# The `app` attribute This is the smallest possible RTIC application: ``` rust {{#include ../../../../examples/smallest.rs}} ``` `static mut` variables declared at the beginning of `init` will be transformed into `&'static mut` references that are safe to access. Notice, this feature may be deprecated in next release, see `task_local` resources. [`rtic::Peripherals`]: ../../api/rtic/struct.Peripherals.html The example below shows the types of the `core`, `device` and `cs` fields, and showcases safe access to a `static mut` variable. The `device` field is only available when the `peripherals` argument is set to `true` (default). In the rare case you want to implement an ultra-slim application you can explicitly set `peripherals` to `false`. ``` rust {{#include ../../../../examples/init.rs}} ``` Running the example will print `init` to the console and then exit the QEMU process. ``` console $ cargo run --example init {{#include ../../../../ci/expected/init.run}} ``` > **NOTE**: Remember to specify your chosen target device by passing a target > triple to cargo (e.g `cargo run --example init --target thumbv7m-none-eabi`) or > configure a device to be used by default when building the examples in `.cargo/config.toml`. > In this case, we use a Cortex M3 emulated in QEMU so the target is `thumbv7m-none-eabi`. > See [`Starting a new project`](./new.md) for more info. ## `idle` A function marked with the `idle` attribute can optionally appear in the module. This function is used as the special *idle task* and must have signature `fn(idle::Context) - > !`. When present, the runtime will execute the `idle` task after `init`. Unlike `init`, `idle` will run *with interrupts enabled* and it's not allowed to return so it must run forever. When no `idle` function is declared, the runtime sets the [SLEEPONEXIT] bit and then sends the microcontroller to sleep after running `init`. [SLEEPONEXIT]: https://developer.arm.com/docs/100737/0100/power-management/sleep-mode/sleep-on-exit-bit Like in `init`, `static mut` variables will be transformed into `&'static mut` references that are safe to access. Notice, this feature may be deprecated in the next release, see `task_local` resources. The example below shows that `idle` runs after `init`. **Note:** The `loop {}` in idle cannot be empty as this will crash the microcontroller due to LLVM compiling empty loops to an `UDF` instruction in release mode. To avoid UB, the loop needs to imply a "side-effect" by inserting an assembly instruction (e.g., `WFI`) or a `continue`. ``` rust {{#include ../../../../examples/idle.rs}} ``` ``` console $ cargo run --example idle {{#include ../../../../ci/expected/idle.run}} ``` ## Hardware tasks To declare interrupt handlers the framework provides a `#[task]` attribute that can be attached to functions. This attribute takes a `binds` argument whose value is the name of the interrupt to which the handler will be bound to; the function adorned with this attribute becomes the interrupt handler. Within the framework these type of tasks are referred to as *hardware* tasks, because they start executing in reaction to a hardware event. The example below demonstrates the use of the `#[task]` attribute to declare an interrupt handler. Like in the case of `#[init]` and `#[idle]` local `static mut` variables are safe to use within a hardware task. ``` rust {{#include ../../../../examples/hardware.rs}} ``` ``` console $ cargo run --example hardware {{#include ../../../../ci/expected/hardware.run}} ``` So far all the RTIC applications we have seen look no different than the applications one can write using only the `cortex-m-rt` crate. From this point we start introducing features unique to RTIC. ## Priorities The static priority of each handler can be declared in the `task` attribute using the `priority` argument. Tasks can have priorities in the range `1..=(1 << NVIC_PRIO_BITS)` where `NVIC_PRIO_BITS` is a constant defined in the `device` crate. When the `priority` argument is omitted, the priority is assumed to be `1`. The `idle` task has a non-configurable static priority of `0`, the lowest priority. > A higher number means a higher priority in RTIC, which is the opposite from what > Cortex-M does in the NVIC peripheral. > Explicitly, this means that number `10` has a **higher** priority than number `9`. When several tasks are ready to be executed the one with highest static priority will be executed first. Task prioritization can be observed in the following scenario: an interrupt signal arrives during the execution of a low priority task; the signal puts the higher priority task in the pending state. The difference in priority results in the higher priority task preempting the lower priority one: the execution of the lower priority task is suspended and the higher priority task is executed to completion. Once the higher priority task has terminated the lower priority task is resumed. The following example showcases the priority based scheduling of tasks. ``` rust {{#include ../../../../examples/preempt.rs}} ``` ``` console $ cargo run --example preempt {{#include ../../../../ci/expected/preempt.run}} ``` Note that the task `gpiob` does *not* preempt task `gpioc` because its priority is the *same* as `gpioc`'s. However, once `gpioc` returns, the execution of task `gpiob` is prioritized over `gpioa` due to its higher priority. `gpioa` is resumed only after `gpiob` returns. One more note about priorities: choosing a priority higher than what the device supports (that is `1 << NVIC_PRIO_BITS`) will result in a compile error. Due to limitations in the language, the error message is currently far from helpful: it will say something along the lines of "evaluation of constant value failed" and the span of the error will *not* point out to the problematic interrupt value -- we are sorry about this!