rtic/book/en/src/by-example/software_tasks.md

Software tasks & spawn

The RTIC concept of a software task shares a lot with that of hardware tasks. 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).

Similarly to hardware tasks, the #[task] attribute used on a function declare it as a task. The absence of a binds = InterruptName argument to the attribute declares the function as a software task.

The static method task_name::spawn() spawns (starts) a software task and given that there are no higher priority tasks running the task will start executing directly.

The software task itself is given as an async Rust function, which allows the user to optionally await future events. This allows to blend reactive programming (by means of hardware tasks) with sequential programming (by means of software tasks).

While hardware tasks are assumed to run-to-completion (and return), software tasks may be started (spawned) once and run forever, on the condition that any loop (execution path) is broken by at least one await (yielding operation).

Dispatchers

All software tasks at the same priority level share an interrupt handler acting as an async executor dispatching the software tasks. This list of dispatchers, dispatchers = [FreeInterrupt1, FreeInterrupt2, ...] is an argument to the #[app] attribute, where you define the set of free and usable interrupts.

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.

Example: The dispatchers = argument needs to have at least 3 entries for an application using three different priorities for software tasks.

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 hardware tasks.

See the following example:

{{#include ../../../../examples/lm3s6965/examples/spawn.rs}}

$ cargo xtask qemu --verbose --example spawn

{{#include ../../../../ci/expected/lm3s6965/spawn.run}}

You may spawn a software task again, given that it has run-to-completion (returned).

In the below example, we spawn the software task foo from the idle task. Since the priority of the software task is 1 (higher than idle), the dispatcher will execute foo (preempting idle). Since foo runs-to-completion. It is ok to spawn the foo task again.

Technically the async executor will poll the foo future which in this case leaves the future in a completed state.

{{#include ../../../../examples/lm3s6965/examples/spawn_loop.rs}}

$ cargo xtask qemu --verbose --example spawn_loop

{{#include ../../../../ci/expected/lm3s6965/spawn_loop.run}}

An attempt to spawn an already spawned task (running) task will result in an error. Notice, the that the error is reported before the foo task is actually run. This is since, the actual execution of the software task is handled by the dispatcher interrupt (SSIO), which is not enabled until we exit the init task. (Remember, init runs in a critical section, i.e. all interrupts being disabled.)

Technically, a spawn to a future that is not in completed state is considered an error.

{{#include ../../../../examples/lm3s6965/examples/spawn_err.rs}}

$ cargo xtask qemu --verbose --example spawn_err

{{#include ../../../../ci/expected/lm3s6965/spawn_err.run}}

Passing arguments

You can also pass arguments at spawn as follows.

{{#include ../../../../examples/lm3s6965/examples/spawn_arguments.rs}}

$ cargo xtask qemu --verbose --example spawn_arguments

{{#include ../../../../ci/expected/lm3s6965/spawn_arguments.run}}

Priority zero tasks

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.

Conceptually, one can see such tasks as running in the main thread of the application, thus the resources associated are not required the Send bound.

{{#include ../../../../examples/lm3s6965/examples/zero-prio-task.rs}}

$ cargo xtask qemu --verbose --example zero-prio-task

{{#include ../../../../ci/expected/lm3s6965/zero-prio-task.run}}

Notice: software task at zero priority cannot co-exist with the [idle] task. The reason is that idle 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 software tasks at the same priority.

---

Application side safety: Technically, the RTIC framework ensures that poll is never executed on any software task with completed future, thus adhering to the soundness rules of async Rust.