C语言中的内存对齐

#include<stdio.h>
enum Season
{ 
    Spring,Summer,Autumn
};
enum ENUM Enum = Autumn;

union UNION
{ 
    int a; 
    char b;
    double c;  
};
union UNION Union;

struct STRUCT
{ 
    char b;   //4字节 
    int a;   //4字节
    char d;   //4字节
    int  c;   //4字节
};
struct STRUCT Struct;

struct STRUCT2
{  
    char b;      //4字节
    int a;      //4字节
    char d;     //8字节
    double  c;   //8字节
};
struct STRUCT2 Struct2;

int main()
{
    printf("%d\n",sizeof(Enum));       //4     枚举变量占4个字节 
    printf("%d\n",sizeof(Union));      //8
    printf("%d\n",sizeof(Struct));     //16   内存对齐 
    printf("%d\n",sizeof(Struct2));    //24   内存对齐
    return 0;
}