Getting Started with QNX Neutrino - QNX Software Systems

Process and thread fundamentals
© 2009, QNX Software Systems GmbH & Co. KG.
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!
As we know from everyday life, things aren’t quite that simple. Now that we’ve seen
the basic characteristics (summary: everything is shared), let’s take a look at where
things get a little more interesting, and why.
The diagram below shows the way that we’ll be representing threads and processes.
The process is the circle, representing the “container” concept (the address space), and
the three squigley lines are the threads. You’ll see diagrams like this throughout the
book.
A process as a container of threads.
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?
It’s done with something called mutual exclusion. It means pretty much what you think
— a number of threads are mutually exclusive when it comes to a particular resource.
If you’re taking a shower, you want to have exclusive access to the bathroom. To do
this, you would typically go into the bathroom and lock the door from the inside.
Anyone else trying to use the bathroom would get stopped by the lock. When you’re
done, you’d unlock the door, allowing someone else access.
This is just what a thread does. A thread uses an object called a mutex (an acronym for
MUTual EXclusion). This object is like the lock on a door — once a thread has the
mutex locked, no other thread can get the mutex, until the owning thread releases
(unlocks) it. Just like the door lock, threads waiting to obtain the mutex will be barred.
Another interesting parallel that occurs with mutexes and door locks is that the mutex
is really an “advisory” lock. If a thread doesn’t obey the convention of using the
mutex, then the protection is useless. In our house analogy, this would be like
someone breaking into the washroom through one of the walls ignoring the convention
of the door and lock.
16 Chapter 1 • Processes and Threads April 30, 2009