Libbson's data structures are NOT thread-safe. You are responsible for accessing and mutating these structures from one thread at a time.
Libbson requires POSIX threads (pthreads) on all UNIX-like platforms. On Windows, the native threading interface is used. Libbson uses your system's threading library to safely generate unique ObjectIds, and to provide a fallback implementation for atomic operations on platforms without built-in atomics.