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Ruby Programming
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Thread Variables

As we described in the previous section, a thread can normally access any variables that are in scope when the thread is created. Variables local to the block of a thread are local to the thread, and are not shared.

But what if you need per-thread variables that can be accessed by other threads---including the main thread? Thread features a special facility that allows thread-local variables to be created and accessed by name. You simply treat the thread object as if it were a Hash, writing to elements using []= and reading them back using []. In this example, each thread records the current value of the variable count in a thread-local variable with the key mycount. (There's a race condition in this code, but we haven't talked about synchronization yet, so we'll just quietly ignore it for now.)

count = 0
arr = []
10.times do |i|
  arr[i] = {
    Thread.current["mycount"] = count
    count += 1
arr.each {|t| t.join; print t["mycount"], ", " }
puts "count = #{count}"
8, 0, 3, 7, 2, 1, 6, 5, 4, 9, count = 10

The main thread waits for the subthreads to finish and then prints out the value of count captured by each. Just to make it more interesting, we have each thread wait a random time before recording the value.
Ruby Programming
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