Read stdout of spawned process that is waiting for input

Question

I have a Swift program that logs info to the stdout while waiting for a \n (that terminates the execution). It requests the input immediately after start running and logs info 1~2 seconds after:

fetchAndLogDataInBackground(); // will print some data in ~1 sec

readLine();

I'm spawning/execFileing it with the following:

const spawn = require('child_process').spawn;

const process = spawn('swift/main');
process.stdout.on('data', data => console.log(data.toString()));

setTimeout(() => {
    process.stdin.write('\n');
}, 5000);

I was expecting to see the logs "live" since I'm using on('data'), but they are only being processed after the process.stdin.write('\n');

There's any way to get the data "live"?

PS: if I run the Swift program in terminal (swift/main), it works as expected.

Answer 1

You may have to either disable stdout buffering or manually flush stdout in Swift.

To manually flush:

fflush(__stdoutp)

To disable output buffering:

setbuf(__stdoutp, nil);