---
minutes: 5
---

# `static`

Static variables will live during the whole execution of the program, and
therefore will not move:

```rust,editable
static BANNER: &str = "Welcome to RustOS 3.14";

fn main() {
    println!("{BANNER}");
}
```

As noted in the [Rust RFC Book][1], these are not inlined upon use and have an
actual associated memory location. This is useful for unsafe and embedded code,
and the variable lives through the entirety of the program execution. When a
globally-scoped value does not have a reason to need object identity, `const` is
generally preferred.

<details>

- `static` is similar to mutable global variables in C++.
- `static` provides object identity: an address in memory and state as required
  by types with interior mutability such as `Mutex<T>`.

# More to Explore

Because `static` variables are accessible from any thread, they must be `Sync`.
Interior mutability is possible through a
[`Mutex`](https://doc.rust-lang.org/std/sync/struct.Mutex.html), atomic or
similar.

It is common to use `OnceLock` in a static as a way to support initialization on
first use. `OnceCell` is not `Sync` and thus cannot be used in this context.

Thread-local data can be created with the macro `std::thread_local`.

</details>

[1]: https://rust-lang.github.io/rfcs/0246-const-vs-static.html
