Ruby Semaphores?

Question

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?

Answer 1

If you are using JRuby, you can import semaphores from Java as shown in this article.

require 'java'

java_import 'java.util.concurrent.Semaphore'

SEM = Semaphore.new(limit_of_simultaneous_threads)
SEM.acquire #To decrement the number available
SEM.release #To increment the number available

Answer 2

There's http://sysvipc.rubyforge.org/SysVIPC.html which gives you SysV semaphores. Ruby is perfect for eliminating the API blemishes of SysV semaphores and SysV semaphores are the best around -- they are interprocess semaphores, you can use SEM_UNDO so that even SIGKILLs won't mess up your global state (POSIX interprocess semaphores don't have this), and you with SysV semaphores you can perform atomic operations on several semaphores at once as long as they're in the same semaphore set.

As for inter-thread semaphores, those should be perfectly emulatable with Condition Variables and Mutexes. (See Bernanrdo Martinez's link for how it can be done).

Answer 3

I also found this code: https://gist.github.com/pettyjamesm/3746457

probably someone might like this other option.