comprehensive-rust/src/unsafe-rust/mutable-static.md

---
minutes: 5
---

# Mutable Static Variables

It is safe to read an immutable static variable:

```rust,editable
static HELLO_WORLD: &str = "Hello, world!";

fn main() {
    println!("HELLO_WORLD: {HELLO_WORLD}");
}
```

However, since data races can occur, it is unsafe to read and write mutable
static variables:

```rust,editable
static mut COUNTER: u32 = 0;

fn add_to_counter(inc: u32) {
    unsafe { COUNTER += inc; }  // Potential data race!
}

fn main() {
    add_to_counter(42);

    unsafe { println!("COUNTER: {COUNTER}"); }  // Potential data race!
}
```

<details>

- The program here is safe because it is single-threaded. However, the Rust compiler is conservative
  and will assume the worst. Try removing the `unsafe` and see how the compiler explains that it is
  undefined behavior to mutate a static from multiple threads.

- Using a mutable static is generally a bad idea, but there are some cases where it might make sense
  in low-level `no_std` code, such as implementing a heap allocator or working with some C APIs.

</details>