Dynamic memory management

Common memory development ：

frequently-used int、long、char Type and output open up space , The size of the open space is fixed .

int a;// The space opened in the stack space is four bytes 
char a;// The space opened in the stack space is one byte 
short a;// The space opened in the stack space is two bytes 
long a;// The space opened up in the stack space is eight bytes 
int arr[10]={0};// Open up on the stack space 40 A contiguous space of bytes 

Dynamic memory exploit function ：

malloc function ：void* malloc(size_t size);

malloc The basic function is to apply for continuous available space from memory , And return a pointer to this deficient space .

If the development is successful , The thief returns a pointer to the opened space .

If the development fails , Then return a null pointer , So it's using malloc Functions are generally used if Function determines the return value of the function .

int *p=(int *)malloc(40);// Open up for 10 An integer space 
// Because the return type is void*, So the specific type should be set by yourself 
if(p==NULL)
{
    printf("%s\n",strerror(errno));
    return 0;
}

free function ：void free(void *ptr);

free The function is used to release dynamically opened memory space .

calloc function ：void *calloc(site_t num,size_t size);

calloc The function function is to num Size is size The elements of open up a space , And initialize each byte of the space to 0.

int *p=(int *)calloc(10,sizeof(int));// open up 10 A space of integer size 
int *p=(int *)malcoc(40);// open up 10 A space of integer size 

malloc Function and calloc Function also opens up 40 Bytes of space , But the difference between the two functions is calloc The opened space will initialize each byte to 0.

So if there are requirements for the initial value of the open space , In general use malloc function , Define the initial value of space by yourself , Not too many requirements can be used calloc Function to open up space .

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

realloc The function adjusts the memory size .

and realloc There may be two situations during capacity increase and adjustment ：

1. After the original space, there is enough space to increase capacity .

2. There is not enough space to increase the capacity after the original space .

  Appearance 1 when , Capacity expansion will not lead to changes in the original spatial data , And the situation 2, In case of capacity increase, find another large enough continuous space on the heap space to use , So the return value of the function is a new memory space .

When something happens 2 when , If you directly assign the increased space to the original space , Return when capacity increase fails NULL The pointer , The original data will be lost , So it's using realloc The function is to verify whether the augmented space is a null pointer , I'm doing an assignment .