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Re: Openat without die

From: Bruno Haible
Subject: Re: Openat without die
Date: Tue, 11 Jan 2011 16:30:18 +0100
User-agent: KMail/1.9.9

Hi Jim,

> Before embarking on changes to (or duplication of) infrastructure like
> the *at functions, please tell us about your motivation.  Why do you care
> about whether openat may abort under unusual circumstances

The reason is so that libposix can offer openat() and getcwd() among the
functions that it offers.

Nothing in libposix should call chdir() without the guaranteed ability to
restore the previous working directory. And nothing in libposix should call
error(), since error reporting is the application's task.

See <http://lists.gnu.org/archive/html/bug-gnulib/2010-12/msg00178.html>
and <http://lists.gnu.org/archive/html/bug-gnulib/2010-12/msg00069.html>.

My proposal to solve the problem in December was naïve. I find it good that
Bastien thinks at a better one.

> As I tried to explain, there does not seem to be a clean way
> to solve the problem

I believe the stuff with chdir() is only needed in order to handle special
cases like
  - long directory and file names that would otherwise lead to ENAMETOOLONG,
  - access relative to a directory where 'cd $(pwd)' would not work, because
    of missing permissions in the parent directories.

Therefore I believe it should be possible to propose an openat() implementation
in libposix that, admittedly, cannot handle these special cases. But that would
be better than nothing. And coreutils, which does not want these restrictions,
would continue to use the current code for openat().

> the target systems that would benefit 
> from this portability improvement are mostly old and dying: not supporting
> POSIX-mandated *at functions is a good sign that they are not evolving.

Huh? Here's the portability state of openat():

$ ./show-portability openat
libc                aix-7.1.0
libc                cygwin-1.7
libcygwin           cygwin-1.7
libc                solaris-2.10
libc                solaris-2.11_2010_11
libc                solaris-2.9
MISSING in          aix-4.3.2 aix-5.1.0 aix-5.2.0 aix-5.3.0a aix-6.1.0 beos 
cygwin-1.5 freebsd-5.2.1 freebsd-6.0 freebsd-6.4 glibc-2.3.6 hpux-10.20 
hpux-11.00 hpux-11.11 hpux-11.23 hpux-11.31 interix-3.5 irix-5.3 irix-6.5 
macosx-10.3 macosx-10.4 macosx-10.5 mingw netbsd-3.0 netbsd-5.0.1 netbsd-5.0 
nsk-G06 openbsd-3.8 osf1-4.0d osf1-5.1a pips solaris-2.4 solaris-2.5.1 
solaris-2.6 solaris-2.7 solaris-2.8

Also, FreeBSD >= 8.0 has it. But OpenBSD and NetBSD do not have it. [1][2]

I wouldn't say that MacOS X, OpenBSD, and NetBSD are "not evolving".


[1] http://www.openbsd.org/cgi-bin/cvsweb/src/sys/sys/fcntl.h?rev=HEAD

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