When I said that the OS executes the processes in time slots, I was not absolutely precise. Every process has one or more threads, and threads are executed. A thread is the smallest execution managed by an external scheduler. Older operating systems did not have the notion of a thread and were executing processes. As a matter of fact, the first thread implementations were simply duplications of processes that were sharing the memory.

You may hear the terminology, lightweight process—it means a thread.

The important thing is that the threads do not have their own memory. They use the memory of the process. In other words, the threads that run in the same process have undistinguished access to the same memory segment. It is an extremely powerful possibility to implement parallel algorithms that make use of the multiple cores in the machine, but at the same time, it may lead to bugs.

Imagine that two threads increment the same long variable. The increment first calculates the incremented...