How to Use `no_std` in Rust

How to Use no_std in Rust

Rust's std library provides a lot of handy features and abstractions that make writing Rust programs a lot easier. However, not all programs require all features from the std library, and in some cases, you may want to reduce the footprint of your code by excluding std. For example in bare-metal, embedded system, and operating system development, you might need to operate without the standard library. This can be achieved with no_std attribute in Rust.

In this guide, we'll delve into what no_std is, why you might want to use it, and how you can utilize it in your Rust code.

Table of Contents

What is no_std?

no_std is an attribute in Rust that allows you to develop software without the use of Rust's standard library (std). But what does that mean? It means that you’re working in an environment where the features provided by the OS or the standard library are either unavailable or won't work.

Why Use no_std?

There are several reasons you might want to use no_std:

  1. Lower Memory Footprint: std contains a lot of stuff you may not need for your program. By not including it, you can significantly reduce the size of your binary.

  2. Better Control: Without std, you have low-level control over what happens in your code.

  3. Compatibility with Bare-Metal Systems: Not all systems have an operating system or support all features of std. no_std is ideal for things like OS development, embedded systems, and more.

How to Declare no_std

To declare a no_std environment in your Rust code, you simply specify #![no_std] at the top of your root file. This tells Rust to not include the std library.

#![no_std]

Core Crate vs Standard Library

When using #![no_std], you're leaving out the standard library std, but not all of Rust library resources. Rust's core crate, which is a subset of std, is still available for use in a no_std environment. The core library is platform-independent and has no dependencies, thus it doesn't require an underlying operating system.

extern crate core;

Code Examples

Now let's look at a simple no_std code example:

#![no_std]

use core::panic::PanicInfo;

#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {
    loop {}
}

pub fn add(a: i32, b: i32) -> i32 {
    a + b
}

In this example, a simple add function is defined that doesn't rely on std at all.

Error Handling

One challenge when using no_std is error handling. Without the std library, we lose availability of Box<dyn std::error::Error>. The usual idiom is to declare an Error enum which can be any error in your application.

pub enum Error {
    Overflow,
    DivisionByZero,
}

Converting a std Codebase to no_std

Transforming a codebase that uses std into one that uses no_std isn't always straightforward. Here's a general guide:

  1. Declare #![no_std] at the top of your root file.
  2. Replace uses of std with core or alloc where possible. Remember that some features that are in std are not available.
  3. Ensure your build targets don't automatically include std.

Conclusion

Now you're more familiar with no_std in Rust. Do remember that while no_std code can be more difficult to write due to the loss of the std library, it gives you more control and can help you create more lean, efficient programs, especially in bare metal or embedded development environments.

While it might not be the right fit for every project, it's a powerful tool to have in your Rust toolbox and presents a unique approach for system-level programming.