diff --git a/util.hpp b/util.hpp
new file mode 100644
index 0000000..2c5eccd
--- /dev/null
+++ b/util.hpp
@@ -0,0 +1,25 @@
+#pragma once
+
+/* This program assumes sensors use the Sensor.Value interface
+ * and for sensor->write() I only implemented sysfs as a type,
+ * but -- how would it know whether to use Control.FanSpeed or Control.FanPwm?
+ *
+ * One could get the interface list for the object and search for Control.*
+ * but, it needs to know the maximum, minimum.  The only sensors it wants to write
+ * in this code base are Fans...
+ */
+enum class IOInterfaceType
+{
+    NONE, // There is no interface.
+    EXTERNAL,
+    DBUSPASSIVE,
+    DBUSACTIVE, // This means for write that it needs to look up the interface.
+    SYSFS,
+    UNKNOWN
+};
+
+/* WriteInterfaceType is different because Dbusactive/passive. how to know... */
+IOInterfaceType GetWriteInterfaceType(const std::string& path);
+
+IOInterfaceType GetReadInterfaceType(const std::string& path);
+