I think that file-mapping is an excellent suggestion because it treats the shared resource “at arm’s length” especially in the sense of avoiding the memory-scribbling that can so easily occur among supposedly well-behaved children who are sharing exactly the same playpen at the same time. Given that bugs will inevitably occur, even due the to slightest typo, this makes things a bit easier to spot. (e.g. as in the preceding sentence.)
You might also seriously consider literally using a shared disk file, knowing that most if not all of the actual access will occur through a shared buffer pool that is helpfully maintained for you by the OS.