Dynamic memory management

• 

  Catalog

  malloc function

  free function

  calloc function  

   realloc function

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  malloc function

• First understand malloc The data type of the function ：1. The return value is void* The pointer , Parameter is unsigned integer  size_t,size Indicates the size of the space you need to open , The unit is byte
• matters needing attention ： 
  If the development is successful , Then return a pointer to open a good space .
  If the development fails , Returns a NULL The pointer , therefore malloc The return value of must be checked .

  The type of return value is void* , therefore malloc Function doesn't know the type of open space , Specifically, when using, the user himself

  To decide .

  malloc The statement in stdlib.h Header file

  If parameters size by 0,malloc The standard is undefined , Depends on the compiler . Better not let size by 0;
• Use up malloc Remember to use free Function frees the space pointed to ;

• free function

• free Function function ：free Function is used to release memory opened dynamically .
• If parameters ptr The pointed space is not opened dynamically , that free The behavior of a function is undefined .
  If parameters ptr yes NULL The pointer , Then the function does nothing .
• free The statement in stdlib.h Header file
• The specific usage is as follows ：

• #include <stdio.h>
  int main()
  {
   // Code 1
   int num = 0;
   scanf("%d", &num);
   int arr[num] = {0};
   // Code 2
   int* ptr = NULL;
   ptr = (int*)malloc(num*sizeof(int));
   if(NULL != ptr)// Judge ptr Is the pointer null 
   {
   int i = 0;
   for(i=0; i<num; i++)
   {
   *(ptr+i) = 0;
   }
   }
   free(ptr);// Release ptr The dynamic memory pointed to 
   ptr = NULL;// Is it necessary to ？
   return 0; }

  Finally, put the pointer ptr Into a NULL highly necessary , Otherwise, it will cause wild pointer .

  • calloc function  

  • 
  •  calloc The return value of void*, Parameters are two unsigned integers ,num It means that you need to open up several units of space , and size Indicates the size of unit space , Unit is byte , And initialize each byte of the space to 0.; for example , I want to open up 40 Bytes of space , May make num=10,size=4, The size of the space opened up is equal to num*size.
  • malloc and calloc The difference is that ,calloc While opening up space , Also initialize each byte of the requested space to zero .
  • Other functions and malloc Almost the same
  • 

  •  realloc function

  • 

    Parameters in ptr The pointer points to Memory address to adjust ,

  size New size after adjustment

  The return value is the starting memory position after adjustment .

  This function adjusts the size of the original memory space , The original data in memory will also be moved to new Space .

  • realloc There are two ways to adjust memory space ：
  • 1. The original space is big enough
  • 2. There is not enough space after the original space ：
  • 

     

   

  situation 1

  When it's the case 1 When , To expand memory, add space directly after the original memory , The original spatial data does not change .

  situation 2

  When it's the case 2 When , When there is not enough space after the original space , The way to expand is ： Find another suitable size on the heap space

  Continuous space to use . This function returns a new memory address .

  Because of the above two situations ,realloc We should pay attention to the use of functions