C bit And Big Little Endian

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The tencent engineer asked a question about the C bit, I could not answer it with a pretty description. so there is a summary for it.

Bit Little Endule

for exmaple: 32bit int（uint32_t） data : 0x12345678，store in address : 0x08004000:

Address Little Endian Big Endian
0x08004003 0x12 0x78
0x08004002 0x34 0x56
0x08004001 0x56 0x34
0x08004000 0x78 0x12

CPU

little-endian

X86
ARM(default)

big-endian

PowerPC
IBM

Host to network bit

htonl()
htons()

Network to host bit

ntohl()
ntohs()

Check Big & Little Endian

Int char check

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int i = 1;   
char *p = (char *)&i;   
if(*p == 1)     
      printf("Little Endian"); 
else
      printf("Big Endian");

union check

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int checkCPUendian()
{
  union
  {
    unsigned int a;
    unsigned char b; 
  }c;
  c.a = 1;
  return (c.b == 1); 
}

Parse bit field

We need to parse the data protocal, and there are many key-value in bit mode, usually we will use the right shift to get the target bit.

D will be : ((pucPktAddr + 3)>>6) & 0x3 –point shift 3 bytes, and right shift 6 bit，then apply mask 0x3

E will be : ((pucPktAddr + 3)>>3) & 0x7

Now we try bit mode

Bit mode

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1 typedef struct XXX {
2     unsigned char D:2
3     unsigned char E:3
4     unsigned char F:3  
5 }

then we can use xxx->D，xxx->E，xxx->F to access the data，but this is only in big end mode, if you want to use light end mode,try to reverse it:

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1 typedef struct XXX {
2     unsigned char F:3
3     unsigned char E:3
4     unsigned char D:2  
5 }

大小端序数据相互转换函数

将unsigned char数组转换成“大端序”整数；

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/*
*    Function:        ConverseArrayToBeUI
*    Description:    将无符号字符数组数据转换成“大端序”整数
*    Parameter:        srcArray    --[in]    源数组数据
*                    desBeData    --[out]    目标“大端序”整数
*    Return            0    成功
*                    非0    失败
*    Note:            
*    Other:
*/
int MULCONVERSE_CALL ConverseArrayToBeUI(unsigned char *srcArray,unsigned int *desBeData)
{
    if (srcArray == NULL_POINT || desBeData == NULL_POINT)
    {
        return ERR_NULL_POINT;
    }
    *desBeData = (unsigned int)(srcArray[0]<<24) + (unsigned int)(srcArray[1]<<16) + 
                    (unsigned int)(srcArray[2]<<8) + (unsigned int)srcArray[3];
    return _SUCCESS;
}

将unsigned char数组转换成“小端序”整数；

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/*
*    Function:        ConverseArrayToLeUI
*    Description:    将无符号字符数组数据转换成“小端序”整数
*    Parameter:        srcArray    --[in]    源数组数据
*                    desLeData    --[out]    目标“小端序”整数
*    Return            0    成功
*                    非0    失败
*    Note:            
*    Other:
*/
int MULCONVERSE_CALL ConverseArrayToLeUI(unsigned char *srcArray,unsigned int *desLeData)
{
    if (srcArray == NULL_POINT || desLeData == NULL_POINT)
    {
        return ERR_NULL_POINT;
    }
    *desLeData = (unsigned int)(srcArray[3]<<24) + (unsigned int)(srcArray[2]<<16) + 
                    (unsigned int)(srcArray[1]<<8) + (unsigned int)srcArray[0];
    return _SUCCESS;
}

将整数按照“大端序”格式存储在数组中；

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/*
*    Function:        ConverseUItoBeArray
*    Description:    将无符号整数转换成“大端序”存储的无符号字符数组
*    Parameter:        srcData        --[in]    源整数
*                    desBeArray    --[out]    目标“大端序”存储的数组数据
*    Return            0    成功
*                    非0    失败
*    Note:            
*    Other:
*/
int MULCONVERSE_CALL ConverseUItoBeArray(unsigned int srcData,unsigned char *desBeArray)
{
    if (desBeArray == NULL_POINT)
    {
        return ERR_NULL_POINT;
    }
    desBeArray[0] = (unsigned char)(srcData>>24);
    desBeArray[1] = (unsigned char)(srcData>>16);
    desBeArray[2] = (unsigned char)(srcData>>8);
    desBeArray[3] = (unsigned char)srcData;
    return _SUCCESS;
}

将整数按照“小端序”格式存储在数组中。

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/*
*    Function:        ConverseUItoLeArray
*    Description:    将无符号整数转换成“小端序”存储的无符号字符数组
*    Parameter:        srcData        --[in]    源整数
*                    desLeArray    --[out]    目标“小端序”存储的数组数据
*    Return            0    成功
*                    非0    失败
*    Note:            
*    Other:
*/
int MULCONVERSE_CALL ConverseUItoLeArray(unsigned int srcData,unsigned char *desLeArray)
{
    if (desLeArray == NULL_POINT)
    {
        return ERR_NULL_POINT;
    }
    desLeArray[3] = (unsigned char)(srcData>>24);
    desLeArray[2] = (unsigned char)(srcData>>16);
    desLeArray[1] = (unsigned char)(srcData>>8);
    desLeArray[0] = (unsigned char)srcData;
    return _SUCCESS;
}

Big-Endian to Little-Endian

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#define BigtoLittle16(A) ((((uint16)(A) & 0xff00) >> 8) | (((uint16)(A) & 0x00ff) << 8))
#define BigtoLittle32(A) ((((uint32)(A) & 0xff000000) >> 24) | (((uint32)(A) & 0x00ff0000) >> 8) | \
             (((uint32)(A) & 0x0000ff00) << 8) | (((uint32)(A) & 0x000000ff) << 24))

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