From: Joel VanderWerf <vjoel@...>
Date: 2009-08-25T05:16:39+09:00
Subject: Re: Asynchronous process to process pipe IO

Pito Salas wrote:
> This is slightly subtle question about how a process that reads from a
> pipe that is being written to by another process may run asynchronously.
> 
> Here's a code snippet:
> 
>  def start_processing
>     open("|-", "r") do |worker|
>       if worker
>         # here we are in the parent
>         i = 0
>         worker.each_line do |line|
>           puts "line #{i}
>           i = i+1
>          end
>       else
>         # here we are in child thread
>         exec("./iacommand.rb")
>       end
>     end
>  end
> 
> iacommand.rb is invoked as a process within ruby. It does a lot of
> processing, outputs a line, does a  more processing, outputs another
> line, and so on.
> 
> I would like to have the "puts" calls occur right when the a line is
> generated by the script running in the process.
> 
> The way it is written above, what happens is that the app waits until
> iacommand.rb exits and then calls the series of puts in immediate
> succession.
> 
> So in other words, how do I asynchronously read from a pipe that is
> being fed by a process?

Hm, the following shows output at 1 sec intervals:

open("|-", "r") do |worker|
   if worker
     i = 0
     worker.each_line do |line|
       puts "line #{i}:#{line}"
       i = i+1
     end
   else
     exec("ruby -e '3.times {|i| p i; sleep 1}'")
   end
end

Does it work that way for you?

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407