lmdb 0.9.35 (The header files)

LMDB is a Btree-based database management library with an API similar
to BerkeleyDB. The library is thread-aware and supports concurrent
read/write access from multiple processes and threads. The DB
structure is multi-versioned, and data pages use a copy-on-write
strategy, which also provides resistance to corruption and eliminates
the need for any recovery procedures. The database is exposed in a
memory map, requiring no page cache layer of its own.

https://symas.com/mdb/