embedded-trainings-2020/embedded-workshop-book/src/parts-embedded-program.md

# Parts of an Embedded Program

We will look at the elements that distinguish an embedded Rust program from a desktop program.


✅ Open the `beginner/apps` folder in VS Code.

``` console
$ # or use "File > Open Folder" in VS Code
$ code beginner/apps
```

✅ Then open the `src/bin/hello.rs` file.


## In the file, you will find the following attributes:

### `#![no_std]`

 The `#![no_std]` attribute indicates that the program will not make use of the standard library, the `std` crate. Instead it will use the `core` library, a subset of the standard library that does not depend on an underlying operating system (OS).

### `#![no_main]`

The `#![no_main]` attribute indicates that the program will use a custom entry point instead of the default `fn main() { .. }` one.

### `#[entry]`

The `#[entry]` attribute declares the custom entry point of the program. The entry point must be a divergent function whose return type is the never type `!`. The function is not allowed to return; therefore the program is not allowed to terminate.