ruby - interact with never-end child process

Question

I wonder how can I interact with never-ending(eternal looping) child process.

source code of loop_puts.rb, child process :

loop do
    str = gets
    puts str.upcase
end

main.rb :

Process.spawn("ruby loop_puts.rb",{:out=>$stdout, :in=>$stdin})

I want to put some letter, not by my hand typing, and get result(not previous result) in variable.

how can I do this?

thanks

Answer 1

There are a number of ways to do this and it's hard to recommend one without more context.

Here's one way using a forked process and a pipe:

# When given '-' as the first param, IO#popen forks a new ruby interpreter.  
# Both parent and child processes continue after the return to the #popen 
# call which returns an IO object to the parent process and nil to the child.
pipe = IO.popen('-', 'w+')
if pipe
  # in the parent process
  %w(please upcase these words).each do |s|
    STDERR.puts "sending:  #{s}"
    pipe.puts s   # pipe communicates with the child process
    STDERR.puts "received: #{pipe.gets}"
  end
  pipe.puts '!quit'  # a custom signal to end the child process
else
  # in the child process
  until (str = gets.chomp) == '!quit'
    # std in/out here are connected to the parent's pipe
    puts str.upcase
  end
end

Some documentation for IO#popen here. Note that this may not work on all platforms.

Other possible ways to approach this include Named Pipes, drb, and message queues.