Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Process and thread fundamentals
Process and thread fundamentals
A process as a house
The occupants as threads
Single threaded
Multi threaded
Back to processes and threads
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.
Let’s base our analogy for processes and threads using a regular, everyday object — a
house.
A house is really a container, with certain attributes (such as the amount of floor space,
the number of bedrooms, and so on).
If you look at it that way, the house really doesn’t actively do anything on its own —
it’s a passive object. This is effectively what a process is. We’ll explore this shortly.
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.
If you’ve ever lived on your own, then you know what this is like — you know that
you can do anything you want in the house at any time, because there’s nobody else in
the house. If you want to turn on the stereo, use the washroom, have dinner —
whatever — you just go ahead and do it.
Things change dramatically when you add another person into the house. Let’s say
you get married, so now you have a spouse living there too. You can’t just march into
the washroom at any given point; you need to check first to make sure your spouse
isn’t in there!
If you have two responsible adults living in a house, generally you can be reasonably
lax about “security” — you know that the other adult will respect your space, won’t
try to set the kitchen on fire (deliberately!), and so on.
Now, throw a few kids into the mix and suddenly things get a lot more interesting.
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
April 30, 2009 Chapter 1 • Processes and Threads 15