[dundarious]

FixedBufferAllocator is just a "bump allocator", i.e., just a size and a pointer that moves forward. You allocate the memory it uses from somewhere else (stack, a big alloc from a general purpose allocator (think malloc), some pages from the OS via mmap or VirtualAlloc, etc. -- zig has simple cross platform functions for all of these).

Individual "allocations" just bump the pointer forward, and it succeeds unless the result is past the end of the fixed size given to it initially. You don't free any individual allocations (except maybe the most recent one by just moving the pointer back again), you just reset the pointer to the very start when you're done with all the allocations.