Section 18.2. Threads and Processes

18.2. Threads and Processes

18.2.1. What Are Processes?

Computer programs are merely executables, binary (or otherwise), which reside on disk. They do not take on a life of their own until loaded into memory and invoked by the operating system. A process (sometimes called a heavyweight process) is a program in execution. Each process has its own address space, memory, a data stack, and other auxiliary data to keep track of execution. The operating system manages the execution of all processes on the system, dividing the time fairly between all processes. Processes can also fork or spawn new processes to perform other tasks, but each new process has its own memory, data stack, etc., and cannot generally share information unless interprocess communication (IPC) is employed.

18.2.2. What Are Threads?

Threads (sometimes called lightweight processes) are similar to processes except that they all execute within the same process, and thus all share the same context. They can be thought of as "mini-processes" running in parallel within a main process or "main thread."

A thread has a beginning, an execution sequence, and a conclusion. It has an instruction pointer that keeps track of where within its context it is currently running. It can be preempted (interrupted) and temporarily put on hold (also known as sleeping) while other threads are runningthis is called yielding.

Multiple threads within a process share the same data space with the main thread and can therefore share information or communicate with one another more easily than if they were separate processes. Threads are generally executed in a concurrent fashion, and it is this parallelism and data sharing that enable the coordination of multiple tasks. Naturally, it is impossible to run truly in a concurrent manner in a single CPU system, so threads are scheduled in such a way that they run for a little bit, then yield to other threads (going to the proverbial "back of the line" to await more CPU time again). Throughout the execution of the entire process, each thread performs its own, separate tasks, and communicates the results with other threads as necessary.

Of course, such sharing is not without its dangers. If two or more threads access the same piece of data, inconsistent results may arise because of the ordering of data access. This is commonly known as a race condition. Fortunately, most thread libraries come with some sort of synchronization primitives that allow the thread manager to control execution and access.

Another caveat is that threads may not be given equal and fair execution time. This is because some functions block until they have completed. If not written specifically to take threads into account, this skews the amount of CPU time in favor of such greedy functions.