C language flexible array

Concept of flexible array

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

struct S
{
    
	int a;
	int arr[0];// there arr[0] Represents an array of unknown size 
};
// Or as follows ：
struct S
{
    
	int a;
	int arr[];
};

The characteristics of flexible arrays

• A flexible array member in a structure must be preceded by at least one other member

struct S
{
    
	int a;
	int arr[];
};

A flexible array must have at least two members , Flexible array must be the last member
The first one above may not compile in some compilers , It can be changed to the second method

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

use sizeof Calculate the size , Flexible arrays are not computed

• 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 , To fit the expected size of the flexible array
  

The use of flexible arrays

typedef struct s
{
    
	int n;
	int arr[];
}s;

int main()
{
    
	s* pd = malloc(sizeof(s) + 40);
	pd->n = 100;
	int i = 0;
	for (i = 0; i < 10; i++)
	{
    
		pd->arr[i] = i;
		printf("%d ", pd->arr[i]);
	}
}

Code results ：

• The above code is given in the form of flexible array arr Allocate space and print
  to arr Distribute 40 Give byte space

Advantages of flexible arrays

Use pointer mode ：

typedef struct s
{
    
	int n;
	int* arr;
}s;

int main()
{
    
	s* pd = malloc(sizeof(s));// To members n Open up space 
	pd->n = 100;
	pd->arr = malloc(40);// To members  arr  Open up space 
	if (pd->arr == NULL)
	{
    
		return 1;
	}
	// Use 
	int i = 0;
	for (i = 0; i < 10; i++)
	{
    
		pd->arr[i] = i;
		printf("%d ", pd->arr);
	}
	// Release 
	free(pd);// What's released is  n  member 
	free(pd->arr);
	pd = NULL;
	return 0;
}

• From the comparison between the above code and the code of flexible array, we can see , The code of flexible array is more convenient , just malloc once
  And the implementation through pointer requires malloc secondary , And release it many times , It is trouble

• Advantages of flexible arrays ：
  1. Convenient memory release
  Write code and do it twice malloc Memory allocation , And return the whole structure , It's going on free When releasing , You may forget to release other members of the structure
  So we should allocate the memory of the structure and its members at one time , Do it once free You can also free up all the memory
  2. Reduce memory fragmentation
  Too much use Dynamic memory functions will fragment memory , It's a waste of space , It will reduce the speed of access