CODEWITHSUNDEEP
rubymultithreading
Wingzero
Wingzero

Reputation: 1

Ruby thread mess up variable

I'm new to ruby, and want to use thread. In purpose, I want thread to spawn thread, I have below code:

require 'thread'
semaphore = Mutex.new

thr = Array.new
outputs = Array.new
scripts = Array.new
for i in 1..3
  thr[i] = Thread.new do
      puts "adding #{i} thread\n"
      puts "ready to create #{i} thread\n"
      scripts[i]= Thread.new do
        puts "in #{i} thread\n"
        puts "X#{i}\n"
        outputs[i] = "a#{i}" 
      end
  end 
end
for i in 1..3
  thr[i].join
end
for i in 1..3
  scripts[i].join
end
for i in 1..3
  puts outputs[i]
end

The output is

adding 1 thread
adding 2 thread
adding 3 thread
ready to create 3 thread
ready to create 1 thread
ready to create 1 thread
in 1 thread
in 1 thread
in 2 thread
X2
X3
X1
C:/Users/user/workspace/ruby-test/test.rb:61: undefined method `join' for nil:NilClass (NoMethodError)
    from C:/Users/liux14/workspace/ruby-test/test.rb:60:in `each'
    from C:/Users/liux14/workspace/ruby-test/test.rb:60

The first three lines are correct, but following i is messed up.

two i = 1 and one i = 2 and one i = 3. And the one of output[i] is nil.

What I missed?

Upvotes: 0

Views: 205

Answers (1)

Denis de Bernardy
Denis de Bernardy

Reputation: 78463

Your use of the for i in 1..3 statement might be making i available outside of the for block, and makes it shared across the parent and child threads.

Try it with a block instead:

(1..3).each do |i|
  # code
end

#!/usr/bin/env ruby

require 'thread'
semaphore = Mutex.new

thr = Array.new
outputs = Array.new
scripts = Array.new
(1..3).each do |i|
  thr[i] = Thread.new do
      puts "adding #{i} thread\n"
      puts "ready to create #{i} thread\n"
      scripts[i]= Thread.new do
        puts "in #{i} thread\n"
        puts "X#{i}\n"
        outputs[i] = "a#{i}" 
      end
  end 
end
(1..3).each do |i|
  thr[i].join
end
(1..3).each do |i|
  scripts[i].join
end
(1..3).each do |i|
  puts outputs[i]
end

denis@DB:~/wk $ ./test.rb 
adding 1 thread
ready to create 1 thread
adding 3 thread
ready to create 3 thread
adding 2 thread
ready to create 2 thread
in 1 thread
X1
in 3 thread
in 2 thread
X3
X2
a1
a2
a3

Upvotes: 1

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