I want to redirect the input/output to/from a running app by:

IO.popen "cmd.exe /C app.exe", "rw" do |io|
  p io.closed?
  ...
end

In mode="rw" there is an ArgumentErrror exception

illegal access mode rw (ArgumentError)

For mode="w" the io is closed immediately (io.closed? is true).

When the mode is just "r", I can read from the io and the output of the app is available inside the ruby script,
but writing data to the app is forbidden.

How can I get the mode="rw" to work properly ?