On Sat, Aug 27, 2022 at 6:05 AM Jeff King [off-list ref] wrote:

Yeah, I saw that. It's a bit vague, and if the call returns ENOSYS or
EISDIR, that would be perfectly fine. It's the "what happens on the
implementations that do support it..." part that I'm more worried about. :)

The history here is that pre-4.2BSD, Unix systems had no mkdir
system call. You used mknod() to make a truly empty directory and
the link() to create the "." and ".." entries within it, and all three of
these operations were restricted to the super-user.  There was no
rmdir either, so again, unlink() as the super-user was permitted to
do the job (with three calls to unlink the "." and ".." entries first and
then remove the directory).

Unlinking a directory when it still contains "." leaves the link count
at 1 and there's no GC, so it sits around occupying an inode.

If you have a mkdir() system call and don't need backwards
compatibility, you get to have these return EISDIR errors...

Chris