参考：
C语言 | 位域的使用详解

测试代码

#include <stdio.h>

struct pack
{
 unsigned int a:2;  // 取值范围为：0~3
 unsigned int b:4;   // 取值范围为：0~15
 unsigned int c:6;   // 取值范围为：0~63
};

union u1  //u1表示联合体类型名
{
   //unsigned int b;
   struct pack f;
   unsigned int b;

};

int main(void)
{
 struct pack pk1;
 struct pack pk2;
 
 // 给pk1各成员赋值并打印输出
 pk1.a = 1;
 pk1.b = 10;
 pk1.c = 50;
 printf("%d, %d, %d\n", pk1.a, pk1.b, pk1.c);
 
 // 给pk2各成员赋值并打印输出
 pk2.a = 5;
 pk2.b = 20;
 pk2.c = 66;
 printf("%d, %d, %d\n", pk2.a, pk2.b, pk2.c);

 printf("\n!!!!!!!!!!!!!!!!!!!!!!!!!!!!!\n");

 union u1 mytest;

 mytest.f.a=3;
 mytest.f.b=15;
 mytest.f.c=63;
 mytest.b = 3456;
 int sum = 3 + 15*4 + 63 * 64;
 printf("@@@@@@@ U2 mytest is %d\n",sum);
 printf("U2 mytest is %d\n",mytest);  

 return 0;
}