Introduction to Tokio

Tokio is a popular helper library that actually knows how to run all those paused async tasks for you.

In this page:

Adding Tokio as a Dependency
The #[tokio::main] Attribute
What Tokio Provides
Why This Example Cannot Run Here

Adding Tokio as a Dependency

Using Tokio starts with adding it to Cargo.toml as an external dependency, since it is not part of the Rust standard library and must be fetched from crates.io.

Example: Adding Tokio as a Dependency

bash

[dependencies]
tokio = { version = "1", features = ["full"] }

⚠️ Run this command in your terminal.

The #[tokio::main] Attribute

The #[tokio::main] macro is placed above async fn main(), transforming it into a real program entry point by setting up Tokio's async runtime behind the scenes. This snippet shows the shape of such code conceptually; it needs the tokio crate and network access to actually run.

Example: The #[tokio::main] Attribute

markup

// Conceptual shape only -- requires the tokio crate (not available offline):
//
// #[tokio::main]
// async fn main() {
//     let result = say_hello().await;
//     println!("{}", result);
// }
//
// async fn say_hello() -> String {
//     String::from("Hello from an async task")
// }

fn main() {
    println!("This shows the shape of #[tokio::main] code without running it");
    println!("Real Tokio programs need the crate fetched from crates.io");
}

What Tokio Provides

Beyond just running futures, Tokio offers async-aware networking, timers, channels, and task spawning, all designed to work efficiently together under one cooperative scheduler.

Example: What Tokio Provides

markup

fn main() {
    let features = ["async TCP/UDP networking", "timers and sleep", "task spawning", "channels for message passing"];
    for f in features.iter() {
        println!("Tokio provides: {}", f);
    }
}

Why This Example Cannot Run Here

Because this course's code runner has no network access to download crates from crates.io, an actual Tokio-based program cannot be compiled or executed in this environment -- only plain standard-library Rust can run here.

Example: Why This Example Cannot Run Here

markup

fn main() {
    println!("Real async runtimes like Tokio require fetching an external crate");
    println!("Without network access, only std-library-only Rust can compile and run here");
}

Common Mistakes

Assuming Tokio is part of the Rust standard library -- it is an external crate that must be added as a dependency via Cargo.
Forgetting #[tokio::main] is what transforms an async fn main() into a regular fn main() that sets up and runs the runtime.
Trying to run networked or timer-based async examples without network access, which will not work in an offline sandbox like this course's code runner.

Chapter Summary

Tokio is a widely used asynchronous runtime crate, added as an external dependency in Cargo.toml.
The #[tokio::main] attribute macro turns an async fn main() into a runnable program by setting up Tokio's executor.
Tokio provides async-friendly versions of I/O, timers, and task spawning built around the Future trait.
Because Tokio must be downloaded from crates.io, it requires network access and cannot run in a fully offline sandbox.

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← Previous Lesson Async/Await Basics Next Lesson → Understanding Cargo.toml

Introduction to Tokio

Adding Tokio as a Dependency

The #[tokio::main] Attribute

What Tokio Provides

Why This Example Cannot Run Here

Chapter Quiz — Complete all 6 topics to unlock

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