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Beginning Linux Programming, 4th Edition by Richard Stones, Neil Matthew

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Chapter 12

POSIX Threads

In Chapter 11, you saw how processes are handled in Linux (and indeed in UNIX). These multiprocessing features have long been a feature of UNIX-like operating systems. Sometimes it may be very useful to make a single program do two things at once, or at least to appear to do so, or you might want two or more things to happen at the same time in a closely coupled way but consider the overhead of creating a new process with fork too great. For these occasions you can use threads, which allow a single process to multitask.

In this chapter, you look at

  • Creating new threads within a process
  • Synchronizing data access between threads in a single process
  • Modifying the attributes of a thread
  • Controlling one thread from another in the same process

What Is a Thread?

Multiple strands of execution in a single program are called threads. A more precise definition is that a thread is a sequence of control within a process. All the programs you have seen so far have executed as a single process, although, like many other operating systems, Linux is quite capable of running multiple processes simultaneously. Indeed, all processes have at least one thread of execution. All the processes that you have seen so far in this book have had just one thread of execution.

It’s important to be clear about the difference between the fork system call and the creation of new threads. When a process executes a fork call, a new copy of the process is created with its own variables and ...

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