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<h1 class="menu-title">Real-Time Interrupt-driven Concurrency</h1>
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<main>
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<h1 id="tasks-with-delay"><a class="header" href="#tasks-with-delay">Tasks with delay</a></h1>
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<p>A convenient way to express miniminal timing requirements is by delaying progression.</p>
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<p>This can be achieved by instantiating a monotonic timer (for implementations, see <a href="https://github.com/rtic-rs/rtic/tree/master/rtic-monotonics"><code>rtic-monotonics</code></a>):</p>
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<pre><code class="language-rust noplayground">...
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#[init]
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fn init(cx: init::Context) -> (Shared, Local) {
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hprintln!("init");
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Mono::start(cx.core.SYST, 12_000_000);
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...</code></pre>
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<p>A <em>software</em> task can <code>await</code> the delay to expire:</p>
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<pre><code class="language-rust noplayground">#[task]
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async fn foo(_cx: foo::Context) {
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...
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Systick::delay(100.millis()).await;
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...
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}
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</code></pre>
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<details>
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<summary>A complete example</summary>
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<pre><code class="language-rust noplayground">//! examples/async-delay.rs
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#![no_main]
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#![no_std]
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#![deny(warnings)]
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#![deny(unsafe_code)]
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#![deny(missing_docs)]
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use panic_semihosting as _;
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#[rtic::app(device = lm3s6965, dispatchers = [SSI0, UART0], peripherals = true)]
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mod app {
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use cortex_m_semihosting::{debug, hprintln};
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use rtic_monotonics::systick::prelude::*;
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systick_monotonic!(Mono, 100);
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#[shared]
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struct Shared {}
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#[local]
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struct Local {}
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#[init]
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fn init(cx: init::Context) -> (Shared, Local) {
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hprintln!("init");
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Mono::start(cx.core.SYST, 12_000_000);
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foo::spawn().ok();
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bar::spawn().ok();
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baz::spawn().ok();
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(Shared {}, Local {})
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}
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#[task]
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async fn foo(_cx: foo::Context) {
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hprintln!("hello from foo");
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Mono::delay(100.millis()).await;
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hprintln!("bye from foo");
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}
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#[task]
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async fn bar(_cx: bar::Context) {
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hprintln!("hello from bar");
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Mono::delay(200.millis()).await;
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hprintln!("bye from bar");
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}
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#[task]
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async fn baz(_cx: baz::Context) {
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hprintln!("hello from baz");
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Mono::delay(300.millis()).await;
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hprintln!("bye from baz");
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debug::exit(debug::EXIT_SUCCESS);
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}
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}</code></pre>
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<pre><code class="language-console">$ cargo xtask qemu --verbose --example async-delay --features test-critical-section
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</code></pre>
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<pre><code class="language-console">init
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hello from bar
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hello from baz
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hello from foo
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bye from foo
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bye from bar
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bye from baz
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</code></pre>
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</details>
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<blockquote>
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<p>Interested in contributing new implementations of <a href="https://docs.rs/rtic-time/latest/rtic_time/trait.Monotonic.html"><code>Monotonic</code></a>, or more information about the inner workings of monotonics?
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Check out the <a href="../monotonic_impl.html">Implementing a <code>Monotonic</code></a> chapter!</p>
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</blockquote>
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<h2 id="timeout"><a class="header" href="#timeout">Timeout</a></h2>
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<p>Rust <a href="https://doc.rust-lang.org/std/future/trait.Future.html"><code>Future</code></a>s (underlying Rust <code>async</code>/<code>await</code>) are composable. This makes it possible to <code>select</code> in between <code>Futures</code> that have completed.</p>
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<p>A common use case is transactions with an associated timeout. In the examples shown below, we introduce a fake HAL device that performs some transaction. We have modelled the time it takes based on the input parameter (<code>n</code>) as <code>350ms + n * 100ms</code>.</p>
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<p>Using the <code>select_biased</code> macro from the <code>futures</code> crate it may look like this:</p>
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<pre><code class="language-rust noplayground noplayground"> // Call hal with short relative timeout using `select_biased`
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select_biased! {
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v = hal_get(1).fuse() => hprintln!("hal returned {}", v),
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_ = Mono::delay(200.millis()).fuse() => hprintln!("timeout", ), // this will finish first
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}
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// Call hal with long relative timeout using `select_biased`
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select_biased! {
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v = hal_get(1).fuse() => hprintln!("hal returned {}", v), // hal finish first
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_ = Mono::delay(1000.millis()).fuse() => hprintln!("timeout", ),
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}</code></pre>
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<p>Assuming the <code>hal_get</code> will take 450ms to finish, a short timeout of 200ms will expire before <code>hal_get</code> can complete.</p>
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<p>Extending the timeout to 1000ms would cause <code>hal_get</code> will to complete first.</p>
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<p>Using <code>select_biased</code> any number of futures can be combined, so its very powerful. However, as the timeout pattern is frequently used, more ergonomic support is baked into RTIC, provided by the <a href="https://github.com/rtic-rs/rtic/tree/master/rtic-monotonics"><code>rtic-monotonics</code></a> and <a href="https://github.com/rtic-rs/rtic/tree/master/rtic-time"><code>rtic-time</code></a> crates.</p>
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<p>Rewriting the second example from above using <code>timeout_after</code> gives:</p>
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<pre><code class="language-rust noplayground"> // get the current time instance
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let mut instant = Mono::now();
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// do this 3 times
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for n in 0..3 {
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// absolute point in time without drift
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instant += 1000.millis();
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Mono::delay_until(instant).await;
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// absolute point in time for timeout
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let timeout = instant + 500.millis();
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hprintln!("now is {:?}, timeout at {:?}", Mono::now(), timeout);
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match Mono::timeout_at(timeout, hal_get(n)).await {
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Ok(v) => hprintln!("hal returned {} at time {:?}", v, Mono::now()),
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_ => hprintln!("timeout"),
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}
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}</code></pre>
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<p>In cases where you want exact control over time without drift we can use exact points in time using <code>Instant</code>, and spans of time using <code>Duration</code>. Operations on the <code>Instant</code> and <code>Duration</code> types come from the [<code>fugit</code>] crate.</p>
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<p><code>let mut instant = Systick::now()</code> sets the starting time of execution.</p>
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<p>We want to call <code>hal_get</code> after 1000ms relative to this starting time. This can be accomplished by using <code>Systick::delay_until(instant).await</code>.</p>
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<p>Then, we define a point in time called <code>timeout</code>, and call <code>Systick::timeout_at(timeout, hal_get(n)).await</code>.</p>
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<p>For the first iteration of the loop, with <code>n == 0</code>, the <code>hal_get</code> will take 350ms (and finishes before the timeout).</p>
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<p>For the second iteration, with <code>n == 1</code>, the <code>hal_get</code> will take 450ms (and again succeeds to finish before the timeout).</p>
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<p>For the third iteration, with <code>n == 2</code>, <code>hal_get</code> will take 550ms to finish, in which case we will run into a timeout.</p>
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<details>
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<summary>A complete example</summary>
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<pre><code class="language-rust noplayground">//! examples/async-timeout.rs
|
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#![no_main]
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#![no_std]
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#![deny(warnings)]
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#![deny(unsafe_code)]
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#![deny(missing_docs)]
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|
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use cortex_m_semihosting::{debug, hprintln};
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use panic_semihosting as _;
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use rtic_monotonics::systick::prelude::*;
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systick_monotonic!(Mono, 100);
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#[rtic::app(device = lm3s6965, dispatchers = [SSI0, UART0], peripherals = true)]
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mod app {
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use super::*;
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use futures::{future::FutureExt, select_biased};
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#[shared]
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struct Shared {}
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#[local]
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struct Local {}
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// ANCHOR: init
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#[init]
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fn init(cx: init::Context) -> (Shared, Local) {
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hprintln!("init");
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Mono::start(cx.core.SYST, 12_000_000);
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// ANCHOR_END: init
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foo::spawn().ok();
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(Shared {}, Local {})
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}
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|
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#[task]
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async fn foo(_cx: foo::Context) {
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// ANCHOR: select_biased
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// Call hal with short relative timeout using `select_biased`
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select_biased! {
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v = hal_get(1).fuse() => hprintln!("hal returned {}", v),
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_ = Mono::delay(200.millis()).fuse() => hprintln!("timeout", ), // this will finish first
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}
|
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|
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// Call hal with long relative timeout using `select_biased`
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select_biased! {
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v = hal_get(1).fuse() => hprintln!("hal returned {}", v), // hal finish first
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_ = Mono::delay(1000.millis()).fuse() => hprintln!("timeout", ),
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}
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// ANCHOR_END: select_biased
|
|
|
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// ANCHOR: timeout_after_basic
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// Call hal with long relative timeout using monotonic `timeout_after`
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match Mono::timeout_after(1000.millis(), hal_get(1)).await {
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Ok(v) => hprintln!("hal returned {}", v),
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_ => hprintln!("timeout"),
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}
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// ANCHOR_END: timeout_after_basic
|
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|
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// ANCHOR: timeout_at_basic
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// get the current time instance
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let mut instant = Mono::now();
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// do this 3 times
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for n in 0..3 {
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// absolute point in time without drift
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instant += 1000.millis();
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Mono::delay_until(instant).await;
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|
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// absolute point in time for timeout
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let timeout = instant + 500.millis();
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hprintln!("now is {:?}, timeout at {:?}", Mono::now(), timeout);
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match Mono::timeout_at(timeout, hal_get(n)).await {
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Ok(v) => hprintln!("hal returned {} at time {:?}", v, Mono::now()),
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_ => hprintln!("timeout"),
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}
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}
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// ANCHOR_END: timeout_at_basic
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debug::exit(debug::EXIT_SUCCESS);
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}
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}
|
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|
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// Emulate some hal
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async fn hal_get(n: u32) -> u32 {
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// emulate some delay time dependent on n
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let d = 350.millis() + n * 100.millis();
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hprintln!("the hal takes a duration of {:?}", d);
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Mono::delay(d).await;
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// emulate some return value
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5
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}</code></pre>
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<pre><code class="language-console">$ cargo xtask qemu --verbose --example async-timeout --features test-critical-section
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</code></pre>
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<pre><code class="language-console">init
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the hal takes a duration of Duration { ticks: 45 }
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timeout
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the hal takes a duration of Duration { ticks: 45 }
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hal returned 5
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the hal takes a duration of Duration { ticks: 45 }
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hal returned 5
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now is Instant { ticks: 213 }, timeout at Instant { ticks: 263 }
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the hal takes a duration of Duration { ticks: 35 }
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hal returned 5 at time Instant { ticks: 249 }
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now is Instant { ticks: 313 }, timeout at Instant { ticks: 363 }
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the hal takes a duration of Duration { ticks: 45 }
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hal returned 5 at time Instant { ticks: 359 }
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now is Instant { ticks: 413 }, timeout at Instant { ticks: 463 }
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|
the hal takes a duration of Duration { ticks: 55 }
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|
timeout
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|
</code></pre>
|
|
</details>
|
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</main>
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