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.