C Programming
In this tutorial we will learn about Pointers in C programming language.
We know that in C if we want to save some value we take help of a variable.
So, a variable stores some value and we access that value by write the variable name.
In the following example we are storing an integer value in a variable num and to access that value we are writing the name of the variable.
num
// assign value to variable int num = 10; // access value stored in the variable printf("%d", num);
Three things happen when we create a variable and assign a value to it.
Pointers are derived data types and they store memory address.
So, pointers are variables that store memory addresses which are memory location of other variables.
Before we can use a pointer variable we first have to create or declare them like any other variable in C programming.
Syntax of a pointer variable declaration is given below.
dataType *pointerVar;
In the above syntax the * asterisk indicates that we are creating a pointer variable.
*
The name of the pointer variable in the above syntax is pointerVar.
pointerVar
The created pointer variable pointerVar will point at the variable of type dataType.
dataType
In the following example we are creating an integer pointer variable which will point at a variable of type int.
int
int *ptr;
Similarly, we can create a character pointer variable which will point at a variable of type char.
char
char *ptr;
We use the address of & operator to get the address of a variable.
&
In the following syntax we will get the address of a variable varName.
varName
&varName
We use the value at the address of * operator to get the value stored at any given address of a variable.
In the following syntax we will get the value stored at the given address.
*varName
In the given example we are printing the address of the variable num and value stored at the address of the variable num.
#include <stdio.h> int main(void) { // creating variable num int num; // assigning value to num num = 10; // printing the value stored in variable num printf("Value in num: %d\n", num); // printing the address of the variable num printf("Address of num: %ld\n", &num); // printing the value stored at the address of the variable num printf("Value at the address of num: %d\n", *(&num)); return 0; }
Note! In the above code &num gives us the address of variable num and *(&num) gives us the value stored at the address of the variable num.
&num
*(&num)
Output:
Value in num: 10 Address of num: 140732733818936 Value at the address of num: 10
By default, when we declare a pointer variable it gets initialized by a random value a.k.a. garbage value.
In the following example we have created a pointer variable of type int. Since we have not assigned any address of any int type variable so, it holds some garbage value.
#include <stdio.h> int main(void) { // pointer variable of type int int *ptr; // by default pointer variable ptr // holds garbage value as no // int variable address was assigned printf("Value stored in ptr: %ld\n", ptr); return 0; }
Value stored in ptr: 140732859951192
We generally initialise a pointer variable to NULL value to avoid the random garbage value.
NULL
#include <stdio.h> int main(void) { // pointer variable of type int int *ptr = NULL; printf("Value stored in ptr: %ld\n", ptr); return 0; }
Value stored in ptr: 0
To initialise pointer variable with the address of another variable we use the & address of operator.
In the following example we have an integer variable num that holds value 10.
We also have a ptr pointer variable that holds the value of integer variable num. So, ptr variable is pointing at the num variable.
ptr
#include <stdio.h> int main(void) { // num variable int num = 10; // pointer variable int *ptr; // assign address of num variable to ptr pointer variable ptr = # // print the value stored in num using the num variable printf("Value stored in num: %d\n", num); // print address of num stored in ptr variable printf("Address of num variable: %ld\n", ptr); // print address of ptr variable printf("Address of ptr variable: %ld\n", &ptr); // print the value stored in num using ptr variable printf("Value stored in num using ptr variable: %d\n", *ptr); return 0; }
Value stored in num: 10 Address of num variable: 140732844468280 Address of ptr variable: 140732844468272 Value stored in num using ptr variable: 10
In the above image we are showing only the last 4 digits of the address.