[arendtio]

It is a joke to say that it is not a real OS.

But if you want to argue: You could say that a 'real OS' must be 'mostly POSIX-compliant' [1]. That way most other OS (Linux, MacOS, iOS) but not Windows would be included in your definition of a 'real OS' ;-)

[1] https://en.wikipedia.org/wiki/POSIX#Mostly_POSIX-compliant

[edgyquant]

Windows is mostly posix compliant as it had a POSIX subsystem until 2000, then it had Windows Services for UNIX/Interix and now days has Windows Subsystem for Linux.

[chrisseaton]

I wouldn’t get why POSIX is the only way to design an OS.

[arendtio]

It is not.

But that way you could pretend to have an objective definition of the term 'real OS' and exclude Windows while including most other major OS.

[rbanffy]

It’s not. It’s an API. IBM’s zOS has a POSIX-compliant Unix subsystem and it has absolutely nothing to do with Unix under its hood.

[cobalt]

regardless of anything else, windows is older than posix... so posix should arguably be windows compliant :P