From 4e65c8f69fd808f252e5a25b5df5e3ad643ebd34 Mon Sep 17 00:00:00 2001
From: Patryk Obara <dreamer.tan@gmail.com>
Date: Tue, 12 Nov 2019 10:45:46 +0100
Subject: [PATCH] Add asserts to prevent a potential bug

Use C++11 static_assert to assure, that code behaves correctly; if
Chip or Channel structs will be changed to non-std layout or a different
first fields will be introduced, that will invalidate the offset
calculation.

Additionally, add asserts to prevent possibility of introducing a bug
when offset stored in one the tables is 0.
---
 src/hardware/adlib.h   |  1 +
 src/hardware/dbopl.cpp | 22 +++++++++++++++++-----
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/src/hardware/adlib.h b/src/hardware/adlib.h
index e22d49ea..7fbc7820 100644
--- a/src/hardware/adlib.h
+++ b/src/hardware/adlib.h
@@ -27,6 +27,7 @@
 #include "pic.h"
 #include "hardware.h"
 
+#include <cmath>
 
 namespace Adlib {
 
diff --git a/src/hardware/dbopl.cpp b/src/hardware/dbopl.cpp
index b4cd183c..c61351ce 100644
--- a/src/hardware/dbopl.cpp
+++ b/src/hardware/dbopl.cpp
@@ -32,15 +32,14 @@
 	//DUNNO Keyon in 4op, switch to 2op without keyoff.
 */
 
+#include "dbopl.h"
 
-
+#include <cassert>
+#include <cstddef>
 #include <math.h>
 #include <stdlib.h>
 #include <string.h>
-#include <stddef.h>
-#include "dosbox.h"
-#include "dbopl.h"
-
+#include <type_traits>
 
 #ifndef PI
 #define PI 3.14159265358979323846
@@ -1450,6 +1449,13 @@ void InitTables( void ) {
 		//Add back the bits for highest ones
 		if ( i >= 16 )
 			index += 9;
+
+		static_assert(std::is_standard_layout<Chip>::value,
+		              "struct Chip is not a standard layout type");
+		static_assert(offsetof(Chip, chan) == 0,
+		              "offset table stores values relative to the start of struct");
+		// values stored in offset tables are artificially increased by 1
+		// to keep macros REGCHAN and REGOP working correctly
 		ChanOffsetTable[i] = 1+(Bit16u)(index*sizeof(DBOPL::Channel));
 	}
 	//Same for operators
@@ -1463,7 +1469,13 @@ void InitTables( void ) {
 		if ( chNum >= 12 )
 			chNum += 16 - 12;
 		Bitu opNum = ( i % 8 ) / 3;
+
+		static_assert(std::is_standard_layout<Channel>::value,
+		              "struct Channel is not a standard layout type");
+		static_assert(offsetof(Channel, op) == 0,
+		              "offset table stores values relative to the start of struct");
 		OpOffsetTable[i] = ChanOffsetTable[chNum]+(Bit16u)(opNum*sizeof(DBOPL::Operator));
+		assert(OpOffsetTable[i] > 0); // needs to be non-zero; see REGOP macro
 	}
 #if 0
 	DBOPL::Chip* chip = 0;