The original NeXT operating system was built on top of Mach, just as OS X is. Mach supported threads from the very start and was one of the first UNIX-like systems to do so. More recently, the POSIX threading APIs have provided a cross-platform way of creating and manipulating threads.
On OS X, POSIX threads are implemented on top of Mach threads and Objective-C threads are implemented on top of POSIX threads. You will almost certainly not want to use Mach threads directly—they do not quite match up to the UNIX process model and are tricky to use correctly—but you may wish to use some of the POSIX threading functions directly.
Threads in Objective-C are encapsulated in
NSThread objects. These wrap ...