For a while I've wanted a nice C library for an mmapped heap with allocation and common data structures (and maybe locks?) - all the usual stuff you'd expect in a standard library, but with support for relative offsets instead of pointers, crash robustness, introspection, and other features required to work well with a persistent file. I do not know any library of this type that currently exists.
"This library also supports distributed, durable, observable collections (Map, List, Set)" "It uses almost no heap, trivial GC impact, can be much larger than your physical memory size (only limited by the size of your disk) and can be shared between processes with better than 1/10th latency of using Sockets over loopback."