On 12/23/2010 05:08 AM, Bruno Haible wrote: > Bastien ROUCARIES wrote: >> If chdir is a problem, why not using a heavy weight approach (better safe >> than sorry particularly if gnulib is used for a lot of project): >> - fork and exec an helper for name resolution >> - pass fd using a socket >> >> It is really heavy but it is on the safe side > > I like your idea. It would be a safe way to implement openat(). > > How does "pass fd using a socket" actually work? There are apparently two > ways to do it [1]: > - On Unix domain sockets, using sendmsg SCM_RIGHTS [2][3]. > Likely portable to glibc, MacOS X, FreeBSD, NetBSD, OpenBSD, AIX, HP-UX, > IRIX, OSF/1, Solaris, Cygwin, Haiku. But not to Interix, mingw, BeOS.
Unfortunately, it is not possible to pass fds on cygwin; it is not possible to implement SCM_RIGHTS on top of what Windows provides. Then again, cygwin's *at functions don't need replacing, so it's a moot point. The whole exercise here is how to cater to systems that don't support native *at correctly. > And on mingw, REPLACE_FCHDIR is 1, which means we assume that we use the > emulated fchdir, which associates a file name to every file descriptor > that maps to a directory. In this case, we could implement openat through > filename concatenation. Indeed; furthermore, mingw has the (in this case, nice) property that absolute pathnames cannot be larger than about 256 bytes, so there's not as much risk of the O(n^2) behavior that filename concatenation would have on other systems with unlimited directory depth. -- Eric Blake ebl...@redhat.com +1-801-349-2682 Libvirt virtualization library http://libvirt.org
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