How do pointer to pointers work in C?

Question

How do pointers to pointers work in C? When would you use them?

Answer 1

Consider the below figure and program to understand this concept better.

As per the figure, ptr1 is a single pointer which is having address of variable num.

ptr1 = #

Similarly ptr2 is a pointer to pointer(double pointer) which is having the address of pointer ptr1.

ptr2 = &ptr1;

A pointer which points to another pointer is known as double pointer. In this example ptr2 is a double pointer.

Values from above diagram :

Address of variable num has : 1000
Address of Pointer ptr1 is: 2000
Address of Pointer ptr2 is: 3000

Example:

#include 

int main ()
{
   int  num = 10;
   int  *ptr1;
   int  **ptr2;

   // Take the address of var 
   ptr1 = #

   // Take the address of ptr1 using address of operator &
   ptr2 = &ptr1;

   // Print the value
   printf("Value of num = %d\n", num );
   printf("Value available at *ptr1 = %d\n", *ptr1 );
   printf("Value available at **ptr2 = %d\n", **ptr2);
}

Output:

Value of num = 10
Value available at *ptr1 = 10
Value available at **ptr2 = 10