The Little Book of Semaphores, 2nd Edition

By Allen B. Downey
Second Edition Revision date July 2007

The Little Book of Semaphores is a free (in both senses of the word) textbook that introduces the principles of synchronization for concurrent programming.

In most computer science curricula, synchronization is a module in an Operating Systems class. OS textbooks present a standard set of problems with a standard set of solutions, but most students don't get a good understanding of the material or the ability to solve similar problems.

The approach of this book is to identify patterns that are useful for a variety of synchronization problems and then show how they can be assembled into solutions. After each problem, the book offers a hint before showing a solution, giving students a better chance of discovering solutions on their own.

The book covers the classical problems, including "Readers-writers", "Producer-consumer", and "Dining Philosophers". In addition, it collects a number of not-so-classical problems, some written by the author and some by other teachers and textbook writers. Readers are invited to create and submit new problems.


Most undergraduate Operating Systems textbooks have a module on Synchronization, which usually presents a set of primitives (mutexes, semaphores, monitors, and sometimes condition variables), and classical problems like readers-writers and producers-consumers.

When I took the Operating Systems class at Berkeley, and taught it at Colby College, I got the impression that most students were able to understand the solutions to these problems, but few would have been able to produce them, or solve similar problems.

One reason students don't understand this material deeply is that it takes more time, and more practice, than most classes can spare. Synchronization is just one of the modules competing for space in an Operating Systems class, and I'm not sure I can argue that it is the most important. But I do think it is one of the most challenging, interesting, and (done right) fun.

I wrote the first edition this book with the goal of identifying synchronization idioms and patterns that could be understood in isolation and then assembled to solve complex problems. This was a challenge, because synchronization code doesn't compose well; as the number of components increases, the number of interactions grows unmanageably.

Nevertheless, I found patterns in the solutions I saw, and discovered at least some systematic approaches to assembling solutions that are demonstrably correct.

I had a chance to test this approach when I taught Operating Systems at Wellesley College. I used the first edition of The Little Book of Semaphores along with one of the standard textbooks, and I taught Synchronization as a concurrent thread for the duration of the course. Each week I gave the students a few pages from the book, ending with a puzzle, and sometimes a hint. I told them not to look at the hint unless they were stumped.

I also gave them some tools for testing their solutions: a small magnetic whiteboard where they could write code, and a stack of magnets to represent the threads executing the code.

The results were dramatic. Given more time to absorb the material, students demonstrated a depth of understanding I had not seen before. More importantly, most of them were able to solve most of the puzzles. In some cases they reinvented classical solutions; in other cases they found creative new approaches.


Copyright 2005, 2006, 2007 Allen B. Downey

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; this book contains no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.


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