Programming with POSIX® Threads

2. Scheduling determines which context (or set of contexts) should execute at any given point in time, and switches between contexts when necessary.

3. Synchronization provides mechanisms for concurrent execution contexts to coordinate their use of shared resources. We use this term in a way that is nearly the opposite of the standard dictionary meaning. You'll find a definition much like "cause to occur at the same time," whereas we usually mean something that might better be expressed as "prevent from occurring at the same time." In a thesaurus, you may find that "cooperate" is a synonym for "synchronize"-and synchronization is the mechanism by which threads cooperate to accomplish a task. This book will use the term "synchronization," though, because that is what you'll see used, almost universally.

There are many ways to provide each of these facilities—but they are always present in some form. The particular choices presented in this book are dictated by the book's subject—Pthreads. Table 1.1 shows a few examples of the three facilities in various systems.

	Execution context	Scheduling	Synchronization
Real traffic	automobile	traffic lights and signs	turn signals and brake lights
UNIX	process	priority (nice)	wait and pipes
(before threads)
Pthreads	thread	policy, priority	condition variables and mutexes

TABLE 1.1Execution contexts, schedulers, and synchronization

A system's scheduling facility may allow each thread to run until it voluntarily yields the processor to another thread ("run until block"). It may provide time-slicing, where each thread is forced to periodically yield so that other threads may run ("round-robin"). It may provide various scheduling policies that allow the application to control how each thread is scheduled according to that thread's function. It may provide a "class scheduler" where dependencies between threads are described so that, for example, the scheduler can ensure that members of a tightly coupled parallel algorithm are scheduled at the same time.

Synchronization may be provided using a wide variety of mechanisms. Some of the most common forms are mutexes, condition variables, semaphores, and events. You may also use message passing mechanisms, such as UNIX pipes, sockets, POSIX message queues, or other protocols for communicating between asynchronous processes—on the same system or across a network. Any form of communication protocol contains some form of synchronization, because passing data around with no synchronization results in chaos, not communication.

The terms thread, mutex, and condition variable are the main topics of this book. For now, it is enough to know that a thread represents an "executable thing" on your computer. A mutex provides a mechanism to prevent threads from colliding unexpectedly, and a condition variable allows a thread, once it has avoided such a collision, to wait until it is safe to proceed. Both mutexes and condition variables are used to synchronize the operation of threads.

1.3 Asynchronous programming is intuitive ...

"In most gardens," the Tiger-lily said,

"they make the beds too soft—so that the flowers are always asleep." This sounded a very good reason, and Alice was quite

pleased to know it. "I never thought of that before!" she said.

— Lewis Carroll, Through the Looking-Glass

If you haven't been involved in traditional realtime programming, asynchronous programming may seem new and different. But you've probably been using

asynchronous programming techniques all along. You've probably used UNIX, for example, and, even as a user, the common UNIX shells from sh to ksh have been designed for asynchronous programming. You've also been using asynchronous "programming" techniques in real life since you were born.

Most people understand asynchronous behavior much more thoroughly than they expect, once they get past the complications of formal and restricted definitions.

1.3.1 ... because UNIX is asynchronous