Attribute Macro tokio::main

#[main]

Marks async function to be executed by selected runtime. This macro helps set up a Runtime without requiring the user to use Runtime or Builder directly.

Options:

If you want to set the number of worker threads used for asynchronous code, use the core_threads option.

• core_threads=n - Sets core threads to n (requires rt-threaded feature).
• max_threads=n - Sets max threads to n (requires rt-core or rt-threaded feature).
• basic_scheduler - Use the basic schduler (requires rt-core).

Function arguments:

Arguments are allowed for any functions aside from main which is special

Usage

Using default

#[tokio::main]
async fn main() {
    println!("Hello world");
}

Equivalent code not using #[tokio::main]

fn main() {
    tokio::runtime::Builder::new()
        .threaded_scheduler()
        .enable_all()
        .build()
        .unwrap()
        .block_on(async {
            println!("Hello world");
        })
}

Using basic scheduler

The basic scheduler is single-threaded.

#[tokio::main(basic_scheduler)]
async fn main() {
    println!("Hello world");
}

Equivalent code not using #[tokio::main]

fn main() {
    tokio::runtime::Builder::new()
        .basic_scheduler()
        .enable_all()
        .build()
        .unwrap()
        .block_on(async {
            println!("Hello world");
        })
}

Set number of core threads

#[tokio::main(core_threads = 2)]
async fn main() {
    println!("Hello world");
}

Equivalent code not using #[tokio::main]

fn main() {
    tokio::runtime::Builder::new()
        .threaded_scheduler()
        .core_threads(2)
        .enable_all()
        .build()
        .unwrap()
        .block_on(async {
            println!("Hello world");
        })
}

NOTE:

If you rename the tokio crate in your dependencies this macro will not work. If you must rename the 0.2 version of tokio because you’re also using the 0.1 version of tokio, you must make the tokio 0.2 crate available as tokio in the module where this macro is expanded.