MdeModulePkg/DriverSampleDxe: Make bit fields aligned in C structure

For a structure with a series of bit fields and used as a storage
in vfr file, and if the bit fields do not add up to the size of
the defined type.In the C code use sizeof() to get size of the
structure, the results may vary form the compiler(VS,GCC...).
But the size of the storage calculated by VfrCompiler is fixed
(calculate with alignment).To avoid the issue cased by above case,
we need to make the total width of the bit fields in the structure
aligned with the size of the defined type for these bit fields.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

diff --git a/MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h b/MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h
index 40fb3d0964787c56ddca25479749edeeaa80d97b..6f092de8b637dae71e2860dc096e08333886cf8c 100644 (file)
--- a/MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h
+++ b/MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h
@@ -35,6 +35,14 @@ Revision History:
 \r
 #pragma pack(1)\r
 \r
+//\r
+// !!! For a structure with a series of bit fields and used as a storage in vfr file, and if the bit fields do not add up to the size of the defined type.\r
+// In the C code use sizeof() to get the size the strucure, the results may vary form the compiler(VS,GCC...).\r
+// But the size of the storage calculated by VfrCompiler is fixed (calculate with alignment).\r
+// To avoid above case, we need to make the total bit width in the structure aligned with the size of the defined type for these bit fields. We can:\r
+// 1. Add bit field (with/without name) with remianing with for padding.\r
+// 2. Add unnamed bit field with 0 for padding, the amount of padding is determined by the alignment characteristics of the members of the structure.\r
+//\r
 typedef struct {\r
   UINT16   NestByteField;\r
   UINT8                    : 1;  // unamed field can be used for padding\r
@@ -84,7 +92,9 @@ typedef struct {
   UINT8   Match2;\r
   UINT8   GetDefaultValueFromCallBackForOrderedList[3];\r
   UINT8   BitCheckbox : 1;\r
+  UINT8   ReservedBits: 7;  // Reserved bit fields for padding.\r
   UINT16  BitOneof    : 6;\r
+  UINT16              : 0;  // Width 0 used to force alignment.\r
   UINT16  BitNumeric  : 12;\r
   MY_BITS_DATA  MyBitData;\r
   MY_EFI_UNION_DATA MyUnionData;\r
@@ -109,6 +119,7 @@ typedef struct {
   UINT32   EfiBitNumeric     : 4;\r
   UINT32   EfiBitOneof       : 10;\r
   UINT32   EfiBitCheckbox    : 1;\r
+  UINT32                     : 0;  // Width 0 used to force alignment.\r
 } MY_EFI_BITS_VARSTORE_DATA;\r
 \r
 //\r