如何提取字节 [英] How to extract the bytes

问题描述

我们如何提取字节.例如，如果int占用4个字节，则必须从中提取第1个字节或第3个字节.

How can we extract byte. For ex, if int takes 4 bytes and from that I have to extract 1st byte or 3 rd byte. How we can do that?

推荐答案

一种优雅"的方式使用了union:

An ''elegant'' way uses union:

union Mixed 
{
  int c;
  unsigned char  b[4];
};

int main()
{
  int i;
  union Mixed m;
  m.c = 65535;
  printf("int: %d.\n");
  for (i=0; i<4; i++)
  {
    printf("byte[%d]: %d\n",i, m.b[i]);
  }
}

您需要在位位置输入值.表示二进制值rt?

使用按位移位运算符可以做到这一点.
假设我是您的整数，您需要获取其第N位值.
请按照以下步骤进行操作
a =我右移(N-1)
b =右移I N
c =左移b 1. R = XOR c;
R是您的Nth位二进制值. 1或0
C函数可能看起来像下面的

You need value at the bit position. means a binary value rt?

this is possible using bitwise shift operator.
suppose I is your integer and you need to get its Nth bit value.
this possible by following steps
a = right shift I by (N-1)
b = right shift I by N
c = Left shift b by 1.
R = a XOR c;
R is your Nth Bit value in binary. either 1 or 0
c function may look liek below

int GetBit( int I, int N)
{
int a = I >> ( N - 1);
int b = I >> N;
b = b << 1;
int r = a ^ b;
return r;
}

而且这仅对N = 32无效.
您需要单独编写该逻辑.
希望这就是您要查找的内容

and this will not work only for N = 32.
that logic you need to write seprate.
Hope this is what you are looking for

您可以将字符指针指向整数变量，然后使用该指针获得所需的字节

函数看起来像这样

You can take a character pointer to the integer variable , and then using the pointer you can have the required byte

Function would look like this

char GetByteFrom( int i, int i_iByteIndex)
{

  char * cPointer = (char*)&i;
  return *(cPointer + i_iByteIndex);
  
}

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