Mutable Static Variables

It is safe to read an immutable static variable:

1. static HELLO_WORLD: &str = "Hello, world!";

3. fn main() {
4. println!("HELLO_WORLD: {HELLO_WORLD}");
5. }

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

1. static mut COUNTER: u32 = 0;

3. fn add_to_counter(inc: u32) {
4. unsafe { COUNTER += inc; }  // Potential data race!
5. }

7. fn main() {
8. add_to_counter(42);

10. unsafe { println!("COUNTER: {COUNTER}"); }  // Potential data race!
11. }

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.