C language custom type explanation - Consortium

Structure ：http://t.csdn.cn/fs4UT

enumeration ：http://t.csdn.cn/GNP51

Catalog

union

1. Definition of joint type

2. Characteristics of the consortium

3. Problem practice

4. Calculation of the size of the consortium

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Preface

Originality is not easy. , Please also give me three points of support , Your little three company has a great effect on me , Will spur us to forge ahead .

union （ Shared body ）

1. Definition of joint type

Federation is also a special custom type

Variables defined by this type also contain a series of members , The feature is that these members share the same space （ So union is also called community ）

example 1：

// Declaration of union type 
union Un
{
	char c;
	int i;
};
int main()
{
	// The definition of joint variables 
	union Un un;
	// Calculate the size of the variables 
	printf("%d\n", sizeof(un));
	return 0;
}

Code results ：

2. Characteristics of the consortium

Members of the union share the same memory space , The size of such a joint variable , At least the size of the largest member （ Because the Union has to be able to keep at least the largest member ）.

example 2：

// Declaration of union type 
union Un
{
	char c;
	int i;
};
int main()
{
	// The definition of joint variables 
	union Un u;
	// Calculate the size of the variables 
	printf("%d\n", sizeof(u));

	printf("%p\n", &u);
	printf("%p\n", &(u.c));
	printf("%p\n", &(u.i));

	return 0;
}

Code results ：

At least on the first byte, everyone overlaps , Not all members of the consortium have their own separate space , But share the same space with everyone , So it's called community . It's like sharing , Have your own separate room , There are also rooms shared with others .

3. Problem practice

// Method 1
#include<stdio.h>

int Cheack_sys()
{
	int a = 1;
	return *(char*)&a;
}
int main()
{
	int ret = Cheack_sys();
	if (1 == ret)
	{
		printf(" The small end \n");
	}
	else
	{
		printf(" Big end \n");
	}
	return 0;
}

// Method 2 -  Consortium solution 
#include<stdio.h>

int Cheack_sys()
{
	union un
	{
		int i;
		char c;
	}u;
	u.i = 1;
	return u.c;
}
int main()
{
	int ret = Cheack_sys();
	if (1 == ret)
	{
		printf(" The small end \n");

	}
	else
	{
		printf(" Big end \n");
	}
	return 0;
}

Code results ：

                                                                       

to i The assignment is 1, There is a total of 4 Bytes ,c and i Share the first byte , return 1 c If it is 1 It's the small end , Return is 0 It's big end .

4. Calculation of the size of the consortium

1. The size of the union is at least the size of the largest member .
2. When the maximum member size is not an integral multiple of the maximum number of alignments , It's about aligning to an integer multiple of the maximum number of alignments .

The consortium also has memory alignment .

example 1：

union Un1
{
	char c[5];//5// The alignment number is 1
	int i;//4// The alignment number is 4
};
union Un2
{
	short c[4];//8// The alignment number is 2
	int i;//4// The alignment number is 4
};
int main()
{
	printf("%d\n", sizeof(union Un1));
	printf("%d\n", sizeof(union Un2));
	return 0;
}

Code results ：

Structure in custom type , Bit segment , Enumerations and consortiums will be finished here , I got a lot of support .