I'd say that's a clear-cut case when we should not raise an exception, especially since it's only a MAY.

On Sat, Aug 10, 2019 at 5:11 PM <xxxxxx@ancell-ent.com> wrote:
Per POSIX, isatty is allowed to set errno to ENOTTY when handed a file descriptor to something that's not a terminal device, like one opened to a regular file.  Should we signal an error, or just return #f in that case?  Ditto if the port handed to tty? does not have a file descriptor, like a string port.

- Harold