double pointers

int **ptr;    // declaring double pointers

Below diagram explains the concept of Double Pointers:
When will use double pointer?

Use ** when you want to preserve (OR retain change in) the Memory-Allocation or Assignment even outside of a function call.

void alloc2(int** p) {
   *p = (int*)malloc(sizeof(int));
   **p = 10;
  printf("address(alloc2):%p and value %d\n ",*p,**p);

  //Output: address(alloc2):0x1c54440 and value 10

void alloc1(int* p) {
   p = (int*)malloc(sizeof(int));
   *p = 10;
  printf("address(alloc1):%p and value %d\n ",p,*p);
// Output: address(alloc1):0x1c54010 and value 10

int main(){
   int *p1,*p2;

   //Pass by value
   alloc1(p1);

   printf("address:%p and value %d\n ",p1,*p1);//value is undefined

 //Output:  address:0x7fffebb11160 and value 1

  
//Pass by reference

  alloc2(&p2);

   printf("addres:%p and value:%d\n ",p2,*p2); 

  //output:   addres:0x1c54440 and value:10

   free(p2);
   return 0;
}