This is a brilliant free textbook about synchronization for concurrent programming. It doesn't only present the essential knowledges, problems and solutions, but also the underlying patterns that are useful for synchronization problems.
Interesting, I was just tackling semaphores on Windows this past month. The result was an open source class, written in VB6, that uses semaphores to communicate across the user boundary in Windows. (http://www.vbforums.com/showthread.php?t=634635)
What?!! VB6 is still alive?!!
Allen Downey writes great books. It's hard for me to overstate that.
Thanks for the kind words about the book! If you are interested in this topic, there is a video of me giving a talk about it at greenteapress.com/semaphores. The thread simulator I demonstrate in the talk is available from allendowney.com/swampy. As always, comments are welcome.
Downey's PDF is presented as a series of puzzles with hints and answers. It makes for an enjoyable few hours sharpening your concurrency skills. Covers much more ground than the "igloo book". Uses Python and C for examples -- much nicer than reading old-style Pascal code.
Great material, I'm studying operating systems and this really helps me. It'd be great to find similar material about other OS topics(scheduling, I/O...).