Using the Rust standard library with the NuttX RTOS

> non-cooperative processes existing on the same device

That sounds like a real-world problem you could avoid by design. Is that because of 3rd-party vendoring?

> Soemtimes it's to get a collection of higher level features, like an allocator, file system etc.

I can relate to those needs, but I’m still unsure why not expose those in your runtime?

Is embassy-rs with enough interaction between tasks an RTOS? Do you need an external API to be an OS? Do you need specific scheduling rules, or is it enough to know that certain tasks will never be blocked arbitrary long time?

The classic role of an RTOS (realtime operating system) is to run applications that are, well, realtime. Like the anti-lock brake controller in your car, or the fly-by-wire system in the F-117A that compensates for its natural aerodynamic instability, or the motor controller for a 2-ton industrial robot that's whizzing around at many meters per second, or a pacemaker that's adjusting your heart rhythm, or the controller for some piece of a bazillion-dollar ASML EUV lithography system. These are applications where a failure to finish your computations on time has Majorly Bad Consequences.

In these kinds of applications, you need hard realtime guarantees, which usually means things like:

- everything is preemptive

- lock order is very explicitly defined

- memory is statically allocated

- every computation (including interrupts) has a bounded (and guaranteed, and often formally proven) processing time

Embedded async Rust (AFAIK) isn't suitable for building applications like these.

Having said that, the meaning of "RTOS" has shifted over time to something like what you're implying: a collection of primitives that's slimmer than a "real" operating system that allows running an application on embedded hardware. And in that sense, embedded async Rust is a viable option.

[1] https://en.wikipedia.org/wiki/Lockheed_F-117_Nighthawk#:~:te...

Async Rust (by default) can allocate memory, which is (almost always) nondeterministic. I'm not sure how hard it is to guarantee that nothing is using it so that all allocations happen deterministically.

Async Rust schedules (mostly) cooperatively, which means one errant task, or too many tasks, can block higher-priority, time-critical work from occurring. Embassy has an interrupt executor, which helps somewhat, but it essentially ties each task to a dedicated interrupt, and AFAICT requires that all high-priority work must happen in interrupt context. Usually you want to stay in an ISR for the absolute shortest possible time, and not be forced to choose between either running in an ISR or missing a deadline.

I'm not saying it's impossible to build a hard realtime system in async Rust if you're really careful, just that it doesn't provide the usual set of primitives that allow you to easily reason about worst-case execution time, which is a primary concern for such systems.

For me—as someone who has written embedded code professionally, but I'm just some guy, so take this with a grain of salt—the line between RTOS and runtime is whether it has a preemptive scheduler and affordances (usually prioritized tasks with support for handling priority inversion) that allow you to easily prioritize different tasks and ensure bounded worst-case runtime. It almost always also comes with synchronization primitives, like mutexes, mailboxes, queues, etc. All facilities provided by the RTOS need to either guarantee (and document) that they run in bounded time, or be easy to identify and avoid if you need bounded worst-case time.

> Async Rust (by default) can allocate memory

Strictly speaking, async rust never allocates. In fact, no language features do; allocation is a library concern in Rust.

What does allocate memory are the executors created for non-embedded environments. But just like any user-written code, you can use different implementations that do something else instead. Like you’d imagine, ones for embedded environments would use some statically allocated memory that’s bounded in size. Heck, you can go so far as to just leave them on the stack, like lilos: https://github.com/cbiffle/lilos/blob/main/doc/intro.adoc#us...

Not trying to say this means that you’re incorrect about any of the other things you’re talking about; I haven’t worked on realtime systems so I can’t speak on them.