Is there really no mremap in Darwin?

If you need to shrink the map, just munmap the part at the end you want to remove.

If you need to enlarge the map, you can mmap the proper offset with MAP_FIXED to the addresses just above the old map, but you need to be careful that you don't map over something else that's already there...

The above text under strikeout is an awful idea; MAP_FIXED is fundamentally wrong unless you already know what's at the target address and want to atomically replace it. If you are trying to opportunistically map something new if the address range is free, you need to use mmap with a requested address but without MAP_FIXED and see if it succeeds and gives you the requested address; if it succeeds but with a different address you'll want to unmap the new mapping you just created and assume that allocation at the requested address was not possible.

If you expand in large enough chunks (say, 64 MB, but it depends on how fast it grows) then the cost of invalidating the old map is negligible. As always, benchmark before assuming a problem.

You can ftruncate the file to a large size (creating a hole) and mmap all of it. If the file is persistent I recommend filling the hole with write calls rather than by writing in the mapping, as otherwise the file's blocks may get unnecessarily fragmented on the disk.