结构体与联合体的区别

1.结构体

每个成员变量都单独占一块内存，各成员在内存中的分布存在"内存对齐",64位机器按8个字节对齐

    #include<stdio.h>
    int main()
    {
        struct _s{
            char a;  //1字节
            int b;  //4字节
            long c;  //8字节
            void* d;  //8字节
            int e;   //4字节
            char* f;  //8字节
        }s;
        s.a = 'a';
        s.b = 1;
        s.c = 2;
        s.d = NULL;
        s.e = 3;
        s.f = &s.a;
        
        printf("size of struct s is %d\n",sizeof(s));
        return 1;
    }
    // size of struct s is 40

2. 联合体 union

所有的成员变量共用一块内存，"内存复用"，一般8字节复用

#include<stdio.h>
int main()
{
    union _u{
        char a;  //1字节
        int b;  //4字节
        long c;  //8字节
        void* d;  //8字节
        int e;   //4字节
        char* f;  //8字节
    }u;
    u.a = 'a';
    u.b = 1;
    u.c = 2;
    u.d = NULL;
    u.e = 3;
    u.f = &s.a;
    
    printf("size of struct u is %d\n",sizeof(u));
    return 1;
}
// size of struct s is 8