Process and thread fundamentals

Before we start talking about threads, processes, time slices, and all the other wonderful “scheduling concepts,” let's establish an analogy.

What I want to do first is illustrate how threads and processes work. The best way I can think of (short of digging into the design of a realtime system) is to imagine our threads and processes in some kind of situation.

A process as a house
Let's base our analogy for processes and threads using a regular, everyday object: a house.
The occupants as threads
The people living in the house are the “active” objects—they're the ones using the various rooms, watching TV, cooking, taking showers, and so on. We'll soon see that's how threads behave.
Back to processes and threads
Just as a house occupies an area of real estate, a process occupies memory. And just as a house's occupants are free to go into any room they want, a processes' threads all have common access to that memory. If a thread allocates something (mom goes out and buys a game), all the other threads immediately have access to it (because it's present in the common address space—it's in the house). Likewise, if the process allocates memory, this new memory is available to all the threads as well. The trick here is to recognize whether the memory should be available to all the threads in the process. If it is, then you'll need to have all the threads synchronize their access to it. If it isn't, then we'll assume that it's specific to a particular thread. In that case, since only that thread has access to it, we can assume that no synchronization is required—the thread isn't going to trip itself up!
Mutual exclusion
If you want to take a shower, and there's someone already using the bathroom, you'll have to wait. How does a thread handle this?
Priorities
What if the bathroom is currently locked and a number of people are waiting to use it? Obviously, all the people are sitting around outside, waiting for whoever is in the bathroom to get out. The real question is, What happens when the door unlocks? Who gets to go next?
Semaphores
Let's move from the bathroom into the kitchen, since that's a socially acceptable location to have more than one person at the same time. In the kitchen, you may not want to have everyone in there at once. In fact, you probably want to limit the number of people you can have in the kitchen (too many cooks, and all that).
A semaphore as a mutex
We just asked the question “Could you do it with a mutex?” in relation to implementing a lock with a count, and the answer was no. How about the other way around? Could we use a semaphore as a mutex?

Related concepts

Threads and processes (System Architecture)

Programming Overview (QNX Neutrino Programmer's Guide)

Processes (QNX Neutrino Programmer's Guide)

Parent topic: Processes and Threads