Pointer to Pointer in C – Double Pointer in C

Introduction:

We have looked at the introduction to pointers, and pointer arithmetics in earlier articles. In today’s article, We will look at the pointer to pointer in c language or Double pointer in C programming language with Example Programs.

What is Pointer to Pointer or Double Pointer in C Language:

As we know a pointer is a special variable that holds the address of another variable.

The pointer is also a variable and it is stored somewhere in memory and it will also have a memory address. We can store this address of the pointer variable in another variable. This variable is called as the Pointer to Pointer variable or Double pointer in C language.

In simple terms, The double pointer is a pointer that stores the address of another pointer variable.

Syntax of double pointer or pointer to pointer in C language:

Here

• The doublePointerName is the double-pointer or pointer to pointer.
• datatype is the data type of the variable pointed by the double-pointer.

Let’s look at a couple of examples of pointer to pointer variable to make our understanding concrete.

In the above example, We Initialized an integer variable named price with value 110.

int price = 110;

Then Created an Integer pointer variable, That stores the address of the price variable.

int *ptr = &price;

Now, To store the address of the pointer variable *ptr, we need to create a pointer to pointer variable or double pointer variable. We can create a double pointer like below.

int **ptr = &ptr;

Here we created a double pointer variable pptr with an integer datatype, Which is storing the address of the pointer variable ptr.

If you notice, The double pointer has two Asterisks before the variable name ( int **ptr). This will let the compiler know that we are working with a double pointer.

Program to understand the double-pointer or pointer to pointer in C:

please look at the program explanation section below for a detailed explanation.

Program Output:

Let’s compile and run the program using the GCC compiler. (Any of your favorite compilers)

Pointer to Pointer Program Explanation in C:

Here is the graphical representation of how the above integer variable, pointer variable, and a pointer to pointer(Double pointer) variables addresses are mapped.

As we can see from the above output and graphical representation of the double-pointer or pointer to pointer variable,

• The price variable is stored at the memory address 0x7ffc7eb45a54 and the value of the price variable is 110.
• The pointer variable ptr is storing the address of the price variable, So the value of the integer pointer ptr is equal to the price variable address i.e 0x7ffc7eb45a54. The ptr variable also has its own memory address which is equal to 0x7ffc7eb45a58 ( &ptr )
• Finally, We created the pointer to the pointer variable pptr ( i.e double pointer variable) which is used to store the address of the pointer variable ptr.
  • So the value of the double-pointer pptr is equal to the address of the ptr. That is 0x7ffc7eb45a58.
  • Again, The double-pointer variable pptr also has its own memory address which is equal to the 0x7ffc7eb45a60.

We can extend the pointer-to-pointer concept further, Let’s say, If you want to store the address of the pptr variable(pointer to pointer), Then we need to create the pointer to pointer to pointer (triple pointer) variable ( int ***ppptr). Which can store the address of the double pointer pptr.

This pointer-to-pointer notion can be expanded indefinitely. But in practice, we usually use up to the double-pointer.

How to get values from pointer to pointer/double pointer in C:

Now, Let’s look at how to get the values from the pointer to pointer variable.

To get the value from the pointer variable, We use the dereference operator or indirection operation ( *). Similarly, To get the value pointed by the pointer to pointer variables we need to use two dereference operators( ** – Two asterisks)

Let’s say we have an integer variable mars with value 4

int mars = 4;

Here are the pointer and double pointers which are storing the address of mars and marsPtr respectively.

int *marsPtr = &mars;

Double pointer

int **marsPPtr = &marsPtr;

Now, The marsPPtr is a pointer to a pointer(aka double pointer). To get the value of the mars, We need to dereference two times.

The first dereference operation on the double pointer( marsPPtr) will give the value of the pointer( marsPtr )

The value of marsPtr is equal to *marsPPtr

Similarly, The two times dereferencing the double pointer( marsPPtr) will give us the value of the original variable(i.e mars )

The value of mars is equal to **marsPPtr;

Let’s look at an example program on getting the value from the double pointer in c

Program to Get the value from pointer to pointer or double pointer in C ( use double dereference):

In the following program, We have created three variables in the above program

• mars integer variable, which holds the value 4
• Pointer variable marsPtr, which is storing the address of the mars
• Double pointer or pointer to pointer variable marsPPtr, which is storing the address of the marsPtr

Program Output:

Compile the program.

$ gcc double-pointer-dereference.c

Run the program.

As we can see from the above program and program output, To get the values of mars and marsPtr using the double pointer, Then

• Dereference once to get the value of the marsPtr using double pointer – *marsPPtr
• Dereference twice to get the integer mars value – **marsPPtr

Related C Language Tutorials:

• C Tutorials Index
• C Programs Index – 300+ Programs
• Bitwise OR ( | )
• Bitwise AND( & )
• Bitwise XOR ( ^ )
• Bitwise Right Shift Operator ( >> )
• Bitwise Left Shift Operator ( << )
• Bitwise Ones Complement Operator ( ~ )
• Set, Clear, Toggle Bit, and Check a single Bit in C Language