** in C

This tutorial teaches how to use the Pointer to Pointer(Double pointer or ** ) to store the address of another pointer variable.

Memory Allocation for Variable in C

On creating a variable, there will be some specific block of memory allocated to that variable for storing the value. For example, we have created a char variable ch and value a. Internally, one byte of memory will be allocated to the variable ch.

C Pointer

In C programming, a pointer is a variable that stores the address of another variable. To access the value present in that address, we use *.

#include <stdio.h>

int main() {
  char ch = 'a';    // create a variable
  char *ptr = &ch;  // create a pointer to store the address of ch

  printf("Address of ch: %p\n", &ch);  // prints address
  printf("Value of ch: %c\n", ch);     // prints 'a'

  printf("\nValue of ptr: %p\n", ptr);  // prints the address of a
  printf("*ptr(value of ch): %c\n",
         *ptr);  // Prints Content of the value of the ptr
}

Output:

Address of ch: 0x7ffc2aa264ef
Value of ch: a

Value of ptr: 0x7ffc2aa264ef
*ptr(value of ch): a

In the above code,

• Created a char variable ch and assigned the character a as a value.
• Created a char pointer ptr and stored the address of the variable ch.
• Printed the address and value of ch.
• Printed the value of the ptr, the value of ptr will be the address of ch
• Printed the value of ch by using *ptr. The value of ptr is the address of the variable ch, in that address, the value 'a' is present, so it will be printed.

Pointer to Pointer (**) in C

To store the address of a variable, we use a pointer. Similarly, to store the address of a pointer, we need to use ** (pointer to pointer). ** denotes a pointer that stores the address of another pointer.

To print the value present in the pointer to pointer variable, we need to use **.

#include <stdio.h>

int main() {
  char ch = 'a';           // create a variable
  char *ptr = &ch;         // create a pointer to store the address of ch
  char **ptrToPtr = &ptr;  // create a pointer to store the address of ch

  printf("Address of ch: %p\n", &ch);  // prints address of ch
  printf("Value of ch: %c\n", ch);     // prints 'a'

  printf("\nValue of ptr: %p\n", ptr);   // prints the address of ch
  printf("Address of ptr: %p\n", &ptr);  // prints address

  printf("\nValue of ptrToPtr: %p\n", ptrToPtr);  // prints the address of ptr
  printf("*ptrToPtr(Address of ch): %p\n",
         *ptrToPtr);  // prints the address of ch
  printf("**ptrToPtr(Value of ch): %c\n", **ptrToPtr);  // prints ch
}

Output:

Address of ch: 0x7fffb48f95b7
Value of ch: a

Value of ptr: 0x7fffb48f95b7
Address of ptr: 0x7fffb48f95b8

Value of ptrToPtr: 0x7fffb48f95b8
*ptrToPtr(Address of ch): 0x7fffb48f95b7
**ptrToPtr(Value of ch): a

In the above code,

• Created a char variable ch and assigned the character a as a value to it.
• Created a char pointer ptr and stored the address of the variable ch.
• Created a char pointer to pointer ptrToPtr and stored the address of the variable ptr.
• The ptr will have the address of the variable ch as value, and the ptrToPtr will have the address of pointer ptr as value.
• When we dereference the ptrToPtr like *ptrToPtr we get address of variable ch
• When we dereference the ptrToPtr like **ptrToPtr we get value of variable ch

Point to Remember

In order to store the address of ptrToPtr we need to create

char ***ptrToPtrToPtr = &ptrToPtr;
printf("***ptrToPtrToPtr : %c\n", ***ptrToPtrToPtr);  // 'a'