المتغيرات الثابتة القابلة للتغيير (Mutable Static Variables)

It is safe to read an immutable static variable:

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:

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}");
    }
}
This slide should take about 5 minutes.

  • 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.