//! The Tokio runtime.
//!
//! Unlike other Rust programs, asynchronous applications require runtime
//! support. In particular, the following runtime services are necessary:
//!
//! * An **I/O event loop**, called the driver, which drives I/O resources and
//! dispatches I/O events to tasks that depend on them.
//! * A **scheduler** to execute [tasks] that use these I/O resources.
//! * A **timer** for scheduling work to run after a set period of time.
//!
//! Tokio's [`Runtime`] bundles all of these services as a single type, allowing
//! them to be started, shut down, and configured together. However, often it is
//! not required to configure a [`Runtime`] manually, and user may just use the
//! [`tokio::main`] attribute macro, which creates a [`Runtime`] under the hood.
//!
//! # Usage
//!
//! When no fine tuning is required, the [`tokio::main`] attribute macro can be
//! used.
//!
//! ```no_run
//! use tokio::net::TcpListener;
//! use tokio::prelude::*;
//!
//! #[tokio::main]
//! async fn main() -> Result<(), Box<dyn std::error::Error>> {
//! let listener = TcpListener::bind("127.0.0.1:8080").await?;
//!
//! loop {
//! let (mut socket, _) = listener.accept().await?;
//!
//! tokio::spawn(async move {
//! let mut buf = [0; 1024];
//!
//! // In a loop, read data from the socket and write the data back.
//! loop {
//! let n = match socket.read(&mut buf).await {
//! // socket closed
//! Ok(n) if n == 0 => return,
//! Ok(n) => n,
//! Err(e) => {
//! println!("failed to read from socket; err = {:?}", e);
//! return;
//! }
//! };
//!
//! // Write the data back
//! if let Err(e) = socket.write_all(&buf[0..n]).await {
//! println!("failed to write to socket; err = {:?}", e);
//! return;
//! }
//! }
//! });
//! }
//! }
//! ```
//!
//! From within the context of the runtime, additional tasks are spawned using
//! the [`tokio::spawn`] function. Futures spawned using this function will be
//! executed on the same thread pool used by the [`Runtime`].
//!
//! A [`Runtime`] instance can also be used directly.
//!
//! ```no_run
//! use tokio::net::TcpListener;
//! use tokio::prelude::*;
//! use tokio::runtime::Runtime;
//!
//! fn main() -> Result<(), Box<dyn std::error::Error>> {
//! // Create the runtime
//! let rt = Runtime::new()?;
//!
//! // Spawn the root task
//! rt.block_on(async {
//! let listener = TcpListener::bind("127.0.0.1:8080").await?;
//!
//! loop {
//! let (mut socket, _) = listener.accept().await?;
//!
//! tokio::spawn(async move {
//! let mut buf = [0; 1024];
//!
//! // In a loop, read data from the socket and write the data back.
//! loop {
//! let n = match socket.read(&mut buf).await {
//! // socket closed
//! Ok(n) if n == 0 => return,
//! Ok(n) => n,
//! Err(e) => {
//! println!("failed to read from socket; err = {:?}", e);
//! return;
//! }
//! };
//!
//! // Write the data back
//! if let Err(e) = socket.write_all(&buf[0..n]).await {
//! println!("failed to write to socket; err = {:?}", e);
//! return;
//! }
//! }
//! });
//! }
//! })
//! }
//! ```
//!
//! ## Runtime Configurations
//!
//! Tokio provides multiple task scheduling strategies, suitable for different
//! applications. The [runtime builder] or `#[tokio::main]` attribute may be
//! used to select which scheduler to use.
//!
//! #### Multi-Thread Scheduler
//!
//! The multi-thread scheduler executes futures on a _thread pool_, using a
//! work-stealing strategy. By default, it will start a worker thread for each
//! CPU core available on the system. This tends to be the ideal configurations
//! for most applications. The multi-thread scheduler requires the `rt-multi-thread`
//! feature flag, and is selected by default:
//! ```
//! use tokio::runtime;
//!
//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
//! let threaded_rt = runtime::Runtime::new()?;
//! # Ok(()) }
//! ```
//!
//! Most applications should use the multi-thread scheduler, except in some
//! niche use-cases, such as when running only a single thread is required.
//!
//! #### Current-Thread Scheduler
//!
//! The current-thread scheduler provides a _single-threaded_ future executor.
//! All tasks will be created and executed on the current thread. This requires
//! the `rt-core` feature flag.
//! ```
//! use tokio::runtime;
//!
//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
//! let basic_rt = runtime::Builder::new_current_thread()
//! .build()?;
//! # Ok(()) }
//! ```
//!
//! #### Resource drivers
//!
//! When configuring a runtime by hand, no resource drivers are enabled by
//! default. In this case, attempting to use networking types or time types will
//! fail. In order to enable these types, the resource drivers must be enabled.
//! This is done with [`Builder::enable_io`] and [`Builder::enable_time`]. As a
//! shorthand, [`Builder::enable_all`] enables both resource drivers.
//!
//! ## Lifetime of spawned threads
//!
//! The runtime may spawn threads depending on its configuration and usage. The
//! multi-thread scheduler spawns threads to schedule tasks and for `spawn_blocking`
//! calls.
//!
//! While the `Runtime` is active, threads may shutdown after periods of being
//! idle. Once `Runtime` is dropped, all runtime threads are forcibly shutdown.
//! Any tasks that have not yet completed will be dropped.
//!
//! [tasks]: crate::task
//! [`Runtime`]: Runtime
//! [`tokio::spawn`]: crate::spawn
//! [`tokio::main`]: ../attr.main.html
//! [runtime builder]: crate::runtime::Builder
//! [`Runtime::new`]: crate::runtime::Runtime::new
//! [`Builder::basic_scheduler`]: crate::runtime::Builder::basic_scheduler
//! [`Builder::threaded_scheduler`]: crate::runtime::Builder::threaded_scheduler
//! [`Builder::enable_io`]: crate::runtime::Builder::enable_io
//! [`Builder::enable_time`]: crate::runtime::Builder::enable_time
//! [`Builder::enable_all`]: crate::runtime::Builder::enable_all
// At the top due to macros
pub
pub
cfg_rt!
cfg_rt_multi_thread!
cfg_rt_multi_thread!
cfg_rt!