Implementing a memory allocator seems to require system calls. What happens when there's no OS? For example on an MCU like Arduino, is the memory allocator packaged along with the program data? If so and in the case of an Arduino, is the code for it available anywhere? I would be curious to see how this is implemented on an embedded system.
You could fairly easily implement a bitmapped memory allocator which hands out chunks of reserved memory. I think you could build a more complex allocator on top of that if needed. For a lot of embedded applications, you really want to avoid dynamic memory allocation though. Use mempools if you need dynamic memory, which will at least limit fragmentation.
Second this. It’s really easy to fragment memory, and for long running embedded systems it’s not uncommon for free() to be a nop. I had to implement malloc just last week to access psram, and after doing a little research decided all I needed was an offset and some byte alignment. Since I never needed to deallocate the memory, it was very simple code.