C tutorial

Pointer to a Pointer(Double Pointer)

Pointers are used to store the address of other variables of similar datatype. But if you want to store the address of a pointer variable, then you again need a pointer to store it. Thus, when one pointer variable stores the address of another pointer variable, it is known as Pointer to Pointer variable or Double Pointer.

Syntax:

int **p1;

Here, we have used two indirection operator(*) which stores and points to the address of a pointer variable i.e, int *. If we want to store the address of this (double pointer) variable p1, then the syntax would become:

int ***p2

Simple program to represent Pointer to a Pointer

#include <stdio.h>

int main() {

    int  a = 10;
    int  *p1;       //this can store the address of variable a
    int  **p2; 
    /*
        this can store the address of pointer variable p1 only. 
        It cannot store the address of variable 'a' 
    */

    p1 = &a;
    p2 = &p1;

    printf("Address of a = %u\n", &a);
    printf("Address of p1 = %u\n", &p1);
    printf("Address of p2 = %u\n\n", &p2);

    // below print statement will give the address of 'a'
    printf("Value at the address stored by p2 = %u\n", *p2);
    
    printf("Value at the address stored by p1 = %d\n\n", *p1);

    printf("Value of **p2 = %d\n", **p2); //read this *(*p2)

    /*
        This is not allowed, it will give a compile time error-
        p2 = &a;
        printf("%u", p2);
    */
    return 0;
}

Address of a = 2686724 Address of p1 = 2686728 Address of p2 = 2686732 Value at the address stored by p2 = 2686724 Value at the address stored by p1 = 10 Value of **p2 = 10

Explanation of the above program

• p1 pointer variable can only hold the address of the variable a (i.e Number of indirection operator(*)-1 variable). Similarly, p2 variable can only hold the address of variable p1. It cannot hold the address of variable a.
• *p2 gives us the value at an address stored by the p2 pointer. p2 stores the address of p1 pointer and value at the address of p1 is the address of variable a. Thus, *p2 prints address of a.
• **p2 can be read as *(*p2). Hence, it gives us the value stored at the address *p2. From above statement, you know *p2 means the address of variable a. Hence, the value at the address *p2 is 10. Thus, **p2 prints 10.