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# The background task `#[idle]`
A function marked with the `idle` attribute can optionally appear in the module. This becomes 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 must never return, as the `-> !` function signature indicates.
[The Rust type `!` means “never”][nevertype].
[nevertype]: https://doc.rust-lang.org/core/primitive.never.html
Like in `init`, locally declared resources will have `'static` lifetimes that are safe to access.
The example below shows that `idle` runs after `init`.
```rust,noplayground
{{#include ../../../../examples/lm3s6965/examples/idle.rs}}
```
```console
$ cargo xtask qemu --verbose --example idle
```
```console
{{#include ../../../../ci/expected/lm3s6965/idle.run}}
```
By default, the RTIC `idle` task does not try to optimize for any specific targets.
A common useful optimization is to enable the [SLEEPONEXIT] and allow the MCU to enter sleep when reaching `idle`.
> **Caution**: some hardware unless configured disables the debug unit during sleep mode.
>
> Consult your hardware specific documentation as this is outside the scope of RTIC.
The following example shows how to enable sleep by setting the
[`SLEEPONEXIT`][SLEEPONEXIT] and providing a custom `idle` task replacing the default [`nop()`][NOP] with [`wfi()`][WFI].
[SLEEPONEXIT]: https://developer.arm.com/docs/100737/0100/power-management/sleep-mode/sleep-on-exit-bit
[WFI]: https://developer.arm.com/documentation/dui0662/b/The-Cortex-M0--Instruction-Set/Miscellaneous-instructions/WFI
[NOP]: https://developer.arm.com/documentation/dui0662/b/The-Cortex-M0--Instruction-Set/Miscellaneous-instructions/NOP
```rust,noplayground
{{#include ../../../../examples/lm3s6965/examples/idle-wfi.rs}}
```
```console
$ cargo xtask qemu --verbose --example idle-wfi
```
```console
{{#include ../../../../ci/expected/lm3s6965/idle-wfi.run}}
```
> **Notice**: The `idle` task cannot be used together with _software_ tasks running at priority zero. 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.
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