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Editorial Team
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Asked: 2026-05-21T03:34:03+00:00

I’m working on an implementation of the Fair Barbershop problem in Ruby. This is

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I’m working on an implementation of the “Fair Barbershop” problem in Ruby. This is for a class assignment, but I’m not looking for any handouts. I’ve been searching like crazy, but I cannot seem to find a Ruby implementation of Semaphores that mirror those found in C.

I know there is Mutex, and that’s great. Single implementation, does exactly what that kind of semaphore should do.

Then there’s Condition Variables. I thought that this was going to work out great, but looking at these, they require a Mutex for every wait call, which looks to me like I can’t put numerical values to the semaphore (as in, I have seven barbershops, 3 barbers, etc.).

I think I need a Counting Semaphore, but I think it’s a little bizarre that Ruby doesn’t (from what I can find) contain such a class in its core. Can anyone help point me in the right direction?

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1 Answer

  1. Editorial Team

    Thanks to @x3ro for his link. That pointed me in the right direction. However, with the implementation that Fukumoto gave (at least for rb1.9.2) Thread.critical isn’t available. Furthermore, my attempts to replace the Thread.critical calls with Thread.exclusive{} simply resulted in deadlocks. It turns out that there is a proposed Semaphore patch for Ruby (which I’ve linked below) that has solved the problem by replacing Thread.exclusive{} with a Mutex::synchronize{}, among a few other tweaks. Thanks to @x3ro for pushing me in the right direction.

    http://redmine.ruby-lang.org/attachments/1109/final-semaphore.patch

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