let run closer command = 
  let (from_command,to_command) = Unix.open_process command in
    closer to_command ;
    try while true do 
          print_endline (input_line from_command) ;
        done
    with End_of_file -> () ;
    ignore (Unix.close_process (from_command,to_command)) ;;

let _ = String.create (Sys.max_string_length) in
run (fun to_command -> Unix.close (Unix.descr_of_out_channel to_command)) "echo 1" ;
run (fun to_command -> Unix.close (Unix.descr_of_out_channel to_command)) "echo 2" ;
run (fun to_command -> close_out to_command)                              "echo 3" ;
run (fun to_command -> close_out to_command)                              "echo 4" ;