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Irix and OS X are POSIX systems because they are fully-POSIX-compliant. However, operating systems like FreeBSD, Haiku, and GNU/Linux, for example, are "mostly-POSIX-compliant". Can people still correctly refer to such systems as POSIX systems?

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up vote 9 down vote accepted

No. The Open Group has an actual POSIX certification process, so if an operating system hasn't been through that, it cannot be referred to as POSIX-compliant.

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Of course, this won't stop people from referring to them as such, and it's still a useful handle. – evilsoup Dec 29 '13 at 21:07

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