diff --git a/external/db_drivers/liblmdb32/CMakeLists.txt b/external/db_drivers/liblmdb32/CMakeLists.txt
index 62b80013..bca7ff51 100644
--- a/external/db_drivers/liblmdb32/CMakeLists.txt
+++ b/external/db_drivers/liblmdb32/CMakeLists.txt
@@ -26,6 +26,10 @@
 # STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+if(FREEBSD)
+  set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DMDB_DSYNC=O_SYNC")
+endif()
+
 set (lmdb_sources
 mdb.c
 midl.c)
diff --git a/external/db_drivers/liblmdb64/CMakeLists.txt b/external/db_drivers/liblmdb64/CMakeLists.txt
index 62b80013..bca7ff51 100644
--- a/external/db_drivers/liblmdb64/CMakeLists.txt
+++ b/external/db_drivers/liblmdb64/CMakeLists.txt
@@ -26,6 +26,10 @@
 # STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+if(FREEBSD)
+  set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DMDB_DSYNC=O_SYNC")
+endif()
+
 set (lmdb_sources
 mdb.c
 midl.c)