Pointer in C

What is Pointer ?

Pointer is a special variable that stores address/memory-location of other variable but of same type. An * (asterisk) symbol followed by the variable name is used to declare a variable as pointer. When we talk about the address of a variable, then we’ll be able to go to that address and retrieve the data stored in it.

Pointer Syntax in C:

datatype *variable_name; Example: you could declare a pointer that stores the address of an integer with the following syntax: int *points_to_integer; Notice the use of the *. This is the key to declaring a pointer; if we add it directly before the variable name, it will declare the variable to be a pointer.

Characteristics of C Pointers

Pointers are special variables that store the memory address of another variable.
It stores the address but of same type.
Pointers always hold addresses as a whole number.
Pointers in C are always initialized to NULL. For example, int *ptr=null;
Null pointer symbol is ''0''.
The asterisk symbol, * is used to retrieve the value of the variable; & ampersand symbol is used for retrieving the address of a variable.
If a C pointer is assigned to the null value, it points nothing.
Only subtraction between pointers are allowed. Addition, division, and multiplication operations are not allowed.
The memory size of a C pointer for a 16-bit system is 2 bytes. Or It depends upon different issues like Operating system, CPU architecture etc. Usually it depends upon the word size of underlying processor for example for a 32 bit computer the pointer size can be 4 bytes for a 64 bit computer the pointer size can be 8 bytes.

Pointer Notation

Consider the declaration
int a = 5;
This declaration tells the C compiler to

Reserve space in memory to hold the integer value.
Associate the name a with this memory location.
Store the value 5 at this location.
We may represent a’s location in memory by the following memory map.

We see that the computer has selected memory location 65539 as the place to store the value 3. The location number 65539 is not a number to be relied upon, because some other time the computer may choose a different location for storing the value 5. The important point is that , a’s address in memory is a number. We can print this address number through the following program:

#include<stdio.h>
void main()
{
int a = 5 ;
printf("\nAddress of a = %u",&a);
printf("\nValue of a = %d",a);
}
The output of the above program would be:

Address of a = 65539
Value of a = 5

Look at the first printf() statement carefully. & used in this statement is C’s "address of" operator. The expression &a returns the address of the variable a, which in this case happens to be 65539. Since 65539 represents an address, there is no question of a sign being associated with it. Hence it is printed out using %u, which is a format specifier for printing an unsigned integer. We have been using the & operator all the time in the scanf() statement.

The other pointer operator available in C is *, called "value at address" operator. It gives the value stored at a particular address. The "value at address" operator is also called "indirection" operator.
Observe carefully the output of the following program:
#include<stdio.h>
void main()
{
int a = 5 ;
printf("\nAddress of a = %u",&a);
printf("\nValue of a = %d",a);
printf("\nValue of a = %d",*(&a));
}
The output of the above program would be:
Address of a = 65539
Value of a = 5
Value of a = 5
Note that printing the value of *(&a) is same as printing the value of a. The expression &a gives the address of the variable a. Another example to get the address of a variable.

Example:

#include<stdio.h>
void main( )
{
int a = 5 ;
int *b ;
b = &a ;
printf ("\nAddress of a = %u",&a) ;
printf ("\nAddress of a = %u",b) ;
printf ("\nAddress of b = %u",&b) ;
printf ("\nValue of b = %u",b) ;
printf ("\nValue of a = %d",a) ;
printf ("\nValue of a = %d",*(&a)) ;
printf ("\nValue of a = %d",*b) ;
}
The output of the above program would be:
Address of a = 65539
Address of a = 65539
Address of b = 65518
Value of b = 65539
Value of a = 5
Value of a = 5
Value of a = 5

Here
b = &a ;
But remember that b is not an ordinary variable like any other integer variable. It is a variable that contains the address of other variable . Since b is a variable the compiler must provide it space in the memory. Once again, the following memory map would illustrate the contents of a and b.

C Language