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why we use double pointer struct Node** head here? can anyone explain with details

 

/* Given a reference (pointer to pointer) to the head of a DLL and an int, appends a new node at the end */

void append(struct Node** head_ref, int new_data)
{
  struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));
  
  struct Node* last = *head_ref; /* used in while loop below*/
  
  new_node->data = new_data;
  /*This new node is going to be the last node, so make next of it as NULL*/
  new_node->next = NULL;
  
  /*node as head */
  if (*head_ref == NULL) 
  {
    new_node->prev = NULL;
    *head_ref = new_node;
    return;
  }

  while (last->next != NULL)
    last = last->next;

  last->next = new_node;
  new_node->prev = last;

  return;
 }
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if suppose $H$ is a node pointer pointing to the $1^{st}$ node of the DLL and suppose address of $1^{st}$ node is $1000$.

So $H$ stores the address $1000$ and let $H$ is stored at the address $9500$ in the memory

Then $ ^{**}head$ is a pointer pointing to a node pointer. So it can point to $H$

if it points to $H$ then $head$ stores the address $9500$.

$ ^{*}head$ means we are dereferencing $head$ one time

= $ ^{*} $(value stored at $head$)

= $^{*} (9500)$ (value stored at 9500)

= $1000$

This means to access the address of  $1^{st}$ node either we can write $H$ or we can write $ ^{*}head$

In your code they have used $ ^{*}head$ instead of using $H$

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