The DB->fd function
copies a file descriptor representative of the underlying database into
the memory referenced by fdp. A file descriptor referencing the
same file will be returned to all processes that call DB->open with
the same file argument. This file descriptor may be safely used
as an argument to the fcntl(2) and flock(2) locking
functions. The file descriptor is not necessarily associated with any of
the underlying files actually used by the access method.
The DB->fd function only supports a coarse-grained form of locking.
Applications should use the lock manager where possible.
The DB->fd function returns a non-zero error value on failure and 0 on success.
The DB->fd function may fail and return a non-zero error for errors specified for other Berkeley DB and C library or system functions.
If a catastrophic error has occurred, the DB->fd function may fail and return
DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail
in the same way.