FreeRTOS-Kernel VS makani

Here is the port - https://github.com/FreeRTOS/FreeRTOS-Kernel/tree/main/portable/ThirdParty/GCC/Posix

Freertos has support for generic cortex m3: https://github.com/FreeRTOS/FreeRTOS-Kernel/tree/main/portable/GCC/ARM_CM3 It could very well work almost out of the box with your mcu.

the only OS written in iso c i found was RTOS and they have open bug reports due to strict aliasing in simple things like their linked list implementation, which they don't bother to fix because somehow that causes even more problems (?!): https://github.com/FreeRTOS/FreeRTOS-Kernel/pull/54

You likely put the requirements of POSIX just to have an excuse. Fyi, FreeRTOS kernel, the only thing you need, is here: https://github.com/FreeRTOS/FreeRTOS-Kernel/

- https://github.com/FreeRTOS/FreeRTOS-Kernel (architecture, C)

FreeRTOS comes with 5 quite simple reference implementations of the heap memory allocator. You can study how it works.

The main branch is single-core. You're looking for the SMP branch.

On non-POSIX systems, malloc & free are implemented differently. EG FreeRTOS has some very simple malloc (and optionally free) implementations, eg heap_2.c (has malloc & free, but doesn't consolidate blocks), or heap_4.c (more complex, tries to prevent fragmentation by consolidating blocks, still less optmized & simpler than the glibc malloc/free used by most Linux systems).

- https://github.com/google/makani/ (Bazel, C, testing, architecture, real-time control, code-generation)

I'd say it's very different with X. I looked at the large number of hardware design docs that was open-sourced[1] when they shut down Makani - hell, even the Makani documentary[2] was mostly about hardware (material science, mechanics, aerodynamics) with a some software sprinkled in.

1. https://github.com/google/makani

2. https://www.youtube.com/watch?v=qd_hEja6bzE

But it's great that they released everything as open source, I didn't know that, that's awesome. Anybody wanna build a kite?