Statically allocated, runtime initialized cell.

Last update: Oct 2, 2022

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Overview

static-cell

Statically allocated, initialized at runtime cell.

StaticCell provides a no-std-compatible, no-alloc way to reserve memory at compile time for a value, but initialize it at runtime, and get a 'static reference to it.

This is useful in the following scenarios:

You need &'static T, but T can't be constructed in const context so you can't simply use a static.
You need &'static mut T, not just &'static T.

Example

use static_cell::StaticCell;

// Statically allocate memory for a `u32`.
static SOME_INT: StaticCell<u32> = StaticCell::new();

// Initialize it at runtime. This returns a `&'static mut`.
let x: &'static mut u32 = SOME_INT.init(42);
assert_eq!(*x, 42);

// Trying to call `.init()` again would panic, because the StaticCell is already initialized.
// SOME_INT.init(42);

Alternatives

If you can use alloc, you can use Box::leak().
If you're OK with unsafe, you can use static mut THING: MaybeUninit<T>.

Interoperability

This crate uses atomic-polyfill, so on targets without native atomics you must import a crate that provides a critical-section implementation. See the critical-section README for details.

Minimum Supported Rust Version (MSRV)

This crate is guaranteed to compile on stable Rust 1.56 and up. It might compile with older versions but that may change in any new patch release.

License

This work is licensed under either of

Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

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Comments

Allow creating initialized `StaticCell`.

Add StaticCell::new_with_value to create an initialized StaticCell which can be taken and restored.

Example

static DMA_BUF: StaticCell<[u8; 1024]> = StaticCell::new_with_value([0; 1024]);

fn start_dma_transfer(buf: &'static mut [u8; 1024]) {
  // ...
}

fn stop_dma_transfer() -> &'static mut [u8; 1024] {
  // ...
}

fn main() {
  start_dma_transfer(DMA_BUF.take().unwrap());
}

fn interrupt() {
  let buf = stop_dma_transfer();
  DMA_BUF.restore(buf);
}

Also, a question regarding init_with, which claims

The advantage over [StaticCell::init] is that this method allows the closure to construct the T value in-place directly inside the StaticCell, saving stack space.

Where/how is this guaranteed? Does it depend on the platform or compiler optimizations?

At least with the following example, it clearly doesn't work as described as it results in a stack overflow:

std::thread::Builder::new()
  .name("init_with".to_string())
  .stack_size(512)
  .spawn(|| {
    static BUF: StaticCell<[u8; 4096]> = StaticCell::new();
    let buf = BUF.init_with(|| [1; 4096]);

    let mut count = 0;
    for b in buf {
      count += *b as usize;
    }

    assert_eq!(count, 4096);
  })
  .unwrap()
  .join();

opened by reitermarkus 4

Add `StaticCell::uninit()`
Sometimes I find myself using something that could be distilled to ARRAY.init([0; BIG_NUMBER]), where

let arr = ARRAY.uninit(); unsafe { arr.as_mut_ptr().write_bytes(0x00, BIG_NUMBER); arr.assume_init_mut() }

Would guarantee no unnecessary memcpys or stack usage.
opened by zRedShift 1
Compare to once_cell

It seems once_cell is what "most people" use for this case (some use cases would also have been served by lazy_static, which would be one of the "related creates" listed on once_cell's readme). Having it as an alternative and listing the ups and downs seems practical.

opened by chrysn 1