Advanced C language (I) dynamic memory allocation

The header file ：stdlib.h  string.h

One . Request dynamic allocation of memory malloc

stdlib.h Define function in void* malloc(size_t size), Apply to the system The size is size Of memory space . The return is void*, Convert to the required pointer type when using . If the application fails , return NULL.

void A pointer is a special kind of pointer , Expressed as “ No type pointer ”, Any type of pointer can be assigned directly to void The pointer , There is no need for other related cast types , Even so , But this does not mean that you can cast void Pointers are directly assigned to other types of pointers , To cast .

Two . Free dynamically allocated memory free()

#include<stdio.h>
#include<stdlib.h>
int main(){
	int n;
	scanf("%d",&n);
	
	//int arr[n];
	//malloc Write input array  
	int* arr = malloc(sizeof(int)*n);
	for(int i=0;i<n;i++){
		scanf("%d",&arr[i]);
	}
	for(int i=0;i<n;i++){
		printf("%d",&arr[i]);
	}
	free(arr);
	arr=NULL;
	// Do not set the pointer to NULL Yes. . But if you happen to new/malloc Apply for allocation to this memory , Unpredictable things will happen .
    // So the released pointer is for safety , To set it to NULL, Otherwise, it is easy to cause the appearance of wild pointer .
}

3、 ... and . Initialize dynamic memory allocation calloc

stay Linux in malloc and calloc All will be assigned

int* arr = (int*)calloc(n,sizeof(int));
free(arr);

Approximate to

int arr[n] = {0};

Four . Resize memory realloc

void* realloc (void* ptr, size_t size);

• If there is enough memory space behind the current memory segment , Then expand this memory directly ,realloc() Return to the original first address ;
• If there is not enough memory space behind the current memory segment , Then the system will re apply to the memory tree for a suitable space , And release the data blocks in the original space , and realloc() The first address of the heap space reapplied will be returned ;
• If the creation fails , return NULL, At this time, the original pointer is still valid ;