From: Bjorn Nordbo <bn@...>
Date: 2004-09-06T02:50:13+09:00
Subject: capturing stdout from so library

I have a package (rrdtool) that provides functionality through a shared
library and a Ruby interface (ruby-rrd) to it.

The problem is that one of the functions in the library (rrd_graph) only
returns data through a file descriptor; a file or standard output.

This is rather dumb in my opinion, but probably hard to fix.  Also, a
a workaround seems easy:

class StringBuffer
  attr_reader :buffer
  
  def initialize
    @buffer = String.new
  end

  def write(string)
    @buffer << string
  end
end

begin
  oldout = $stdout
  $stdout = StringBuffer.new
  RRD.graph(... tons of options ...)
  image = $stdout.buffer
  $stdout = oldout
end

According to my theory, the StringBuffer object should have captured all
data sent to standard output while $stdout points to it, and image sould
contain a beautiful graph produced by RRD.graph.

However, what happens is that RRD.graph blatantly ignores the fact that
$stdout is redirected, and bangs its data to standard output anyway.

I assume this means that there's nothing I can do in Ruby to capture the
output to standard out from a library written in C.  If this is true,
I have two options:

- Do something (but what?) in the Ruby/C interface
- Fix the library

Please tell me I'm wrong. :-)


-- 
Bj�rn