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# 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 the resource
structures defined by the user.
The `init` task executes after system reset (after the optionally defined [pre-init] and internal RTIC
initialization). The `init` task runs *with interrupts disabled* and has exclusive access to Cortex-M (the
`bare_metal::CriticalSection` token is available as `cs`) while device specific peripherals are available through
the `core` and `device` fields of `init::Context`.
[pre-init]: https://docs.rs/cortex-m-rt/latest/cortex_m_rt/attr.pre_init.html
## 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 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
{{#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
{{#include ../../../../rtic/ci/expected/init.run}}
```
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