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
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