On Unix-like platforms? sysv shared memory (shmget, etc). https://github.com/gnzlbg/slice_deque/blob/045fb28701d3b674b... does this. (Though that code looks racy to me; unmapping the second half's virtual address space before mapping again is wrong. I guess it does that because shmat's SHM_REMAP flag is Linux-specific. At least it will fail rather than silently overwrite some other mapping. edit: oh, and it retries too, so not bad.)

Seems like you could use shm_open + mmap also.

On Linux, you could probably also do some juggling with mremap + MREMAP_DONTUNMAP, but I don't know why you'd prefer this over memfd_create.