详解下c语言中struct和union的对齐规则

接触过c语言的同学应该都知道字节对齐。有些时候我们很容易弄错字节对齐的方式，特别是涉及到struct（结构体）和union（联合体）时。今天我们通过详细例子来说明下struct和union的对齐规则，以便了解各种struct和union所占字节具体计算方式。

一、基础环境信息

我本地计算机系统各类基础类型所占字节如下：

cpp 复制代码

int main() {
    printf("the sizeof char      is %d\n",sizeof(char));
    printf("the sizeof int       is %d\n",sizeof(int));
    printf("the sizeof unsigned  is %d\n",sizeof(unsigned));
    printf("the sizeof short     is %d\n",sizeof(short));
    printf("the sizeof long      is %d\n",sizeof(long));
    printf("the sizeof float     is %d\n",sizeof(float));
    printf("the sizeof double    is %d\n",sizeof(double));
    printf("the sizeof longlong  is %d\n",sizeof(long long));

}

二、结构体字节对齐

2.1，结构体对齐规则

结构体中寻找所有成员中占字节数最大成员，其余成员根据占字节数最大成员拼凑或者插入空位构成n个（n>=1)最大成员字节。

2.2，实例

上图例子实测：

cpp 复制代码

int main() {
    struct stu{
        char a;
        int b;
        short c;
        char d;
    };

    struct  stu s;

    printf("the sizeof s is %d\n", sizeof s);
    printf("the address a is %x\n", &s.a);
    printf("the address b is %x\n", &s.b);
    printf("the address c is %x\n", &s.c);
    printf("the address d is %x\n", &s.d);
}

对于如下结构体：最大类型为b，占4字节，c，d，a拼凑占4字节，总共8字节

复制代码

struct stu{
    int b;
    short c;
    char d;
    char a;
};

cpp 复制代码

int main() {
    struct stu{
        int b;
        short c;
        char d;
        char a;
    };

    struct  stu s;

    printf("the sizeof s is %d\n", sizeof s);
    printf("the address b is %x\n", &s.b);
    printf("the address c is %x\n", &s.c);
    printf("the address d is %x\n", &s.d);
    printf("the address a is %x\n", &s.a);
}

对于如下结构体： f为最大类型，占8字节，成员e需要以8字节对齐，所以stu1占16字节；成员b，c，d拼凑占8字节；成员a分配8字节进行对齐；总共占32字节

复制代码

struct stu{
    int b;
    short c;
    char d;
    struct stu1{
        char e;
        double f;
    } stu1;
    char a;
};

cpp 复制代码

int main() {
    struct stu{
        int b;
        short c;
        char d;
        struct stu1{
            char e;
            double f;
        } stu1;
        char a;
    };

    struct  stu s;

    printf("the sizeof s     is %d\n", sizeof s);
    printf("the address b    is %x\n", &s.b);
    printf("the address c    is %x\n", &s.c);
    printf("the address d    is %x\n", &s.d);
    printf("the address stu1 is %x\n", &s.stu1);
    printf("the address e    is %x\n", &s.stu1.e);
    printf("the address f    is %x\n", &s.stu1.f);
    printf("the address a    is %x\n", &s.a);
}

三、联合体字节对齐

3.1，联合体对齐规则

联合体字节对齐计算非常简单，为所有成员中所占字节最大成员的字节数。

3.2，实例

上图例子实测：

cpp 复制代码

int main() {
    union stu{
        char a;
        int b;
        short c;
        char d;
    };

    union  stu s;

    printf("the sizeof s     is %d\n", sizeof s);
    printf("the address b    is %x\n", &s.b);
    printf("the address c    is %x\n", &s.c);
    printf("the address d    is %x\n", &s.d);
    printf("the address a    is %x\n", &s.a);
    s.b = 0b00000011000000110000001100000011;
    printf("the value of b    is %d\n", s.b);
    printf("the value of c    is %d\n", s.c);
    printf("the value of d    is %d\n", s.d);
    printf("the value of a    is %d\n", s.a);
}

对于如下联合体：stu中成员stu1按照struct对齐规则占8字节，所以联合体stu占8字节。

复制代码

union stu{
    char a;
    int b;
    short c;
    struct stu1{
        char e;
        int f;
    }stu1;
    char d;
};

cpp 复制代码

int main() {
    union stu{
        char a;
        int b;
        short c;
        struct stu1{
            char e;
            int f;
        }stu1;
        char d;
    };

    union  stu s;

    printf("the sizeof s     is %d\n", sizeof s);
    printf("the address a    is %x\n", &s.a);
    printf("the address b    is %x\n", &s.b);
    printf("the address c    is %x\n", &s.c);
    printf("the address e    is %x\n", &s.stu1.e);
    printf("the address f    is %x\n", &s.stu1.f);
    printf("the address d    is %x\n", &s.d);


}