From: Stephan Wehner <stephanwehner@...>
Date: 2007-05-26T02:28:42+09:00
Subject: Pipes and flush

So I wrote a little Ruby script using pipes roughly like this

-- BEGIN SCRIPT ----

#!/usr/bin/ruby -n

  def process(line)
    f = IO.popen('some_executable', 'w')
    f.puts <<END_INPUT
some executable commands
#{line}
some executable commands
END_INPUT

    f.close

    puts
    puts line
  end

process $_  unless $_ =~ /#/

-- END SCRIPT -----

Now this works nicely, I invoke it with

  ./script input_file > result

the "some_executable" prints to STDOUT results as I expect (given the
lines from input_file) and I can read the results in the "result" file

However, the output of the puts / puts line at the end of
the process method does not occur at the places I expect.

The output of "some_executable" for several "lines" shows up on STDOUT,
then the output of the puts / puts line for several "lines" shows up on
STDOUT, whereas they should simply alternate (depending on the "line"
value).

Am I missing something about pipes, and how the input/output streams are
hooked up?

I am guessing that a calling STDOUT.flush before and after the puts/puts
line statements will repair this, but I'm wondering why this would be
necessary.

This is with ruby 1.8.4 (2005-12-24) [i386-linux]

Thanks

Stephan

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