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14: Multiple threads

Thursday Mar 1st 2001

Objects provide a way to divide a program up into independent sections. Often, you also need to turn a program into separate, independently-running subtasks.

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Objects provide a way to divide a program up into independent sections. Often, you also need to turn a program into separate, independently-running subtasks.

Each of these independent subtasks is called a thread, and you program as if each thread runs by itself and has the CPU to itself. Some underlying mechanism is actually dividing up the CPU time for you, but in general, you don’t have to think about it, which makes programming with multiple threads a much easier task.

Some definitions are useful at this point. A process is a self-contained running program with its own address space. A multitasking operating system is capable of running more than one process (program) at a time, while making it look like each one is chugging along by periodically providing CPU cycles to each process. A thread is a single sequential flow of control within a process. A single process can thus have multiple concurrently executing threads.

There are many possible uses for multithreading, but in general, you’ll have some part of your program tied to a particular event or resource, and you don’t want to hang up the rest of your program because of that. So you create a thread associated with that event or resource and let it run independently of the main program. A good example is a “quit” button – you don’t want to be forced to poll the quit button in every piece of code you write in your program and yet you want the quit button to be responsive, as if you were checking it regularly. In fact, one of the most immediately compelling reasons for multithreading is to produce a responsive user interface.

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