struct 位域结构名
　　{ 位域列表 };
eg：struct weiyu {
         int a:2;
         int b:5;
         int  :5;//此五位为空域，不能使用    
         int c:3;
          }

#include "stdio.h"
void main(int argn ,char *argv)
{
  struct   test {
    unsigned a:10;
    unsigned b:10;
    unsigned c:6;
    unsigned :2;//this two bytes can't use
    unsigned d:4;
    }data,*pData;
  data.a=0x177;
  data.b=0x111;
  data.c=0x7;
  data.d=0x8;
  
  pData=&data;
  printf("data.a=%x data.b= %x data.c=%x data.d=%xn",pData->a,pData->b,pData->c,pData->d);//位域可以使用指针
  printf("sizeof(data)=%dn",sizeof(data));  //4 bytes ，最常用的情况
  struct testLen{
  char a:5;
  char b:5;
  char c:5;
  char d:5;
  char e:5;
  }len;
  
  printf("sizeof(len)=%dn",sizeof(len));   //5bytes 规则2
  struct testLen1{
    char a:5;
    char b:2;
    char d:3;
    char c:2;
    char e:7;
    }len1;
  printf("sizeof(len1) =%dn",sizeof(len1));  //3bytes 规则1
  struct testLen2{
    char a:2;
    char :3;
    char b:7;
    long d:20; //4bytes
    char e:4;
    }len2;
  printf("sizeof(len2)=%dn",sizeof(len2)); //12 规则3，4,5，总长为4的整数倍，2+3 占1byte，b占1bye 由于与long对其，2+3+7 占4字节，后面 d 与 e进行了优化 占一个4字节
  struct testLen3{
    char a:2;
    char :3;
    char b:7;
    long d:30;
    char e:4;
    }len3;
  printf("sizeof(len3)=%dn",sizeof(len3));//12 规则3，4，5，总长为4的整数倍，2+3 占1byte，b占1bye 由于与long对其，2+3+7 占4字节，后面 d占一个4字节，为了保证与long对其e独占一个4字节
}