lk VS FreeRTOS-Kernel

This is based on little kernel from recollection (noted somewhere in the docs)

https://github.com/littlekernel/lk

I've worked in LK (Little Kernel), which is the basis of the Android and WebOS boot loaders, but is also a reasonable stand-alone operating system, ThreadX, which is a very lightweight RTOS, and one or two others. These were all written in C, but it was also quite a few years ago, so I suspect a good chunk of the industry has moved to C++.

You might also want to look into LK ("little kernel"). It's the basis for both the Android and WebOS bootloaders, which I've also worked on. It has multi-threading and I/O but doesn't support user processes. Might be good for embedded systems.

what about LK ?

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).