Thread.join blocks the main thread

Question

Calling Thread.join blocks the current (main) thread. However not calling join results in all spawned threads to be killed when the main thread exits. How does one spawn per

Answer 1

You simply accumulate the threads in another container, then join them one-by-one after they've all been created:

my_threads = []
for i in 1..100 do
    puts "Creating thread #{i}"
    my_threads << Thread.new(i) do |j|
        sleep 1
        puts "Thread #{j} done"
    end
end
puts "#{Thread.list.size} threads"

my_threads.each do |t|
    t.join
end

You also can't bind the thread to the i variable because i gets constantly overwritten, and your output will be 100 lines of "Thread 100 done"; instead, you have to bind it to a copy of i, which I have cleverly named j.

Answer 2

You need to join the threads outside of the loop.

for i in 1..100 do
  puts "Creating thread #{i}"
  t = Thread.new(i) do |mi|
    sleep 1
    puts "Thread #{mi} done"
  end
end

# Wait for all threads to end
Thread.list.each do |t|
  # Wait for the thread to finish if it isn't this thread (i.e. the main thread).
  t.join if t != Thread.current
 end