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!
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 squiggly lines are the threads. You'll see diagrams like this throughout the book.
Figure 1. A process as a container of threads.