There are four major code sets in the standard btree source code:
-btree2q.c Single Threaded/MultiProcess with latching supplied by advisory file locking. Works with network file systems.
+btree2s.c Single Threaded/MultiProcess version that removes keys all the way back to an original empty btree, placing removed nodes on a free list.
threads2h.c Multi-Threaded/Multi-Process with latching implemented by a latch manager with pthreads/SRW latches in the first few btree pages.