Introduction to flexible array

Flexible array ：

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

Some compilers will report errors and cannot compile , This can be written in the form left .

struct S                        struct S
{                                {
int n;                             int n;
int arr[];                         int arr[0];// Unknown size , It can be resized 
};                                };

Here is an example of using a flexible array ：

struct S
{
int n;
int arr[];
};
int main()
{
struct S *ps=(struct*)malloc(sizeof(struct S)+5*sizeof(int));
ps->n=100;
int i=0;
for(i=0;i<5;i++)
{
ps->arr[i]=i;
}
struct S *ptr=realloc(ps,44);
if(ptr!=NULL)
{
ps=ptr;
}
for(i=5;i<10;i++)
{
ps->arr[i]=i;
}
for(i=0;i<10;i++)
{
pintf("%d ',ps->arr[i]);
}
free(ps)
ps=NULL;
return 0;
}

Here is an example of using a pointer to resize ：

struct S
{int n;
int *arr;
};
int mian()
{
struct S*ps=(struct S*)malloc(sizeof(struct S));
ps->arr=malloc(5*sizeof(int));
int i=0;
for(i=0;i<5;i++)
{
ps->arr[i]=i;
}
int *ptr=realloc(ps->arr,10*sizeof(int));
if(ptr!=NULL)
{
ps=ptr;
}
for(i=5;i<10;i++)
{
ps->arr[i]=i;
}
for(i=0;i<10;i++)
{
printf("%d ",ps->arr[i]);
}
free(ps->arr);
ps->arr=NULL;
free(ps);
ps=NULL;
return 0;
}

 

  The characteristics of flexible arrays ：

• A flexible array member in a structure must be preceded by at least one other member
• sizeof The size of the structure returned does not include the memory of the flexible array
• Structures that contain flexible array members use malloc Function for dynamic memory allocation , And the allocated memory should be smaller than the size of the structure , To fit the expected size of the flexible array .

advantage ：

① Convenient memory release

② Improve access speed , Reduce memory fragmentation  

Video for detailed introduction https://www.bilibili.com/video/BV1454y1v7wA?p=63&share_source=copy_web