The Truth About OS X’s Kernel: It’s Not Unix

By Daniel Miessler on March 1st, 2007: Tagged as Apple | BSD | Leopard | Mach | OS X | Unix

Leopard

OS X’s kernel isn’t strictly Unix-based. It’s called XNU, and it’s a monolithic (non micro) hybrid of both Mach And BSD kernel components. What’s quite interesting is how many people (including myself up until a little while ago) think that OS X has a pure Unix (BSD) kernel.

But that’s not the case. In fact, XNU is actually an acronym for “X is Not Unix”.

In other words, while Apple’s marketing team loves to play the Unix card at every turn, its developers explicitly made a distinction between the OS X’s kernel and a true Unix kernel.

Wikipedia goes on to mention, however, that this distinction will become far less dramatic in the event that OS X becomes POSIX compliant in Leopard.

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Comments

  • cji - it's a "hybrid" because it's actually, well, two kernels. At least from what I remember when I last looked at the OS X architecture. As I recall, it's the Mach microkernel (which actually shares a lot of history / influence with the Windows NT microkernel that powers modern Windows OSes) with a FreeBSD kernel running on top of it in some Frankensteining arrangement. As I understand it, the convoluted threading model that arises from this design is one of the more annoying things about programming on OS X.
  • > Lay off the sauce during business hours! ;-)

    Indeed. Score one for Rick. :)
  • cji
    This is a hybrid. This is a cross, ah, of Blue Mach, Kentucky BSD, I/O Kit , and Reality Distorting Sensemilia. The amazing stuff about this is, that you can play 36 holes on it in the afternoon, take it home and just get stoned to the bejeezus-belt that night on this stuff.
    -- Carl Spackler (sort of)

    I swear every time I hear "hybrid" I'm reminded of Caddyshack...
  • Lay off the sauce during business hours!
    ;-)
  • frijole
    yup. solid reporting there chap. just like GNU's Not Unix too.

    And Apple's already announced that Leopard is UNIX compliant and will no longer be "UNIX-based" (as the advertising to date has always been) and will be "a UNIX variant" certified by The Open Group.
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