In some concurrent programming systems, global variables are shared by all threads. In others, each newly created thread has a separate copy of the global variables, commonly initialized to the values of the globals of the creating thread. Under this private globals approach, shared data must be allocated from a special heap. In still other programming systems, the programmer can specify which global variables are to be private and which are to be shared.

Discuss the tradeoffs between private and shared global variables. Which would you prefer to have available, for which sorts of programs? How would you implement each? Are some options harder to implement than others? To what extent do your answers depend on the nature of processes provided by the operating system?