lilos: A minimal async RTOS

This is a wee RTOS written to support the async style of programming in Rust. It's a research project, a playground, and a proof of concept.

(lilos 1 was a wee RTOS written to support multithreaded message-passing programs on AVR; this is technically lilos 2 but who's counting.)

About

lilos is unusual for an RTOS:

• async-based cooperative multitasking
• No dynamic memory allocation
• Low memory consumption
• Applications can be written entirely in safe code

Concrete example: code to blink an LED on the STM32F407's pin D13. (Note that a real application also needs a main, this just shows the one task.)

```rust use core::time::Duration; use os::exec::sleep_for; use stm32f4xx::stm32f407::GPIOD;

async fn blinkytask(gpiod: &GPIOD) -> ! { let period = Duration::frommillis(500); loop { gpiod.bsrr.write(|w| w.bs13().setbit()); sleepfor(period).await; gpiod.bsrr.write(|w| w.br13().setbit()); sleepfor(period).await; } } ```

For complete worked examples, see the examples directory, particularly:

• examples/stm32f4/minimal is the simplest LED blinky program.
• examples/stm32f4/blinky is a fancier blinky program.
• examples/stm32f4/uart_echo does UART I/O across tasks with a queue.

If you'd like to read about this at length, see the technical report.

Features

By default, lilos includes the core OS and some useful data structures for inter-task communication. You can turn those off with --no-default-features and go a la carte:

• mutex enables the lilos::mutex module.
• spsc enables the lilos::spsc module.

Toolchain version

As of 0.3.0, lilos builds on stable Rust. Previously, lilos required the nightly toolchain to get access to the never type, which is necessary to write the type of a future that loops forever, Future<Output = !>. When I initially published lilos in 2019 this looked like it would stabilize in the near future; however, true to its name, it's never made it.

And so in the interest of compatibility with stable, lilos uses core::convert::Infallible in place of !. So, your task functions need to be written as

rust async fn my_task(whatever: Args) -> Infallible

...instead of -> !.

Contact and License

If you have questions, or you use it for something, I'd love to find out! Send me an email.

I'm experimenting with using the MPL-2 for subversive reasons. I'm open to relicensing this code if MPL-2 doesn't work for your organization, we'll just need to discuss your labor practices. ;-)