How to send Message from Node JS to C++ code

I am running C++ program using Node js and the C++ program will start with one process ID in Linux. Now I would like to send a message from Node JS to the C++ program using the process Id which was started.

Below is the code sample for calling C++ from Node JS

childout = cp.spawn('/home/build/eth/eth',['-i'], function(error, stdout, stderr){
   console.log("stdout :: " +stdout);
   console.log("stderror :: " +stderr);
   console.log("error :: " +error);
});

The above code is starting C++ and having process Id. Now I need to send a message using this process ID.

Can any one please guide me how to achieve this task?