https://www.runoob.com/cprogramming/c-bit-fields.html
#include <stdio.h>
#include <string.h>
struct
{
unsigned int age : 3;
} Age;
int main( )
{
Age.age = 4;
printf( "Sizeof( Age ) : %d\n", sizeof(Age) );
printf( "Age.age : %d\n", Age.age );
Age.age = 7;
printf( "Age.age : %d\n", Age.age );
Age.age = 8; // 二进制表示为 1000 有四位,超出
printf( "Age.age : %d\n", Age.age );
return 0;
}