[advanced C language] user defined type

1、 Structure

1.1、 Structure declaration

A special case ：
If the name of the structure is ignored in the declaration , Anonymous structure （ It's best not to be anonymous to the structure ）

1.2、 Structure self reference

The correct way of structure self reference

struct Node
{
    
	int data;
	struct Node* next;
};

int main()
{
    
	return 0;
}

1.3、 Structure variable definition and initialization

Various types of structure initialization methods

#include <stdio.h>

typedef struct Node
{
    
	int data;
	struct Node* next;
}Node;

struct stu
{
    
	char name[20];
	char sex[5];
	int age;
	int height;
}s2,s3,s4;// Variable list (s2,s3,s4 Is the global structure variable )

struct Data// Nested structures within structures 
{
    
	struct stu s;
	char ch;
	double;
};

int main()
{
    
	struct Node n2 = {
     100,NULL };// Initialize with braces 
	struct stu s1 = {
     "zhangsan","nan",20,180 };

	struct Data d = {
     {
    "lisi","nv",30,166},'w',3.14 };
	// How nested structures are initialized 
	return 0;
}

1.4、 Structure memory alignment （ a key ）

1、 The first member of the structure , Stored at the beginning of the structure variable 0 Offset

2、 Start with the second member , Align to Align numbers At an integer multiple address of

3、VS Default in phase Align numbers by 8, The smaller value of the member's own size and the default alignment number , Is the alignment number

4、 The total size of the structure must be Maximum number of alignments Integer multiple

5、 Maximum number of alignments It refers to the largest alignment number of all members

6、 If the structure is nested , The nested structure is aligned to an integral multiple of its maximum alignment , The overall size of the structure is the maximum number of alignments （ The number of alignments with nested structures ） Integer multiple

7、 We can go through #pragma pack To change the default alignment number of structure memory

#pragma pack (1)// The default number of alignments is changed to 4, Change it to  1  That is, it is not aligned , The general revision is 2,4,6……
struct S 
{
    
	double d;
	char c;
};
#pragma pack()

PS:Linux There is no default alignment number in the environment , The number of alignments is the size of the member itself

1.5、 Structural parameters

1、 There are two ways to transfer parameters from a structure , The first is value transfer , The other is the address

2、 Passing values will cause a lot of memory space waste , Therefore, generally choose the transmission address

Little knowledge

1、typedef yes C Keywords in language , Its effect is Define a new name for a data type

2、 Why is there structure memory alignment

Performance reasons ： To access unaligned memory , The processor needs to do two memory accesses , The aligned memory access only needs one access , So structure memory alignment is Trade space for time How to do it , So when we design the structure , We have to satisfy the alignment , And save space , Need to let Members who take up less space try to get together

3、ofsetof（） Is a macro , Returns the offset of the structure member in memory

#include<stdio.h>
#include<stddef.h>
struct S
{
    
	char c1;
	int a;
	char c2;
};
int main()
{
    
	//offsetof() return   Structural members   Offset in memory 
	printf("%d\n", offsetof(struct S, c1));//0
	printf("%d\n", offsetof(struct S, a));//4
	printf("%d\n", offsetof(struct S, c2));//8
	return 0;
}