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Actually, both popular mobile operating systems have a POSIX-y system programming interface under the hood. Whether application developers can get at it is a different matter.

Even Symbian recently got a POSIX-layer: http://en.wikipedia.org/wiki/P.I.P.S._Is_POSIX_on_Symbian I have no idea how complete it is, but they did manage to port Qt.

It's been successful in keeping most server OSes similar, I suppose.

even if that's so, 20-plus years is a good run for any standard.