Pointer to Pointer in C with Example

Question: What is Pointer to Pointer in C Programming?

Answer: A pointer is a variable that holds address, address of a variable of any basic types, address of a structure, address of an array or address of some other pointer variable. For example,

    int val = 10;
    int *pi = &val;    /* pi is a pointer pointing to integer val */
    int **ppi = π   /* ppi is a pointer to pointer to int */

Can you figure out results of following statements?

    val;
    *pi;
    **ppi;

val is simplest gives value 10. What about value of *pi? We initialized pi address of integer val. Therefore, pi is pointing to val and we know that indirection on a pointer to something is that thing.

    *pi;

results 10. Similarly, ppi is pointing to pi, single indirection on ppi, i.e.

    *ppi;

results pi which is pointer to val. Therefore, double indirection on ppi, i.e.

    **ppi;

or equivalently,

    *(*ppi);

or equivalently,

    *(pi);

which is *pi; and this results 10.

Why do we ever need a pointer to pointer when simple assignment works? Actually, consider the instance of insertion of new value in the singly linked list, fragment of code I’ve written below for easy understanding,

/* let's insert new value in the ordered list */
void insert_value(Node **linkp, int new_value)
{
    Node *new_node;
    Node *current;
 
    /* 
     * walk down the list to find appropriate place to insert
     * the new value
     */
    while ((current = *linkp) != NULL && current->data < new_value)
            linkp = &current->link;
    /* Let's allocate new node dynamically */
    new_node = (Node *)malloc(sizeof(Node));
 
    /* Let's confirm if new_node allocated successfully */
    if (new_node == NULL) {
            puts("ERROR: new_node allocation!");
            exit(2);
    }
 
    /* write-in new_value into value field of new_node */
    new_node->data = new_value;
 
    /* Let's insert new_node into the list */
    new_node->link = current; /* new_node points to current node */
    *linkp = new_node;        /* previous node points to new_node */
    puts("**New value successfully added into the list!**\n");
}

Notice here that a simple variable is not known within the function scope and linkp which is pointer to pointer to type Node makes insertion easy and efficient.

Sanfoundry Global Education & Learning Series – 1000 C Tutorials.

If you wish to look at all C Tutorials, go to C Tutorials.