Various libraries, like pThread.h (and its associated library) in C, give languages exposure to the underlying systems

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Various libraries, like pThread.h (and its associated library) in C, give languages exposure to the underlying system’s threading system. How does working with Grand Central Dispatch differ from other threading systems you have used (if you have never used threading, read up on Java threading for a comparison)? Be sure to talk about the activities that take place on the main thread versus those that take place on the background threads.