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C language flexible array

2022-07-22 01:06:00 【East east!】

Catalog

One 、 Flexible array （flflexible array）

Two 、 The characteristics of flexible arrays ：

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

Four 、 The advantages of flexible arrays

One 、 Flexible array （flflexible array）

 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 
struct s
{
	int num;
	double d;
	int arr[];// Flexible array members 
};

perhaps Some compilers will report errors and cannot compile , Add one to the array 0

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

Two 、 The characteristics of flexible arrays ：

1. Before the flexible array member in the structure must 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 , Adapt to Expected size of the flexible array . 
typedef struct st_type
{
 int i;
 int a[0];// Flexible array members 
}type_a;
printf("%d\n", sizeof(type_a));// The output is 4
struct s
{
	int num;
	double d;
	int arr[];// Flexible array members 
};

int main()
{
	malloc(sizeof(struct s) + 40);
	return 0;
}

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

int i = 0;
type_a *p = (type_a*)malloc(sizeof(type_a)+100*sizeof(int));
// Business processing 
p->i = 100;
for(i=0; i<100; i++) {
 p->a[i] = i; }
free(p);
So flexible array members a, It's equivalent to getting 100 A continuous space of integer elements

Four 、 The advantages of flexible arrays

First advantage ：

If our code is in a function for others , You do a secondary memory allocation in it , And return the whole structure to
user . use User call 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 send Now this thing . therefore , If we allocate the memory of the structure and the memory of its members at one time
了 , And return a structure to the user The pointer , 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 , It also helps reduce memory fragmentation .
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