Small Swift 2.0 pthread example that does not work

pthread.swift

func foo(x:UnsafeMutablePointer<Void>) -> UnsafeMutablePointer<Void> {
    print("Hello World")
    return nil
}

let thread:UnsafeMutablePointer<pthread_t> = nil

//
pthread_create(thread, nil, foo, nil)
pthread_join(thread.memory, nil)

func foo(x: UnsafeMutablePointer<Void>) -> UnsafeMutablePointer<Void> {
    print("Hello pthread")

    return nil
}

var pid: pthread_t = 0
pthread_create(&pid, nil, foo, nil)
pthread_join(pid, nil)

This can run