Pad the configuration struct to 250 bytes, so the SDK.DLL ABI doesn't change when fields are added.

sdk/SMX.h

@@ -215,8 +215,14 @@ struct SMXConfig
     uint8_t panelThreshold5Low, panelThreshold5High;
     uint8_t panelThreshold6Low, panelThreshold6High;
     uint8_t panelThreshold8Low, panelThreshold8High;
+
+    // Pad the struct to 250 bytes.  This keeps this struct size from changing
+    // as we add fields, so the ABI doesn't change.  Applications should leave
+    // any data in here unchanged when calling SMX_SetConfig.
+    uint8_t padding[166];
 };
-static_assert(sizeof(SMXConfig) == 84, "Expected 84 bytes");
+static_assert(offsetof(SMXConfig, padding) == 84, "Expected 84 bytes"); // includes one padding byte
+static_assert(sizeof(SMXConfig) == 250, "Expected 250 bytes");
 
 // The values (except for Off) correspond with the protocol and must not be changed.
 enum SensorTestMode {

smx-config/SMX.cs

@@ -60,6 +60,10 @@ namespace SMX
         public Byte panelThreshold6Low, panelThreshold6High;
         public Byte panelThreshold8Low, panelThreshold8High;
 
+        // Pad this struct to exactly 250 bytes.
+        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 166)]
+        public Byte[] padding;
+
         // enabledSensors is a mask of which panels are enabled.  Return this as an array
         // for convenience.
         public bool[] GetEnabledPanels()
@@ -291,6 +295,15 @@ namespace SMX
         {
             if(!DLLAvailable()) return;
 
+            // Sanity check SMXConfig, which should be 250 bytes.  If this is incorrect,
+            // check the padding array.
+            {
+                SMXConfig config = new SMXConfig();
+                int bytes = Marshal.SizeOf(config);
+                if(bytes != 250)
+                    throw new Exception("SMXConfig is " + bytes + " bytes, but should be 250 bytes");
+            }
+
             // Make a wrapper to convert from the native enum to SMXUpdateCallbackReason.
             InternalUpdateCallback NewCallback = delegate(int PadNumber, int reason, IntPtr user) {
                 SMXUpdateCallbackReason ReasonEnum = (SMXUpdateCallbackReason) Enum.ToObject(typeof(SMXUpdateCallbackReason), reason);