Darwin is based on BSD and is open source, but Aqua (the GUI) and other parts of OSX are not. That's what CL has to be directing their programming towards. You can write a cool 'Unix' app as long as you know C and Darwin's kernel commands, but Aqua and other OSX code requires information from Apple (as Mosca states). Kuroyume

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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