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 <[email protected]> Cc: Liming Gao <[email protected]> Contributed-under: TianoCore Contribution Agreement 1.1 Signed-off-by: Dandan Bi <[email protected]> --- MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h b/MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h index 40fb3d0..af3d4bc 100644 --- a/MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h +++ b/MdeModulePkg/Universal/DriverSampleDxe/NVDataStruc.h @@ -33,10 +33,18 @@ Revision History: #define CONFIGURATION_VARSTORE_ID 0x1234 #define BITS_VARSTORE_ID 0x2345 #pragma pack(1) +// +// !!! 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 the size the strucure, 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 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: +// 1. Add bit field (with/without name) with remianing with for padding. +// 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. +// typedef struct { UINT16 NestByteField; UINT8 : 1; // unamed field can be used for padding UINT8 NestBitCheckbox : 1; UINT8 NestBitOneof : 2; @@ -82,11 +90,13 @@ typedef struct { EFI_HII_TIME Time; UINT8 RefreshGuidCount; UINT8 Match2; UINT8 GetDefaultValueFromCallBackForOrderedList[3]; UINT8 BitCheckbox : 1; + UINT8 ReservedBits: 7; // Reserved bit fields for padding. UINT16 BitOneof : 6; + UINT16 : 0; // Width 0 used to force alignment. UINT16 BitNumeric : 12; MY_BITS_DATA MyBitData; MY_EFI_UNION_DATA MyUnionData; } DRIVER_SAMPLE_CONFIGURATION; @@ -107,10 +117,11 @@ typedef struct { MY_BITS_DATA BitsData; UINT32 EfiBitGrayoutTest : 5; UINT32 EfiBitNumeric : 4; UINT32 EfiBitOneof : 10; UINT32 EfiBitCheckbox : 1; + UINT32 : 0; // Width 0 used to force alignment. } MY_EFI_BITS_VARSTORE_DATA; // // Labels definition // -- 1.9.5.msysgit.1 _______________________________________________ edk2-devel mailing list [email protected] https://lists.01.org/mailman/listinfo/edk2-devel
