C dereference pointer

Dereferencing a pointer means getting the value that is stored in the memory location pointed by the pointer. The dereference operator * also known as indirection operator is used to do this, and is called the dereferencing operator. When indirection operator (*) is used with the pointer variable, then it is known as dereferencing a pointer.

Usage of dereferencing pointer

Dereference a pointer is used because of the following reasons.

• It can be used to access or manipulate the data stored at the memory location, which is pointed by the pointer.
• Any operation applied to the dereferenced pointer will directly affect the value of the variable that it points to.

In the below example steps to dereference a pointer is given.

1) Declare the integer variable to which the pointer points.

int x =9;

2) Declare the integer pointer variable.

int *ptr;

3) Store the address of 'x' variable to the pointer variable 'ptr'.

ptr=&x;

Change the value of 'x' variable by dereferencing a pointer 'ptr' as given below:

*ptr =8;

The above code changes the value of 'x' variable from 9 to 8 because 'ptr' points to the 'x' location and dereferencing of 'ptr', i.e., *ptr=8 will update the value of x.

Below example combines all the above steps

#include <stdio.h>
int main()
{
    int x=9;
    int *ptr;
    ptr=&x;
    *ptr=8;
    printf("value of x is : %d", x);
    return 0;}

Output

Let's consider another example.

#include <stdio.h>
int main()
{
    int x=4;
    int y;
    int *ptr;
    ptr=&x; 
    y=*ptr;
    *ptr=5;
    printf("The value of x is : %d",x);
    printf("\n The value of y is : %d",y);
    return 0;
}

In the above code.

• We declare two variables 'x' and 'y' where 'x' is holding a '4' value.
• We declare a pointer variable 'ptr'.
• Assign the address of the 'x' variable to the pointer 'ptr'.
• As the 'ptr' contains the address of 'x' variable, so '*ptr' is the same as 'x'.
• Now assign the value of 'x' to 'y' with the help of 'ptr' variable, i.e., y=*ptr instead of using the 'x' variable.

Output

Let's consider another scenario.

#include <stdio.h>
int main()
{
   int a=90;
   int *ptr1,*ptr2;
   ptr1=&a;
   ptr2=&a;
   *ptr1=7;
   *ptr2=6;
    printf("The value of a is : %d",a);
    return 0;
}

In the above code:

• Both the pointers ptr1 and ptr2 contain the address of 'a' variable.
• So when we assign the '7' value to the *ptr1 and '6' to the *ptr2. The final value of 'a' would be '6'.

Output