What's wrong with print! macro?

[daniiarov-alym]

I might be not understanding something, but take following code:

use tokio::net::{TcpListener, TcpStream};
use tokio::time::{sleep, Duration};
use std::io;

#[tokio::main]
async fn main() -> io::Result<()> {
    let listener = TcpListener::bind("127.0.0.1:8080").await?;

    loop {
        let (socket, address) = listener.accept().await?;

        tokio::task::spawn(async move {	
            println!("new client: {:?}", &address);	
            sleep(Duration::from_secs(5)).await;
            println!("test");
        });
    }
}

launch it, and start several connections. You will see expected output, like this

new client: 127.0.0.1:37084
test
new client: 127.0.0.1:44448
new client: 127.0.0.1:44456
test
test

everything correct, tasks processed concurrently.

But now take the same code but with one change

use tokio::net::{TcpListener, TcpStream};
use tokio::time::{sleep, Duration};
use std::io;

#[tokio::main]
async fn main() -> io::Result<()> {
    let listener = TcpListener::bind("127.0.0.1:8080").await?;

    loop {
        let (socket, address) = listener.accept().await?;

        tokio::task::spawn(async move {	
            println!("new client: {:?}", &address);	
            sleep(Duration::from_secs(5)).await;
            print!("test"); // print! macro here
        });
    }
}

and output is

new client: 127.0.0.1:37746
new client: 127.0.0.1:37758
testnew client: 127.0.0.1:50376
testtestnew client: 127.0.0.1:39636

like print! macro slept before accepting new connection or something like that. Is is valid behavior?

[Darksonn]

This is unrelated to Tokio. Generally, stdout is line buffered which means that the program doesn't actually send the bytes until you write a newline or flush stdout.

[daniiarov-alym]

Thanks! Should've looked at print! documentation.