elixir - Port messages returning to an unexpected process -

i have simple ports app (literally example erlang -- ports documentation) , genserver controlling use.

the genserver can communicate c app fine doesn't receive responses, iex or supervisor does. if call flush iex, see expected message.

if create separate module , spawn receive loop it, still doesn't receive port response messages.

i have feeling i'm incorrectly opening port cannot prove it. there obvious i'm screwing up?

port = port.open({:spawn, "./extprg"}, [{:packet, 2}, :exit_status]) collect = fun () -> collect_results(port) end spawn(collect)   def collect_results(port)    receive       {^port, {:data, data}} ->            #never gets called despite matching messages in flush       {^port, {:exit_status, status}} ->           #never gets called...        {:increment, value} ->           port.command(port, [1, value])           collect_results(port)    end end 

when opening port module uses genserver, ensure calling port.open in init function , not start or start_link functions. init ran new process, start , start_link both still calling process.

here example of genserver uses port:

https://github.com/fhunleth/elixir_ale/blob/master/lib/i2c.ex