Basic Thread Concepts

This chapter starts by reviewing what threads are and how you can control them.


A thread is the fundamental unit of execution within an application: A running application consists of at least one thread. Each thread has its own stack and runs independently from the application’s other threads. By default, threads share their resources, such as file handles or memory. Problems can occur when access to shared resources is not properly controlled. Data corruption is a common side effect of having two threads simultaneously write data to the same block of memory, for example.

Threads can be implemented in different ways. On most systems, threads are created and managed by the operating system: These are called native threads or kernel-level threads. Sometimes the threads are implemented by a software layer above the operating system, such as a virtual machine: These are called green threads. Both types of threads have essentially the same behavior. Some thread operations are faster on green threads, but they typically cannot take advantage of multiple processor cores, and implementation of blocking I/O is difficult. As multicore systems have become prevalent, most virtual machines have shifted away from green threads. The remainder of this chapter assumes that the threads are native threads.

Because the number of threads that can be executed at any given instant is limited by the number of cores in the computer, the operating system rapidly switches from ...

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