From: Jacob Burkhart <igotimac@...>
Date: 2007-09-05T06:41:14+09:00
Subject: Catching and redirecting STDERR output

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So ruby has this libxml that writes validation errors directly to standard
error.  The only way I've been able to figure out how to capture this output
and then use it programmatically (send to logger and/or display to the user)
is to do something like this:

require 'xml/libxml'
require 'tempfile'

def capture_stderr()
  begin
    err_file = Tempfile.new("err_file")
    err_file.close
    old_stderr = STDERR.dup
    STDERR.reopen(err_file.path)
    STDERR.sync = true
    result = yield
    STDERR.flush
    err_file.open
    err_file.seek(0)
    err_text = err_file.read
    err_file.close
    err_file.unlink
    return result, err_text
  ensure
    STDERR.reopen(old_stderr)
  end
end

And then I can call what I want to call like this:

result, errors = capture_stderr{ ...do something with XML::Document and
XML::Dtd here... }

I've previously tried:
Altering $stderr variable or overriding the IO class to overrride the puts
or write methods, or re-opening STDERR to a StringIO.
I figure none of these work because the STDERR call I want to intercept is
actually in the C code that implements libxml as an fprintf
So no extent of ruby object space manipulation is going to change that.

Can anybody see any danger of breakage here? Or perhaps have some other
ideas on how to do this?

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