About Bit Field in C/C++

In the C/C++, 

usually we use the "Bit Field" to optimize code or communicate with other process.

What is the Bit Filed?

I have attached "Example Code".

Example code

// Test Code, Bit Field

// ----------------------------------

{

 #define u8 unsigned char

 #define FILL_STRUCT1 u8:0; u8:8; u8:0;

 #define FILL_STRUCT2 u8:0; u8:8; u8:8; u8:0;

 #define FILL_STRUCT3 u8:0; u8:8; u8:8; u8:8; u8:0;

 typedef struct tag_SIMCommand {

 u8 Class;

 u8 Instruction;

 u8 P1;

 u8 P2;

 u8 P3;

 FILL_STRUCT3 // Bit Field

 u8 CData[8];

 } SIMCommandType;

 SIMCommandType SIMCommand;

 memset(&SIMCommand, 0xFF, sizeof(SIMCommandType));

 memset(&SIMCommand, 0x00, sizeof(SIMCommandType));

 // SELECT

 SIMCommand.Class = 0xA0;

 SIMCommand.Instruction = 0xA4;

 SIMCommand.P1 = 0x00;

 SIMCommand.P2 = 0x00;

 SIMCommand.P3 = 0x02;

 memset(SIMCommand.CData, 0xFF, sizeof(u8)*8);

}

// ----------------------------------

// Real Memory, Dump 1

// ----------------------------------

0012F3B0  A0 A4 00 00  

0012F3B4  02 00 00 00  

0012F3B8  FF FF FF FF  

0012F3BC  FF FF FF FF  

// ----------------------------------

// Test Code, Bit Field

// ----------------------------------

{

 #define u8 unsigned char

 #define FILL_STRUCT1 u8:0; u8:8; u8:0;

 #define FILL_STRUCT2 u8:0; u8:8; u8:8; u8:0;

 #define FILL_STRUCT3 u8:0; u8:8; u8:8; u8:8; u8:0;

 typedef struct tag_SIMCommand {

 u8 Class:4;

 u8 Instruction:2;

 u8 P1:1;

 FILL_STRUCT1

 u8 P2:4;

 FILL_STRUCT2

 u8 P3:1;

 FILL_STRUCT3 // Bit Field

 u8 CData[8];

 } SIMCommandType;

 SIMCommandType SIMCommand;

 memset(&SIMCommand, 0xFF, sizeof(SIMCommandType));

 memset(&SIMCommand, 0x00, sizeof(SIMCommandType));

 // SELECT

 SIMCommand.Class = 0x0F;

 SIMCommand.Instruction = 0x03;

 SIMCommand.P1 = 0x01;

 SIMCommand.P2 = 0x0F;

 SIMCommand.P3 = 0x01;

 memset(SIMCommand.CData, 0xFF, sizeof(u8)*8);

}

// ----------------------------------

// Real Memory, Dump 2

// ----------------------------------

0012F39C  7F 00 0F 00  

0012F3A0  00 01 00 00  

0012F3A4  00 FF FF FF  

0012F3A8  FF FF FF FF  

0012F3AC  FF CC CC CC  

// ----------------------------------

If you have any question? Please let me know :-)