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Hi Masayuki, thanks for the response (I thought I replied to this earlier but maybe I only imagined it--I've been on vacation).
I agree with just about everything you write here.
I'm aware of the FreeBSD setmode(), but I figured os.setmode() could do different things on different platforms as applicable. But that would also be very confusing.
Your patch using ctypes looks fine to me--I considered doing the same, but what I'm not sure is if it's kosher to use ctypes here, given that it's techically an optional module. Since _pyio is, as I understand it, mainly used for testing it's probably fine? But I think we'll need some core maintainers' comments here...
Implementing a cygwin module would be really nice, actually, even if it isn't included in the stdlib. There are a few other cygwin-specific APIs that it is useful to have wrappers around: https://cygwin.com/cygwin-api/cygwin-functions.html But that would be outside the scope of fixing this issue.