author ： A collection of old dreams 186

special column ：C Language programming ---- Little bit's Growth Diary

Related links ：C Detailed explanation of language custom types ： Structure

 

Daily inspirational ：

Some people , Only do two things in a lifetime , Refuse to accept , strive for , So it's getting better and better . There are also people. , Only do two things in a lifetime , wait for , regret , So the more mixed, the worse . 

Preface ：

This article is also the final chapter of the structure , Xiaobian takes you to know the flexible array in the structure . 

Catalog

One . Definition ：

Two . The characteristics of flexible arrays  

  3、 ... and . The use of flexible arrays

  Four . The advantages of flexible arrays

One . Definition ：

Maybe you've never heard of Flexible array （flexible array） The concept , But it does exist .

C99 in , The last element in the structure is allowed to be an array of unknown size , This is called 『 Flexible array 』 member .

for example ：

typedef struct st_type
{
 int i;
 int a[0];// Flexible array members 
}type_a;

Some compilers will report errors and cannot compile. They can be changed to ：

typedef struct st_type
{
 int i;
 int a[];// Flexible array members 
}type_a;

 

Two . The characteristics of flexible arrays  

1. Flexible array members in structure Must be preceded by at least one other member .

2.sizeof The size of the structure returned does not include the memory of the flexible array .

3. Structures that contain flexible array members use malloc () Function to dynamically allocate memory , And the allocated memory should be larger than the size of the structure Small , To fit the expected size of the flexible array .

for example ：

#include<stdio.h>
#include<stdlib.h>
typedef struct st_type
{
	int i;
	int a[0];// Flexible array members 
}type_a;
int main()
{
	printf("%d\n", sizeof(type_a));// The output is 4
}

 

 

  3、 ... and . The use of flexible arrays

#include<stdio.h>
#include<stdlib.h>
struct pep
{
	int n;
	int a[];
};
int main()
{
	struct pep* ps=(struct peo*)malloc(sizeof(struct pep)+40);
	if (ps == NULL)
	{
		return 1;
	}
	ps->n = 10;
	
	for (int i = 0; i < 10; i++)
	{
		
		ps->a[i] = i;
	}
	for (int i = 0; i < 10; i++)
	{

		printf("%d",(ps->a[i]));
		printf("\n");
	}
	free(ps);
	return 0;
}

It is equivalent to applying space for integer and flexible arrays in the heap . 

  Four . The advantages of flexible arrays

We can compare the two codes

Code 1：

#include<stdio.h>
#include<stdlib.h>
struct pep
{
	int n;
	int a[];
};
int main()
{
	struct pep* ps=(struct peo*)malloc(sizeof(struct pep)+40);
	if (ps == NULL)
	{
		return 1;
	}
	ps->n = 10;
	
	for (int i = 0; i < 10; i++)
	{
		
		ps->a[i] = i;
	}
	for (int i = 0; i < 10; i++)
	{

		printf("%d",(ps->a[i]));
		printf("\n");
	}
	free(ps);
	return 0;
}

Code 2：

#include<stdio.h>
#include<stdlib.h>
struct pep
{
	int n;
	int *a;
};
int main()
{
	struct pep* ps = (struct pep*)malloc(sizeof(struct pep) + 40);
	if (ps == NULL)
	{
		return 1;
	}
	ps->n = 10;
	ps->a = (int*)malloc(40);
	if (ps->a == NULL)
	{
		return 1;
	}
	for (int i = 0; i < 10; i++)
	{

		ps->a[i] = i;
	}
	for (int i = 0; i < 10; i++)
	{

		printf("%d", (ps->a[i]));
		printf("\n");
	}
	free(ps->a);
	free(ps);
	ps = NULL;
	return 0;
}

Above Code 1 and Code 2 It can do the same thing , however Method 1 There are two benefits to the implementation of  

The first advantage is ： Convenient memory release

If our code is in a function for others , You do a secondary memory allocation in it , And return the whole structure to

user . The user calls free You can release the structure , But the user doesn't know that the members in the structure also need free, So you

You can't expect users to find out . therefore , If we allocate the memory of the structure and the memory of its members at one time

了 , And return a structure pointer to the user , The user does it once free You can also free up all the memory .

 

The second advantage is ： This is good for access speed .

Continuous memory is good for improving access speed , Also beneficial to reduce Inside save broken slice .（ Actually , Personally, I don't think it's much higher , Anyway

You can't run. You have to add offsets to address ）

 

  Conclusion ：

Everyone's growth is ability and what he wants , Continuous matching process , When your talent doesn't match your desire , You should calm down and study , If Xiaobian's summary can help you , I hope you guys will pay more attention for three consecutive days , Your support is the biggest motivation for Xiaobian's creation .