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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="software-tasks--spawn"><a class="header" href="#software-tasks--spawn">Software tasks & spawn</a></h1>
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<p>The RTIC concept of a software task shares a lot with that of <a href="./hardware_tasks.html">hardware tasks</a>. The core difference is that a software task is not explicitly bound to a specific interrupt vector, but rather bound to a “dispatcher” interrupt vector running at the intended priority of the software task (see below).</p>
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<p>Similarly to <em>hardware</em> tasks, the <code>#[task]</code> attribute used on a function declare it as a task. The absence of a <code>binds = InterruptName</code> argument to the attribute declares the function as a <em>software task</em>.</p>
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<p>The static method <code>task_name::spawn()</code> spawns (starts) a software task and given that there are no higher priority tasks running the task will start executing directly.</p>
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<p>The <em>software</em> task itself is given as an <code>async</code> Rust function, which allows the user to optionally <code>await</code> future events. This allows to blend reactive programming (by means of <em>hardware</em> tasks) with sequential programming (by means of <em>software</em> tasks).</p>
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<p>While <em>hardware</em> tasks are assumed to run-to-completion (and return), <em>software</em> tasks may be started (<code>spawned</code>) once and run forever, on the condition that any loop (execution path) is broken by at least one <code>await</code> (yielding operation).</p>
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<h2 id="dispatchers"><a class="header" href="#dispatchers">Dispatchers</a></h2>
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<p>All <em>software</em> tasks at the same priority level share an interrupt handler acting as an async executor dispatching the software tasks. This list of dispatchers, <code>dispatchers = [FreeInterrupt1, FreeInterrupt2, ...]</code> is an argument to the <code>#[app]</code> attribute, where you define the set of free and usable interrupts.</p>
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<p>Each interrupt vector acting as dispatcher gets assigned to one priority level meaning that the list of dispatchers need to cover all priority levels used by software tasks.</p>
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<p>Example: The <code>dispatchers =</code> argument needs to have at least 3 entries for an application using three different priorities for software tasks.</p>
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<p>The framework will give a compilation error if there are not enough dispatchers provided, or if a clash occurs between the list of dispatchers and interrupts bound to <em>hardware</em> tasks.</p>
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<p>See the following example:</p>
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<pre><code class="language-rust noplayground">//! examples/spawn.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])]
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mod app {
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use cortex_m_semihosting::{debug, hprintln};
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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(_: init::Context) -> (Shared, Local) {
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hprintln!("init");
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foo::spawn().unwrap();
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(Shared {}, Local {})
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}
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#[task]
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async fn foo(_: foo::Context) {
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hprintln!("foo");
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debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
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}
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}</code></pre>
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<pre><code class="language-console">$ cargo xtask qemu --verbose --example spawn
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</code></pre>
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<pre><code class="language-console">init
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foo
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</code></pre>
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<p>You may <code>spawn</code> a <em>software</em> task again, given that it has run-to-completion (returned).</p>
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<p>In the below example, we <code>spawn</code> the <em>software</em> task <code>foo</code> from the <code>idle</code> task. Since the priority of the <em>software</em> task is 1 (higher than <code>idle</code>), the dispatcher will execute <code>foo</code> (preempting <code>idle</code>). Since <code>foo</code> runs-to-completion. It is ok to <code>spawn</code> the <code>foo</code> task again.</p>
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<p>Technically the async executor will <code>poll</code> the <code>foo</code> <em>future</em> which in this case leaves the <em>future</em> in a <em>completed</em> state.</p>
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<pre><code class="language-rust noplayground">//! examples/spawn_loop.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])]
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mod app {
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use cortex_m_semihosting::{debug, hprintln};
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|
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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(_: init::Context) -> (Shared, Local) {
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hprintln!("init");
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(Shared {}, Local {})
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}
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#[idle]
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fn idle(_: idle::Context) -> ! {
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for _ in 0..3 {
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foo::spawn().unwrap();
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hprintln!("idle");
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}
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debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
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loop {}
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}
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#[task(priority = 1)]
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async fn foo(_: foo::Context) {
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hprintln!("foo");
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}
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}</code></pre>
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<pre><code class="language-console">$ cargo xtask qemu --verbose --example spawn_loop
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</code></pre>
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<pre><code class="language-console">init
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foo
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idle
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foo
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idle
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foo
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idle
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</code></pre>
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<p>An attempt to <code>spawn</code> an already spawned task (running) task will result in an error. Notice, the that the error is reported before the <code>foo</code> task is actually run. This is since, the actual execution of the <em>software</em> task is handled by the dispatcher interrupt (<code>SSIO</code>), which is not enabled until we exit the <code>init</code> task. (Remember, <code>init</code> runs in a critical section, i.e. all interrupts being disabled.)</p>
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<p>Technically, a <code>spawn</code> to a <em>future</em> that is not in <em>completed</em> state is considered an error.</p>
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<pre><code class="language-rust noplayground">//! examples/spawn_err.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])]
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mod app {
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use cortex_m_semihosting::{debug, hprintln};
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|
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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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|
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#[init]
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fn init(_: init::Context) -> (Shared, Local) {
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hprintln!("init");
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foo::spawn().unwrap();
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match foo::spawn() {
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Ok(_) => {}
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Err(()) => hprintln!("Cannot spawn a spawned (running) task!"),
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}
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(Shared {}, Local {})
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}
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#[task]
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async fn foo(_: foo::Context) {
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hprintln!("foo");
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debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
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}
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}</code></pre>
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<pre><code class="language-console">$ cargo xtask qemu --verbose --example spawn_err
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</code></pre>
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<pre><code class="language-console">init
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Cannot spawn a spawned (running) task!
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|
foo
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</code></pre>
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<h2 id="passing-arguments"><a class="header" href="#passing-arguments">Passing arguments</a></h2>
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<p>You can also pass arguments at spawn as follows.</p>
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<pre><code class="language-rust noplayground">//! examples/spawn_arguments.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)]
|
|
|
|
use panic_semihosting as _;
|
|
|
|
#[rtic::app(device = lm3s6965, dispatchers = [SSI0])]
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|
mod app {
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|
use cortex_m_semihosting::{debug, hprintln};
|
|
|
|
#[shared]
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|
struct Shared {}
|
|
|
|
#[local]
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|
struct Local {}
|
|
|
|
#[init]
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|
fn init(_: init::Context) -> (Shared, Local) {
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|
foo::spawn(1, 1).unwrap();
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|
assert!(foo::spawn(1, 4).is_err()); // The capacity of `foo` is reached
|
|
|
|
(Shared {}, Local {})
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|
}
|
|
|
|
#[task]
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async fn foo(_c: foo::Context, x: i32, y: u32) {
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|
hprintln!("foo {}, {}", x, y);
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|
debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
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|
}
|
|
}</code></pre>
|
|
<pre><code class="language-console">$ cargo xtask qemu --verbose --example spawn_arguments
|
|
</code></pre>
|
|
<pre><code class="language-console">foo 1, 1
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|
</code></pre>
|
|
<h2 id="priority-zero-tasks"><a class="header" href="#priority-zero-tasks">Priority zero tasks</a></h2>
|
|
<p>In RTIC tasks run preemptively to each other, with priority zero (0) the lowest priority. You can use priority zero tasks for background work, without any strict real-time requirements.</p>
|
|
<p>Conceptually, one can see such tasks as running in the <code>main</code> thread of the application, thus the resources associated are not required the <a href="https://doc.rust-lang.org/nomicon/send-and-sync.html">Send</a> bound.</p>
|
|
<pre><code class="language-rust noplayground">//! examples/zero-prio-task.rs
|
|
|
|
#![no_main]
|
|
#![no_std]
|
|
#![deny(warnings)]
|
|
#![deny(unsafe_code)]
|
|
#![deny(missing_docs)]
|
|
|
|
use core::marker::PhantomData;
|
|
use panic_semihosting as _;
|
|
|
|
/// Does not impl send
|
|
pub struct NotSend {
|
|
_0: PhantomData<*const ()>,
|
|
}
|
|
|
|
#[rtic::app(device = lm3s6965, peripherals = true)]
|
|
mod app {
|
|
use super::NotSend;
|
|
use core::marker::PhantomData;
|
|
use cortex_m_semihosting::{debug, hprintln};
|
|
|
|
#[shared]
|
|
struct Shared {
|
|
x: NotSend,
|
|
}
|
|
|
|
#[local]
|
|
struct Local {
|
|
y: NotSend,
|
|
}
|
|
|
|
#[init]
|
|
fn init(_cx: init::Context) -> (Shared, Local) {
|
|
hprintln!("init");
|
|
|
|
async_task::spawn().unwrap();
|
|
async_task2::spawn().unwrap();
|
|
|
|
(
|
|
Shared {
|
|
x: NotSend { _0: PhantomData },
|
|
},
|
|
Local {
|
|
y: NotSend { _0: PhantomData },
|
|
},
|
|
)
|
|
}
|
|
|
|
#[task(priority = 0, shared = [x], local = [y])]
|
|
async fn async_task(_: async_task::Context) {
|
|
hprintln!("hello from async");
|
|
}
|
|
|
|
#[task(priority = 0, shared = [x])]
|
|
async fn async_task2(_: async_task2::Context) {
|
|
hprintln!("hello from async2");
|
|
|
|
debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
|
|
}
|
|
}</code></pre>
|
|
<pre><code class="language-console">$ cargo xtask qemu --verbose --example zero-prio-task
|
|
</code></pre>
|
|
<pre><code class="language-console">init
|
|
hello from async
|
|
hello from async2
|
|
</code></pre>
|
|
<blockquote>
|
|
<p><strong>Notice</strong>: <em>software</em> task at zero priority cannot co-exist with the [idle] task. The reason is that <code>idle</code> is running as a non-returning Rust function at priority zero. Thus there would be no way for an executor at priority zero to give control to <em>software</em> tasks at the same priority.</p>
|
|
</blockquote>
|
|
<hr />
|
|
<p>Application side safety: Technically, the RTIC framework ensures that <code>poll</code> is never executed on any <em>software</em> task with <em>completed</em> future, thus adhering to the soundness rules of async Rust.</p>
|
|
|
|
</main>
|
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