C language register skills (struct and union)

Reference resources ：
C Language | Detailed explanation of the use of bit fields

Test code

#include <stdio.h>

struct pack
{
    
 unsigned int a:2;  //  The value range is ：0~3
 unsigned int b:4;   //  The value range is ：0~15
 unsigned int c:6;   //  The value range is ：0~63
};

union u1  //u1 Indicates the consortium type name 
{
    
   //unsigned int b;
   struct pack f;
   unsigned int b;

};

int main(void)
{
    
 struct pack pk1;
 struct pack pk2;
 
 //  to pk1 Assign values to members and print out 
 pk1.a = 1;
 pk1.b = 10;
 pk1.c = 50;
 printf("%d, %d, %d\n", pk1.a, pk1.b, pk1.c);
 
 //  to pk2 Assign values to members and print out 
 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;
}