maintains a cache of file descriptors to directories, so it uses more
file descriptors than before, but it adjusts to system limits on
the number of file descriptors. The new checks are implemented via
-the openat, dirfd, fdopendir, fstatat, and readlinkat calls
+the openat, fdopendir, fstatat, and readlinkat calls
standardized by POSIX.1-2008. On an older system where these calls do
not exist or do not return useful results, tar emulates the calls at
some cost in efficiency and reliability.