In the past I've abused virtual memory systems to block off a bunch of pages after my array. This lets you use an array data structure, have guard pages to prevent out of bounds access, and to have stable pointers in the data structure.
And, if you find you didn't reserve enough address space, Linux has mremap() which can grow the reserved region. Or map the region to two places at once (the original place and a new, larger place).
The space I needed was too large to be added to the heap, so I used mmap. Because of the nature of the processing (mmap, process, yeet mmap) I put the system under a lot of pressure. Maintaining the set of mapped blocks and reusing them around fixed the issue.
I ended up in this situation like I end up in most, by just learning as I go.
With this I learned that the libc allocator has a max amount that it'll add to the heap, and how mmap works, and that doing frequent munmap is slow.
Now I learned why it is slow!