C - Pointer to Pointer (Double Pointer)

In C, double pointers are those pointers which stores the address of another pointer. The first pointer is used to store the address of the variable, and the second pointer is used to store the address of the first pointer. That is why they are also known as a pointer to pointer.

#include <stdio.h>

int main()
{

    // A variable
    int var = 10;

    // Pointer to int
    int *ptr1 = &var;

    // Pointer to pointer (double pointer)
    int **ptr2 = &ptr1;

    printf("var: %d\n", var);
    printf("*ptr1: %d\n", *ptr1);
    printf("**ptr2: %d", **ptr2);

    return 0;
}

var: 10
*ptr1: 10
**ptr2: 10

Explanation: In this code, ptr1 is a pointer that stores the address of the integer variable var. ptr2 is a double pointer that stores the address of the pointer ptr1. **ptr2 dereferences ptr2 to get the value of ptr1 (which is the address of var) and then dereferences that address to get the value of var itself.

Size of Double Pointer

Size of a double pointer is same as size of any other pointer as it also stored address.

#include <stdio.h>

int main()
{

    // Defining single and double pointers
    int a = 5;
    int *ptr = &a;
    int **d_ptr = &ptr;

    // Size of double pointer
    printf("%d bytes\n", sizeof(d_ptr));
    
    
    // Size of a single pointer
    printf("%d bytes", sizeof(ptr));

    return 0;
}

8 bytes
8 bytes

Note: The output of the above code also depends on the type of machine which is being used.

Create a Dynamic 2D Array

Double pointers are useful in creating a dynamically sized 2D array where every row can have different number of elements.

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int m = 2, n = 3;

    // Create a double pointer
    int **arr;

    // Allocate memory for rows
    arr = (int **)malloc(m * sizeof(int *));

    // Allocate memory for each row
    for (int i = 0; i < m; i++)
        arr[i] = (int *)malloc(n * sizeof(int));

    // Initialize with some values
    for (int i = 0; i < m; i++)
    {
        for (int j = 0; j < n; j++)
        {
            arr[i][j] = i * n + j + 1;
        }
    }

    // Print the array
    for (int i = 0; i < m; i++)
    {
        for (int j = 0; j < n; j++)
        {
            printf("%d ", arr[i][j]);
        }
        printf("\n");
    }

    // Free allocated memory
    for (int i = 0; i < m; i++)
    {
        free(arr[i]);
    }
    free(arr);

    return 0;
}

1 2 3 
4 5 6 

Explanation: A 2D array is an array where each element is essentially a 1D array. This can be implemented using double pointers. The double pointer points to the first element of the 2D array, and each pointer it references points to a dynamically allocated 1D array using malloc().

Pass Array of Strings to Function

#include <stdio.h>

// Function that takes array of strings as argument
void print(char **arr, int n)
{
    for (int i = 0; i < n; i++)
        printf("%s\n", *(arr + i));
}

int main()
{
    char *arr[10] = {"Geek", "Geeks", "Geekfor"};
    print(arr, 3);
    return 0;
}

Geek
Geeks
Geekfor

Explanation: Array of strings are generally stored as array of pointer to strings. This can be passed using double pointers to function.

Application of Double Pointers in C

Following are the main uses of pointer to pointers in C:

• They are used in the dynamic memory allocation of multidimensional arrays.
• They can be used to store multilevel data such as the text document paragraph, sentences, and word semantics.
• They are used in data structures to directly manipulate the address of the nodes without copying.
• They can be used as function arguments to manipulate the address stored in the local pointer.

Multilevel Pointers in C

Double Pointers are not the only multilevel pointers supported by the C language. What if we want to change the value of a double pointer? In this case, we can use a triple pointer, which will be a pointer to a pointer to a pointer i.e, int ***t_ptr.

#include <stdio.h>

int main()
{
    int num = 10;
    int *ptr = &num;
    int **ptr2 = &ptr;

    // accessing values
    printf("Value of num: %d\n", num);
    printf("Value using single pointer: %d\n", *ptr);
    printf("Value using double pointer: %d\n", **ptr2);

    return 0;
}

Value of num: 10
Value using single pointer: 10
Value using double pointer: 10

Note: We can use any level pointer in C. There is no restriction about it but it makes the program very complex and vulnerable to errors.

C - Pointer to Pointer

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