` is called, or as an absolute pathname.
> >>
> >> Thanks for taking a look at this. However, I *think* your analysis is
> >> wrong, and my proposed changes is right. But, still I'd like some
> >> further confirmation. Please take a look at the the program below, and
> >> the output produced when it runs.
> >
> > Yeah, that's because you're doing chdir()s during ftw, which is
> > undefined behaviour as documented in the manpage already IIRC.
>
> True. That is documented in the POSIX page, but not currently in
> man-pages. I've fixed that now. Thanks.
>
> > The point is, ftw() /may/ decide to do chdir() by itself sometimes, and
> > then the path is relative to the current working directory as set by
> > ftw().
>
> I'm not sure why you think it may decide to do this. If this was
> unpredictable, then it would create difficulties for the application,
> as far as I can tell, since it would take quite some effort to
> correctly interpret the pathname supplied to 'fn'. And POSIX seems
> fairly clear on the point (at least for nftw()):
>
> FTW_CHDIR
> If set, nftw() shall change the current working
> directory to each directory as it reports files in
> that directory. If clear, nftw() shall not change
> the current working directory.
>
> > I'm pretty sure it's what POSIX authorizes ftw() to perform chdirs.
>
> See above.
>
> > And when I look at ftw.c in the glibc, it's also pretty much what
> > happens in the case when you set FTW_CHDIR in the flags: ftw() forces a
> > chdir before the fn() call, and makes the path relative to this cwd.
>
> Yes to the first part, but as far as I can see, still no to the last
> part ("and makes the path relative to this cwd" ). See below.
>
> > It happens that the glibc doesn't seem to perform any kind of chdir() in
> > the other cases (IOW when FTW_CHDIR isn't set), but I'm pretty sure
> > POSIX allows ftw() to do so.
>
> I don't see anywhere that POSIX authorizes that, and it wouldn't seem
> sensible to do so. See above.
>
> Here's a variation on my earlier test program that uses nftw()
> instead. If "c" is provided in the second command line argument, it
> uses "FTW_CHDIR". Looking at the results, do you agree with my
> analysis?
>
> $ cat n.c
> #define _GNU_SOURCE
> #define _XOPEN_SOURCE 500
> #include <ftw.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
>
>
> static int
> displayFileInfo(const char *fpath, const struct stat *sb,
> int tflag, struct FTW *ftwbuf)
> {
> printf("%-3s %2d %7lld %-40s %d %s\n",
> (tflag == FTW_D) ? "d" : (tflag == FTW_DNR) ? "dnr" :
> (tflag == FTW_DP) ? "dp" : (tflag == FTW_F) ? "f" :
> (tflag == FTW_NS) ? "ns" : (tflag == FTW_SL) ? "sl" :
> (tflag == FTW_SLN) ? "sln" : "???",
> ftwbuf->level, (long long) sb->st_size,
> fpath, ftwbuf->base, fpath + ftwbuf->base);
> #ifdef DO_CHDIR
> /* Let's mess with the curent directory during the ftw() call,
> to see what value is passed to 'pathname' in successive calls
> to displayFileInfo() */
>
> chdir("..");
> #endif
> system("pwd");
> return 0; /* To tell nftw() to continue */
> }
>
> int
> main(int argc, char *argv[])
> {
> int flags = 0;
>
> if (argc > 2 && strchr(argv[2], 'd') != NULL)
> flags |= FTW_DEPTH;
> if (argc > 2 && strchr(argv[2], 'p') != NULL)
> flags |= FTW_PHYS;
> if (argc > 2 && strchr(argv[2], 'c') != NULL)
> flags |= FTW_CHDIR;
>
> if (nftw((argc < 2) ? "." : argv[1], displayFileInfo, 20, flags) == -1) {
> perror("nftw");
> exit(EXIT_FAILURE);
> }
>
> exit(EXIT_SUCCESS);
> }
>
> $ cc -o n n.c
> $ cd dir1
> /home/mtk/tlpi/dl/dir1
> $ find ../dir2
> .../dir2
> .../dir2/sub
> .../dir2/sub/d
> .../dir2/sub/b
> .../dir2/sub/c
> .../dir2/sub/a
> .../dir2/bbb
> $ ../n ../dir2 c
> d 0 4096 ../dir2 3 dir2
> /home/mtk/tlpi/dirs_links
> d 1 4096 ../dir2/sub 8 sub
> /home/mtk/tlpi/dirs_links/dir2
> f 2 0 ../dir2/sub/d 12 d
> /home/mtk/tlpi/dirs_links/dir2/sub
> f 2 0 ../dir2/sub/b 12 b
> /home/mtk/tlpi/dirs_links/dir2/sub
> f 2 0 ../dir2/sub/c 12 c
> /home/mtk/tlpi/dirs_links/dir2/sub
> f 2 0 ../dir2/sub/a 12 a
> /home/mtk/tlpi/dirs_links/dir2/sub
> f 1 38 ../dir2/bbb 8 bbb
> /home/mtk/tlpi/dirs_links/dir2
> $ ../n ../dir2
> d 0 4096 ../dir2 3 dir2
> /home/mtk/tlpi/dl/dir1
> d 1 4096 ../dir2/sub 8 sub
> /home/mtk/tlpi/dl/dir1
> f 2 0 ../dir2/sub/d 12 d
> /home/mtk/tlpi/dl/dir1
> f 2 0 ../dir2/sub/b 12 b
> /home/mtk/tlpi/dl/dir1
> f 2 0 ../dir2/sub/c 12 c
> /home/mtk/tlpi/dl/dir1
> f 2 0 ../dir2/sub/a 12 a
> /home/mtk/tlpi/dl/dir1
> f 1 38 ../dir2/bbb 8 bbb
> /home/mtk/tlpi/dl/dir1
>
> Thanks,
>
> Michael
--
·O· Pierre Habouzit
··O madcoder@xxxxxxxxxx
OOO http://www.madism.org
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