CODEWITHSUNDEEP
rubymultithreading
user3358302
user3358302

Reputation: 2111

Sleep and Threading (Ruby)

I am trying the following code from a threading example in Ruby:

count = 0
arr = []

10.times do |i|
  arr[i] = Thread.new {
  sleep(count*10)
  Thread.current["mycount"] = count
  count += 1
}
end

arr.each {|t| t.join; print t["mycount"], ", " }
puts "count = #{count}"

Because I increase the sleep on each thread, I expect the output to be in order from 1-10, However in almost all runs, the order is random. Why ?

Upvotes: 0

Views: 1900

Answers (2)

SparkyRobinson
SparkyRobinson

Reputation: 1522

The order is random because the access of the object count is not synchronized amongst threads.

You are encountering what is called a Race Condition

A race condition occurs when two or more threads can access shared data and they try to change it at the same time.

You can stop this through using mutex, condition variables and queue objects within ruby.

EDIT: Also, see Jeremy's answer

Upvotes: 0

Jeremy Roman
Jeremy Roman

Reputation: 16355

You are only updating count after the thread finishes sleeping, so all of the threads read the initial value of count, which is 0, when they go to sleep.

It's also worth noting that accessing count in this way is not threadsafe.

Upvotes: 1

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