On Sat Jun 3 20:13:31 2023 +0000, Chip Davis wrote:
Turns out, 4.2BSD [*did*, in fact, implement this behavior](https://minnie.tuhs.org/cgi-bin/utree.pl?file=4.2BSD/usr/src/sys/sys/ufs_sys...). Its [man page](https://minnie.tuhs.org/cgi-bin/utree.pl?file=4.2BSD/usr/man/man2/open.2) also lists `EOPNOTSUPP` as a possible error, with the same parenthetical caveat.
Thank you for testing on macOS. I have added an extra condition to account for `EOPNOTSUPP`.
My reading of the docs being a misinterpretation certainly clears things up. I was baffled that returning `EOPNOTSUPP` would have been unsupported for that long.