Does aysnc fn gets called without await?

help
1

Say I have:

use tokio::net::TcpListener;
let listener = TcpListener::bind("127.0.0.1:6142"); // line 2
// sleep 100s
listener.await.unwrap(); // line 4

Does the listener start to listen on "127.0.0.1:6142" since line 2 or line 4?
I vaguely remember future does not do any work unless polled. And I assume await is the syntax sugar for polling. So is the correct answer "line 4"?

2

Yes, nothing happens until line four. You should however be aware that you can't tell without looking at the signature of the function. For example, I might define a function like this:

fn foo(arg: &str) -> impl Future<Output = u32> {
    println!("Before await: {}", arg);
    async {
        println!("After await.");
        10
    }
}

playground with main

This function will first print something, and then create a future that will not run until it is awaited. That said, a function created with the async keyword will implicitly be an -> impl Future function with an async block around the entire body, so those can never do anything until polled.

Therefore, since bind has the signature

pub async fn bind<A: ToSocketAddrs>(addr: A) -> Result<TcpListener>

it cannot do anything until awaited.

2 Likes
3

That's a really useful detail! Thanks!

4

On a single thread program this is definitely true, due to there is only one control flow.
What about two threads where the other thread called await on another unrelated future?

5

Multiple threads do not really have anything to do with this. Futures do nothing until polled.

6

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