The White Rabbit put on his spectacles,
"Where shall I begin, please your Majesty?" he asked.
"Begin at the beginning," the King said, very gravely,
"and go on till you come to the end: then stop."
***-Lewis Carroll, Alice's Adventures in Wonderland
This book is about "threads" and how to use them. "Thread" is just a name for a basic software "thing" that can do work on a computer. A thread is smaller, faster, and more maneuverable than a traditional process. In fact, once threads have been added to an operating system, a "process" becomes just data--address space, files, and so forth--plus one or more threads that do something with all that data.
With threads, you can build applications that utilize system resources more efficiently, that are more friendly to users, that run blazingly fast on multiprocessors, and that may even be easier to maintain. To accomplish all this, you need only add some relatively simple function calls to your code, adjust to a new way of thinking about programming, and leap over a few yawning chasms. Reading this book carefully will, I hope, help you to accomplish all that without losing your sense of humor.
The threads model used in this book is commonly called "Pthreads," or "POSIX threads." Or, more formally (since you haven't yet been properly introduced), the POSIX 1003.1cn1995 standard. I'll give you a few other names later-but for now, "Pthreads" is all you need to worry about.
As I write this, Sun's Solaris, Digital's Digital UNIX, and SGI's IRIX already support Pthreads. The other major commercial UNIX operating systems will soon have Pthreads as well, maybe even by the time you read this, including IBM's AIX and Hewlett-Packard's HP-UX. Pthreads implementations are also available for Linux and other UNIX operating systems.
In the personal computer market, Microsoft's Win32 API (the primary programming interface to both Windows NT and Windows 95) supports threaded programming, as does IBM's OS/2. These threaded programming models are quite different from Pthreads, but the important first step toward using them productively is understanding concurrency, synchronization, and scheduling. The rest is (more or less) a matter of syntax and style, and an experienced thread programmer can adapt to any of these models.
The threaded model can be (and has been) applied with great success to a wide range of programming problems. Here are just a few:
- Large scale, computationally intensive programs
- High-performance application programs and library code that can take advantage of multiprocessor systems
- Library code that can be used by threaded application programs
- Realtime application programs and library code
- Application programs and library code that perform I/O to slow external devices (such as networks and human beings).
This book assumes that you are an experienced programmer, familiar with developing code for an operating system in "the UNIX family" using the ANSI C language. I have tried not to assume that you have any experience with threads or other forms of asynchronous programming. The Introduction chapter provides a general overview of the terms and concepts you'll need for the rest of the book. If you don't want to read the Introduction first, that's fine, but if you ever feel like you're "missing something" you might try skipping back to get introduced.
Along the way you'll find examples and simple analogies for everything. In the end I hope that you'll be able to continue comfortably threading along on your own. Have fun, and "happy threading."
About the author
I have been involved in the Pthreads standard since it began, although I stayed at home for the first few meetings. I was finally forced to spend a grueling week in the avalanche-proof concrete bunker at the base of Snowbird ski resort in Utah, watching hard-working standards representatives from around the world wax their skis. This was very distracting, because I had expected a standards meeting to be a formal and stuffy environment. As a result of this misunderstanding, I was forced to rent ski equipment instead of using my own.
After the Pthreads standard went into balloting, I worked on additional thread synchronization interfaces and multiprocessor issues with several POSIX working groups. I also helped to define the Aspen threads extensions, which were fast-tracked into X/Open XSH5.
I have worked at Digital Equipment Corporation for (mumble, mumble) years, in various locations throughout Massachusetts and New Hampshire. I was one of the creators of Digital's own threading architecture, and I designed (and implemented much of) the Pthreads interfaces on Digital UNIX 4.0. I have been helping people develop and debug threaded code for more than eight years.
My unofficial motto is "Better Living Through Concurrency." Threads are not sliced bread, but then, we're programmers, not bakers, so we do what we can.
This is the part where I write the stuff that I'd like to see printed, and that my friends and coworkers want to see. You probably don't care, and I promise not to be annoyed if you skip over it-nbut if you're curious, by all means read on.
No project such as this book can truly be accomplished by a single person, despite the fact that only one name appears on the cover. I could have written a book about threads without any help-I know a great deal about threads, and I am at least reasonably competent at written communication. However, the result would not have been this book, and this book is better than that hypothetical work could possibly have been.
Thanks first and foremost to my manager Jean Fullerton, who gave me the time and encouragement to write this book on the job-and thanks to the rest of the DECthreads team who kept things going while I wrote, including Brian Keane, Webb Scales, Jacqueline Berg, Richard Love, Peter Portante, Brian Silver, Mark Simons, and Steve Johnson.
Thanks to Garret Swart who, while he was with Digital at the Systems Research Center, got us involved with POSIX. Thanks to Nawaf Bitar who worked with Garret to create, literally overnight, the first draft of what became Pthreads, and who became POSIX thread evangelist through the difficult period of getting everyone to understand just what the heck this threading thing was all about anyway. Without Garret, and especially Nawaf, Pthreads might not exist, and certainly wouldn't be as good as it is. (The lack of perfection is not their responsibility-that's the way life is.)
Thanks to everyone who contributed to the design of cma, Pthreads, UNIX98, and to the users of DCE threads and DECthreads, for all the help, thought-provoking discourse, and assorted skin-thickening exercises, including Andrew Birrell, Paul Borman, Bob Conti, Bill Cox, Jeff Denham, Peter Gilbert, Rick Greer, Mike Grier, Kevin Harris, Ken Hobday, Mike Jones, Steve Kleiman, Bob Knighten, Leslie Lamport, Doug Locke, Paula Long, Finnbarr P. Murphy, Bill Noyce, Simon Patience, Harold Seigel, Al Simons, Jim Woodward, and John Zolnowsky.
Many thanks to all those who patiently reviewed the drafts of this book (and even to those who didn't seem so patient at times). Brian Kernighan, Rich Stevens, Dave Brownell, Bill Gallmeister, Ilan Ginzburg, Will Morse, Bryan O'Sullivan, Bob Robillard, Dave Ruddock, Bil Lewis, and many others suggested or motivated improvements in structure and detail-and provided additional skin-thickening exercises to keep me in shape. Devang Shah and Bart Smaalders answered some Solaris questions, and Bryan O'Sullivan suggested what became the "bailing programmers" analogy.
Thanks to John Wait and Lana Langlois at Addison Wesley Longman, who waited with great patience as a first-time writer struggled to balance writing a book with engineering and consulting commitments. Thanks to Pamela Yee and Erin Sweeney, who managed the book's production process, and to all the team (many of whose names I'll never know), who helped. Thanks to my wife, Anne Lederhos, and our daughters Amy and Alyssa, for all the things for which any writers may thank their families, including support, tolerance, and just being there. And thanks to Charles Dodgson (Lewis Carroll), who wrote extensively about threaded programming (and nearly everything else) in his classic works Alice's Adventures in Wonderland, Through the Looking-Glass, and The Hunting of the Snark.
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