From: "Ara.T.Howard" <ahoward@...>
Date: 2004-06-04T22:23:45+09:00
Subject: Re: playing with popen again

On Fri, 4 Jun 2004, Hal Fulton wrote:

> Why doesn't this work?
>
> prog1 calls prog2... the latter starts by writing a line, then reading
> a line. The former reads first, then writes.
>
> It hangs indefinitely. When I kill prog1, it appears that I interrupted
> a gets on each side.
>
> What am I doing that's stupid?
>
> Thanks,
> Hal
>
>
> # prog1
> io = IO.popen("ruby prog2.rb","r+")
> str = io.gets
> puts "got: #{str}"
> io.puts "junk"
>
> str = io.gets
> puts "got: #{str}"
> io.puts "junk"
>
> str = io.gets
> puts "got: #{str}"
> io.puts "junk"
>
>
> #!/usr/local/bin/ruby
> # prog2
> puts "This is prompt 1"
> gets
> puts "This is 2nd prompt"
> gets
> puts "3rd/final prompt"
> gets
>
>
> $ ruby prog1.rb   # and then ctl-C
> prog1.rb:2:in `gets': Interrupt from prog1.rb:2
> prog2.rb:3:in `gets': Interrupt from prog2.rb:3

stdout is only line buffered when it's a tty.  in the case of an external
process it is completely buffered - you are seeing the operating system chose
not to send the output of prog2 because the buffer has not yet been filled.

   ~ > cat prog1
   io = IO.popen("ruby prog2","r+")
   3.times do |i|
     print "read #{ i } => "
     str = io.gets
     puts "<#{ str.inspect }>"
     io.puts
   end

   ~ > cat prog2
   #
   # magic bullet
   #
   STDOUT.sync = true

   3.times do |i|
     puts "prompt #{ i }"
     gets
   end

   ~ > ruby prog1
   read 0 => <"prompt 0\n">
   read 1 => <"prompt 1\n">
   read 2 => <"prompt 2\n">


cheers.

-a
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