From: Chris Spiegel <usenet-20040603@...>
Date: 2004-06-04T14:18:46+09:00
Subject: Re: playing with popen again

On Fri, 04 Jun 2004 04:43 GMT, Hal Fulton <hal9000@hypermetrics.com> wrote:
> It hangs indefinitely. When I kill prog1, it appears that I interrupted
> a gets on each side.
>
> # 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

This would be a side-effect of buffering.  prog2.rb won't be sending
anything over the pipe until it's flushed (standard out is normally
line-buffered when connected to a terminal, but fully buffered when it's
not).  If you add $stdout.flush after each puts in your prog2.rb then it
should work as expected.

You can check the status of whether autoflushing is on with IO#sync.
io.sync in prog1.rb should be true, $stdout.sync in prog2.rb should be
false (which means instead of flushing manually, you could do:
$stdout.sync = true in prog2.rb.)

I'm not extremely well-versed in Ruby so please accept my apologies if
this information is more Unix-centric than Ruby-centric.

Chris