# App initialization and the `#[init]` task

An RTIC application requires an `init` task setting up the system. The corresponding `init` function must have the
signature `fn(init::Context) -> (Shared, Local)`, where `Shared` and `Local` are resource structures defined by the user.

The `init` task executes after system reset, [after an optionally defined `pre-init` code section][pre-init] and an always occurring internal RTIC initialization.  [pre-init]: https://docs.rs/cortex-m-rt/latest/cortex_m_rt/attr.pre_init.html

The `init` and optional `pre-init` tasks runs *with interrupts disabled* and have exclusive access to Cortex-M (the `bare_metal::CriticalSection` token is available as `cs`).

Device specific peripherals are available through the `core` and `device` fields of `init::Context`.

## Example

The example below shows the types of the `core`, `device` and `cs` fields, and showcases the use of a `local` variable with `'static` lifetime. Such variables can be delegated from the `init` task to other tasks of the RTIC application.

The `device` field is only available when the `peripherals` argument is set to the default value `true`.
In the rare case you want to implement an ultra-slim application you can explicitly set `peripherals` to `false`.

``` rust,noplayground
{{#include ../../../../rtic/examples/init.rs}}
```

Running the example will print `init` to the console and then exit the QEMU process.

``` console
$ cargo run --target thumbv7m-none-eabi --example init
```

``` console
{{#include ../../../../rtic/ci/expected/init.run}}
```