How do I declare a static mutable variable without assignment?

Question

I tried the following

struct mbuf
{
  cacheline: *mut [u64],               // great amount of rows follows below
  // ..........
}
static mut arr: [mbuf; 32];                // Q1 my main aim
// something as before but using Vec;      // Q2 also main aim

fn main() {
 // let static mut arr: [mbuf; 32];        // Q3 also doesn't work
 // static mut arr: [mbuf; 32];            // Q3 also doesn't work
}

and got error

src/main.rs:74:29: 74:30 error: expected one of `+` or `=`, found `;`
src/main.rs:74   static mut arr: [mbuf; 32];
                                           ^

Q1,Q2,Q3 - Is it possible and how?

Well surely you want to use the variables, and surely you don't want to leave them uninitialized. So what do you want? — user395760
– user395760, Commented May 9, 2015 at 23:24
I want to initialize and work with such variables in main. I rewrite С application to check it's speed in rust. So I need to have similar architecture design. Also I am beginner in rust :) — maki
– maki, Commented May 10, 2015 at 8:09

Chris Morgan · Accepted Answer · 2015-05-09 23:29:42Z

9

A static or constant must be assigned when declared; they can never be assigned to after that.

A static must be purely literals; it cannot have any function calls.

A constant must at present be purely literals, but when RFC 911, const fn is implemented it will be possible to do things more like how you desire.

Inside a function, you can have static or const items, just as outside a function, and there is no difference—placing items (trait and type definitions, functions, &c.) inside a function purely hides them from the outside scope. Therefore you typically might as well use let foo.

answered May 9, 2015 at 23:29

Chris Morgan

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1 Comment

dcorking Over a year ago

static variables can be mutable although they need to be marked unsafe. rustbyexample.com/custom_types/constants.html says "static: A possibly mutable variable with 'static lifetime."

ArtemGr · Accepted Answer · 2020-04-18 07:44:50Z

You can use lazy-static to initialize the static array on first access, even though it might incur a minimal overhead (it seems to invoke Once::call_once every time you access the static variable).

For example, Cargo.toml:

[package]
name = "arr"
version = "0.0.1"

[[bin]]
name = "arr"
path = "arr.rs"

[dependencies]
lazy_static = "*"

arr.rs:

#[macro_use]
extern crate lazy_static;
use std::mem;
use std::ptr;

#[derive(Debug)]
struct Mbuf {
    cacheline: *mut u64,
}
// Let's pretend it's thread-safe to appease the lazy_static! constrains.
unsafe impl Sync for Mbuf { }

lazy_static! {
    static ref ARR: [Mbuf; 32] = {
        let mut tmp: [Mbuf; 32] =  = unsafe { mem::MaybeUninit::uninit().assume_init() };
        for idx in 0..tmp.len() {
            tmp[idx] = Mbuf { cacheline: ptr::null_mut() };
        }
        tmp
    };
}

fn main() {
    println!("{:?}", *ARR);
}

Alternatively, just make your own lazy accessor:

use std::mem;
use std::ptr;

#[derive(Debug)]
struct Mbuf {
    cacheline: *mut u64,
}

static mut ARR: Option<[Mbuf; 32]> = None;
fn arr() -> &'static mut [Mbuf; 32] {
    unsafe {
        if ARR.is_none() {
            let mut tmp: [Mbuf; 32] = mem::MaybeUninit::uninit().assume_init();
            for idx in 0..tmp.len() {
                tmp[idx] = Mbuf { cacheline: ptr::null_mut() };
            }
            ARR = Some(tmp);
        }
        mem::transmute(ARR.as_mut().unwrap())
    }
}

fn main() {
    println!("{:?}", arr());
}

Needless to say, this code isn't thread-safe and thus shuns some of the Rust safety guarantees, but for a speed comparison port it shall suffice.

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