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) {
    // SAFETY: There are no other threads which could be accessing `COUNTER`.
    unsafe {
        COUNTER += inc;
    }
}

fn main() {
    add_to_counter(42);

    // SAFETY: There are no other threads which could be accessing `COUNTER`.
    unsafe {
        println!("COUNTER: {COUNTER}");
    }
}
```

<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>