C – Pointer To Pointer (Double Pointer)

C – Pointer To Pointer (Double Pointer)

A pointer-to-pointer (double pointer) is used to store the address of another pointer. The first pointer stores the address of a variable, and the second pointer stores the address of the first pointer. This makes them useful for changing the values of normal pointers or creating variable-sized 2-D arrays. Importantly, a double pointer occupies the same amount of memory as a regular pointer on the stack.

Declaration of Pointer to a Pointer in C

We know that declaring a pointer will be similar to declaring a double pointer in C. The only difference, we can notice is the adding of * just before the pointer name.

data_type_of_pointer **name_of_variable = & normal_pointer_variable;
int val = 5;
int *ptr = &val;    // storing address of val to pointer ptr.
int **d_ptr = &ptr; // pointer to a pointer declared
                    // which is pointing to an integer. 

Therefore, this diagram illustrates the first pointer ptr 1 will store the address of the variable and the second pointer ptr 2 can store the address of the first pointer.

Example of Double Pointer in C

// C program to demonstrate pointer to pointer
#include <stdio.h>
 
int main()
{
    int var = 789;
 
    // pointer for var
    int* ptr2;
 
    // double pointer for ptr2
    int** ptr1;
 
    // storing address of var in ptr2
    ptr2 = &var;
 
    // Storing address of ptr2 in ptr1
    ptr1 = &ptr2;
 
    // Displaying value of var using
    // both single and double pointers
    printf("Value of var = %d\n", var);
    printf("Value of var using single pointer = %d\n", *ptr2);
    printf("Value of var using double pointer = %d\n", **ptr1);
 
    return 0;
}

Output

Value of var = 789
Value of var using single pointer = 789
Value of var using double pointer = 789

How Double Pointer Works?

Int var =10;

int*ptr =&var;

*ptr=20;

Int**ptr=&ptr;

**ptr=30;

• Declare a double-pointer with the appropriate syntax.
• Store the address of another pointer as the value of the double-pointer.
• To manipulate or dereference to any of its levels, use the asterisk (*) operator the number of times needed to reach the desired level.

Size of Pointer to Pointer in C

A double-pointer acts similarly to a normal pointer in C. Also, the size of the pointer is equal to a double pointer.

Example 1: C Program to find the size of a pointer to a pointer

// C program to find the size of pointer to pointer
#include <stdio.h>
 
int main()
{
    // defining single and double pointers
    int a = 5;
    int* ptr = &a;
    int** d_ptr = &ptr;
 
    // size of single pointer
    printf(" Size of normal Pointer: %d \n", sizeof(ptr));
 
    // size of double pointer
    printf(" Size of Double Pointer: %d \n", sizeof(d_ptr));
 
    return 0;
}

Output

 Size of normal Pointer: 8 
 Size of Double Pointer: 8 

The size of a pointer in C is not fixed and can vary based on factors like CPU architecture and the operating system. Typically, for a 64-bit operating system, pointer sizes are 8 bytes, while for a 32-bit operating system, they are 4 bytes. This size variation is important to consider when working with pointers, especially when dealing with memory allocation and ensuring your code is compatible across different platforms and systems.

Application of Double Pointers in C

1. Dynamic Memory Allocation: Double-pointers are used to allocate and manage memory for multidimensional arrays, enabling flexible data structures.
2. Multilevel Data: They are used to store and manage complex hierarchical data structures, like text documents with paragraphs, sentences, and words.
3. Data Structures: In data structures, double-pointers allow direct manipulation of node addresses without copying data, improving efficiency.
4. Function Arguments: Double-pointers can be used as function arguments to modify the addresses stored in local pointers, facilitating efficient data manipulation.

Multilevel Pointers in C

• In C, we have multilevel pointers beyond double pointers.
• For changing the value of a double pointer, we can use a triple pointer, such as int ***t_ptr.
• Syntax: pointer_type ***pointer_name;

Syntax of Triple Pointer

pointer_type *** pointer_name;  

• This concept extends further, allowing us to use “level – x + 1” pointers to change the value of a “level – x” variable, and this can be carried to higher levels as needed.

FAQ- C – Pointer To Pointer (Double Pointer)