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sizeof of struct with bit Field

please explain me why the sizeof (C51) returns 6 bytes for this struct instead of 5:

typedef struct DEKO_COMM_HEADER
{ UINT16 m_uiMsgOpcode;
UCHAR8 m_sblMsgPriority :2;
UINT16 m_sblDataLength :11;
UINT16 m_sblMsgTimeStamp :11;
}DEKO_COMM_HEADER, *PDEKO_COMM_HEADER;

UINT16 is typedef for unsigned int
UCHAR8 is typedef for unsigned char

thanks

Parents

0 Gp F over 18 years ago

Which compiler are you using ? Compiling your example shows a size of 7 which is correct since an UINT16 can hold a max. of 16 bits. Please look at the symbol listing for details on size and layout of the fields.

C51 COMPILER V8.08a, COMPILATION OF MODULE Q
OBJECT MODULE PLACED IN q.OBJ
COMPILER INVOKED BY: c51.exe q.c CODE SB

line level    source

   1          typedef unsigned char  UCHAR8;
   2          typedef unsigned int   UINT16;
   3
   4          typedef struct DEKO_COMM_HEADER  {
   5            UINT16     m_uiMsgOpcode;
   6            UCHAR8  m_sblMsgPriority : 2;
   7            UINT16   m_sblDataLength :11;
   8            UINT16 m_sblMsgTimeStamp :11;
   9          } DEKO_COMM_HEADER;
  10
  11
  12          unsigned char  z;
  13
  14          void main (void)  {
  15   1        z = sizeof (DEKO_COMM_HEADER);
  16   1      }
ASSEMBLY LISTING OF GENERATED OBJECT CODE
             ; FUNCTION main (BEGIN)
                                           ; SOURCE LINE # 14
                                           ; SOURCE LINE # 15
0000 750007      R     MOV     z,#07H
                                           ; SOURCE LINE # 16
0003 22                RET
             ; FUNCTION main (END)


NAME                                    CLASS   MSPACE  TYPE    OFFSET  SIZE
====                                    =====   ======  ====    ======  ====
main . . . . . . . . . . . . . . . . .  PUBLIC   CODE   PROC     0000H  -----
UINT16 . . . . . . . . . . . . . . . .  TYPEDEF  -----  U_INT    -----  2
UCHAR8 . . . . . . . . . . . . . . . .  TYPEDEF  -----  U_CHAR   -----  1
DEKO_COMM_HEADER . . . . . . . . . . .  TYPEDEF  -----  STRUCT   -----  7
  m_uiMsgOpcode. . . . . . . . . . . .  MEMBER   -----  U_INT    0000H  2
  m_sblMsgPriority . . . . . . . . . .  MEMBER   -----  FIELD    0002H  2.0
  m_sblDataLength. . . . . . . . . . .  MEMBER   -----  FIELD    0003H  11.0
  m_sblMsgTimeStamp. . . . . . . . . .  MEMBER   -----  FIELD    0005H  11.0
DEKO_COMM_HEADER . . . . . . . . . . .  * TAG *  -----  STRUCT   -----  7
  m_uiMsgOpcode. . . . . . . . . . . .  MEMBER   -----  U_INT    0000H  2
  m_sblMsgPriority . . . . . . . . . .  MEMBER   -----  FIELD    0002H  2.0
  m_sblDataLength. . . . . . . . . . .  MEMBER   -----  FIELD    0003H  11.0
  m_sblMsgTimeStamp. . . . . . . . . .  MEMBER   -----  FIELD    0005H  11.0
z. . . . . . . . . . . . . . . . . . .  PUBLIC   DATA   U_CHAR   0000H  1

Reply

0 Gp F over 18 years ago

C51 COMPILER V8.08a, COMPILATION OF MODULE Q
OBJECT MODULE PLACED IN q.OBJ
COMPILER INVOKED BY: c51.exe q.c CODE SB

line level    source

   1          typedef unsigned char  UCHAR8;
   2          typedef unsigned int   UINT16;
   3
   4          typedef struct DEKO_COMM_HEADER  {
   5            UINT16     m_uiMsgOpcode;
   6            UCHAR8  m_sblMsgPriority : 2;
   7            UINT16   m_sblDataLength :11;
   8            UINT16 m_sblMsgTimeStamp :11;
   9          } DEKO_COMM_HEADER;
  10
  11
  12          unsigned char  z;
  13
  14          void main (void)  {
  15   1        z = sizeof (DEKO_COMM_HEADER);
  16   1      }
ASSEMBLY LISTING OF GENERATED OBJECT CODE
             ; FUNCTION main (BEGIN)
                                           ; SOURCE LINE # 14
                                           ; SOURCE LINE # 15
0000 750007      R     MOV     z,#07H
                                           ; SOURCE LINE # 16
0003 22                RET
             ; FUNCTION main (END)


NAME                                    CLASS   MSPACE  TYPE    OFFSET  SIZE
====                                    =====   ======  ====    ======  ====
main . . . . . . . . . . . . . . . . .  PUBLIC   CODE   PROC     0000H  -----
UINT16 . . . . . . . . . . . . . . . .  TYPEDEF  -----  U_INT    -----  2
UCHAR8 . . . . . . . . . . . . . . . .  TYPEDEF  -----  U_CHAR   -----  1
DEKO_COMM_HEADER . . . . . . . . . . .  TYPEDEF  -----  STRUCT   -----  7
  m_uiMsgOpcode. . . . . . . . . . . .  MEMBER   -----  U_INT    0000H  2
  m_sblMsgPriority . . . . . . . . . .  MEMBER   -----  FIELD    0002H  2.0
  m_sblDataLength. . . . . . . . . . .  MEMBER   -----  FIELD    0003H  11.0
  m_sblMsgTimeStamp. . . . . . . . . .  MEMBER   -----  FIELD    0005H  11.0
DEKO_COMM_HEADER . . . . . . . . . . .  * TAG *  -----  STRUCT   -----  7
  m_uiMsgOpcode. . . . . . . . . . . .  MEMBER   -----  U_INT    0000H  2
  m_sblMsgPriority . . . . . . . . . .  MEMBER   -----  FIELD    0002H  2.0
  m_sblDataLength. . . . . . . . . . .  MEMBER   -----  FIELD    0003H  11.0
  m_sblMsgTimeStamp. . . . . . . . . .  MEMBER   -----  FIELD    0005H  11.0
z. . . . . . . . . . . . . . . . . . .  PUBLIC   DATA   U_CHAR   0000H  1

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