Re: [BUG] Generic syscalls -- chmod vs. fchmodat

On Tuesday 25 January 2011 18:45:15 Roland McGrath wrote:

quoted

 I know of no reason to
think that the current treatment of the empty string was ever intended at
the creation of the *at interfaces.

I always assumed that this was done so that the *at syscalls can replace
both the ones that take a file descriptor (e.g. fstat) and the ones that
take a pathname (e.g. stat), which is sensible for the non-AT_FDCWD case,
although not documented in the man pages.

I see your point.  That is, having the empty string relative to a file
descriptor work means it can replace f* calls on non-directories, whereas
the standard method of passing "." for descriptor-relative resolution can
only work on a file descriptor open on a directory.  Is that what you mean?

I don't think this was part of the original intent when the calls were
added, but I suppose it makes sense.

Treating the empty string special for AT_FDCWD is rather pointless, but
at least consistent.

I agree about the consistency point.  However, one could also call it
consistent if the empty string fails to resolve when operating on either a
directory file descriptor or AT_FDCWD but works on a non-directory file
descriptor.  POSIX does not mandate that *at calls fail with ENOTDIR when
passed a non-directory file descriptor (it's a "may fail" error, not a
"shall fail" error).  So that behavior would be consistent both with the
POSIX requirements as I read them, and with the desire you mentioned to let
the fblahat system call serve to implement fblah as well as blah.  Then
libc would not have to wrap the *at calls with any special check to conform
to POSIX.


Thanks,
Roland

On Tuesday 25 January 2011 19:34:37 Roland McGrath wrote:

I don't think this was part of the original intent when the calls were
added, but I suppose it makes sense.

More importantly, even if it was never meant this way, anyone could have
assumed that it was and started using the system call in this way.

quoted

Treating the empty string special for AT_FDCWD is rather pointless, but
at least consistent.

I agree about the consistency point.  However, one could also call it
consistent if the empty string fails to resolve when operating on either a
directory file descriptor or AT_FDCWD but works on a non-directory file
descriptor. 

Yes.

POSIX does not mandate that *at calls fail with ENOTDIR when
passed a non-directory file descriptor (it's a "may fail" error, not a
"shall fail" error).  So that behavior would be consistent both with the
POSIX requirements as I read them, and with the desire you mentioned to let
the fblahat system call serve to implement fblah as well as blah.  Then
libc would not have to wrap the *at calls with any special check to conform
to POSIX.

Makes sense.

	Arnd