For the functions here: #include <libkern/OSAtomic.h> there are OSAtomic and OSAtomicBarrier versions. However, the documentation does not show sample code for: When is it safe to use just OSAtomic, without the OSAtomicBarrier version When is it that OSAtomic would be unsafe, but OSAtomiBarrier would be safe. Can anyone provide explainations + sample codes? [Random ramblings of "your opinion" without actual code is useless. Readers: please down vote such answers; and vigrously upvote answers with actual code.] [C/C++ code preferred; Assembly okay too.]