From 7a4bbc6eab2062e1118b8b277abb3da96e1f42eb Mon Sep 17 00:00:00 2001
From: Glenn Maynard <glenn@steprevolution.com>
Date: Thu, 1 Nov 2018 16:17:21 -0500
Subject: [PATCH] Pad the configuration struct to 250 bytes, so the SDK.DLL ABI
 doesn't change when fields are added.

---
 sdk/SMX.h         |  8 +++++++-
 smx-config/SMX.cs | 13 +++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/sdk/SMX.h b/sdk/SMX.h
index 43f52e0..41b1a7a 100644
--- a/sdk/SMX.h
+++ b/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 {
diff --git a/smx-config/SMX.cs b/smx-config/SMX.cs
index c501157..a6a70a7 100644
--- a/smx-config/SMX.cs
+++ b/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);