Talking about Custom Type-Union

joint

  Union is a special custom type. The variables defined by this type also contain a series of members. The characteristic is that these members share the same space, so it is also called a union.

Calculation of joint size

1. The size of the union is at least the size of the largest member, so as to be able to save the largest member.

2. When the maximum member size is not an integral multiple of the maximum alignment, it must be aligned to an integral multiple of the maximum alignment.

    #include <stdio.h>
    //联合的声明
    union Un
    {
    	short s[7];
    	int n;
    };
    int main()
    {
    	//联合的定义
    	union Un un;
    	//计算联合的大小
    	printf("%d\n", sizeof(union Un));//16
    	printf("%d\n", sizeof(un));//16
    	return 0;
    }
   

  Because my computer has 32 data buses, the maximum alignment number of my computer is 4. We find the largest member short s[7] in the union, which requires 14 bytes, and then fill it up to the maximum alignment number 4. Integer multiples, the result is 16.

Use joint judgment of the computer

//大端返回1，小端返回0，要求借用联合体的特性
union Data
{
	int i;
	char ch;
};

int isBigSystem()
{
	union Data d;
	//d.i = 0x00000001;
	d.i = 1;

	if (d.ch == 1)
	{
		return 0;
	}
	else
	{
		return 1;
	}
}

//小端机低位放低位，高位放高位，小端机
//大端机低位放高位，高位放低位，大端机
int main()
{
	printf("%d\n", isBigSystem());

	return 0;
}