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GNU tar implements an early draft of the POSIX 1003.1
ustar standard which is different from the final standard.
Adding support for the new changes in a backward-compatible fashion
is not trivial.
Certain old versions of tar cannot handle additional
information recorded by newer tar programs. To create an
archive in V7 format (not ANSI), which can be read by these old
versions, specify the `--old-archive' (`-o') option in
conjunction with the `--create' (`-c'). tar also
accepts `--portability' for this option. When you specify it,
tar leaves out information about directories, pipes, fifos,
contiguous files, and device files, and specifies file ownership by
group and user IDs instead of group and user names.
When updating an archive, do not use `--old-archive' (`-o') unless the archive was created with using this option.
In most cases, a new format archive can be read by an old
tar program without serious trouble, so this option should
seldom be needed. On the other hand, most modern tars are
able to read old format archives, so it might be safer for you to
always use `--old-archive' (`-o') for your distributions.
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