David Kastrup wrote on 24 Jun 2005 23:01:50 +0200:
> I fail to see the advantage of using chown, or using fopen and
> fchown. In both cases the file name can be changed to refer to
> something else before the operation starts.
>
> The only situation where fchown offers any advantage is where you
> _already_ have a file open, like when you write the file after fopen,
> and then change its permissions.
Yes, that is true. But we are exactly in the situation were fchown is
useful.
> That is, the owner change must be accomplished in something like
> write-region, or it is pointless. As an isolated operation,
> fopen/fchown offers no advantage whatsoever.
Yes, fopen/fchown would be the same as calling chown. But here we have
just created a new file, and we want to change it's owner, so this
should be done with fchown.
The code is:
| Fcopy_file (file, newname,
| /* We have already prompted if it was an integer,
| so don't have copy-file prompt again. */
| NILP (ok_if_already_exists) ? Qnil : Qt,
| Qt, Qnil);
|
| /* Preserve owner and group, if possible (if we are root). */
| if (stat (SDATA (encoded_file), &data) >= 0)
| chown (SDATA (encoded_file), data.st_uid, data.st_gid);
|
| Fdelete_file (file);
So, we first open a file and write something in it inside Fcopy_file,
then we call chown on the same path. But the file that is at this path
could be something else that what the we created in Fcopy_file. We
should keep that file open, and call fchown on its filedescriptor,
instead of calling chown on it's path.
This will need some change in Fcopy_file, to be able to do something
with the filedescriptor before the file is closed; the most easy way
seems to make a subroutine that does the job of Fcopy_file and returns
the fd before closing it, and use this subroutine both in Fcopy_file and
here in Frename_file.
We also have the same kind of thing inside Fcopy_file (and this one is
also in stable emacs) where we call chmod on the file path just after
closing the file.
[I have replaced some part of the code with //... to make it shorter]
| #ifdef VMS
| // ...
| #else
| #ifdef MSDOS
| // ...
| #else /* not MSDOS */
| ofd = emacs_open (SDATA (encoded_newname),
| O_WRONLY | O_TRUNC | O_CREAT
| | (EQ (mustbenew, Qexcl) ? O_EXCL : 0),
| 0666);
| #endif /* not MSDOS */
| #endif /* VMS */
| if (ofd < 0)
| report_file_error ("Opening output file", Fcons (newname, Qnil));
|
| record_unwind_protect (close_file_unwind, make_number (ofd));
|
| immediate_quit = 1;
| QUIT;
| while ((n = emacs_read (ifd, buf, sizeof buf)) > 0)
| if (emacs_write (ofd, buf, n) != n)
| report_file_error ("I/O error", Fcons (newname, Qnil));
| immediate_quit = 0;
|
| /* Closing the output clobbers the file times on some systems. */
| if (emacs_close (ofd) < 0)
| report_file_error ("I/O error", Fcons (newname, Qnil));
|
| if (input_file_statable_p)
| {
| if (!NILP (keep_time))
| {
| // ...
| }
| #ifndef MSDOS
| chmod (SDATA (encoded_newname), st.st_mode & 07777);
| #else /* MSDOS */
| // ...
| #endif /* MSDOS */
| }
Here the change is very easy: we should just do fchmod(ofd,...) before
emacs_close(ofd).
--
Gaëtan LEURENT
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