Pointers in C-Programming

Pointer

"Pointer points the address of the variable which stores in the memory". Pointer is basically used for the dynamic memory allocation.

Pointer is a user defined data type which creates special types of variables which can hold the address of primitive data type like char, int, float, double or user defined data type like function, pointer etc. or derived data type like array, structure, union, enum.

Examples:

int *ptr;

int (*ptr)();

int (*ptr)[2];

As you know, every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator, which denotes an address in memory. Consider the following example, which will print the address of the variables defined:

#include <stdio.h>

int main ()
{
   int  var1;
   char var2[10];

   printf("Address of var1 variable: %x\n", &var1  );
   printf("Address of var2 variable: %x\n", &var2  );

   return 0;
}

When the above code is compiled and executed, it produces result something as follows:

Address of var1 variable: bff5a400
Address of var2 variable: bff5a3f6

So you understood what is memory address and how to access it, so base of the concept is over. Now let us see what is a pointer.

Address Operator

An address-of operator is a mechanism within C++ that returns the memory address of a variable. These addresses returned by the address-of operator are known as pointers, because they "point" to the variable in memory.

The address-of operator is a unary operator represented by an ampersand (&). It is also known as an address operator.

Address operators commonly serve two purposes: 

1. To conduct parameter passing by reference, such as by name
2. To establish pointer values. Address-of operators point to the location in the memory because the value of the pointer is the memory address/location where the data item resides in memory.

For example, if the user is trying to locate age 26 within the data, the integer variable would be named age and it would look like this: int age = 26. Then the address operator is used to determine the location, or the address, of the data using "&age".

From there, the Hex value of the address can be printed out using "cout << &age". Integer values need to be output to a long data type. Here the address location would read "cout << long (&age)".

The address-of operator can only be applied to variables with fundamental, structure, class, or union types that are declared at the file-scope level, or to subscripted array references. In these expressions, a constant expression that does not include the address-of operator can be added to or subtracted from the address-of expression.

What Are Pointers?

A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before you can use it to store any variable address. The general form of a pointer variable declaration is:

type *var-name;

Here, type is the pointer's base type; it must be a valid C data type and var-name is the name of the pointer variable. The asterisk * you used to declare a pointer is the same asterisk that you use for multiplication. However, in this statement the asterisk is being used to designate a variable as a pointer. Following are the valid pointer declaration:

int    *ip;    /* pointer to an integer */
double *dp;    /* pointer to a double */
float  *fp;    /* pointer to a float */
char   *ch     /* pointer to a character */

The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to.

How to use Pointers?

There are few important operations, which we will do with the help of pointers very frequently. (a) we define a pointer variable (b) assign the address of a variable to a pointer and (c) finally access the value at the address available in the pointer variable. This is done by using unary operator * that returns the value of the variable located at the address specified by its operand. Following example makes use of these operations:

#include <stdio.h>

int main ()
{
   int  var = 20;   /* actual variable declaration */
   int  *ip;        /* pointer variable declaration */

   ip = &var;  /* store address of var in pointer variable*/

   printf("Address of var variable: %x\n", &var  );

   /* address stored in pointer variable */
   printf("Address stored in ip variable: %x\n", ip );

   /* access the value using the pointer */
   printf("Value of *ip variable: %d\n", *ip );

   return 0;
}

When the above code is compiled and executed, it produces result something as follows:

Address of var variable: bffd8b3c
Address stored in ip variable: bffd8b3c
Value of *ip variable: 20

NULL Pointers in C

It is always a good practice to assign a NULL value to a pointer variable in case you do not have exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NULL is called a null pointer.

The NULL pointer is a constant with a value of zero defined in several standard libraries. Consider the following program:

#include <stdio.h>

int main ()
{
   int  *ptr = NULL;

   printf("The value of ptr is : %x\n", ptr  );
 
   return 0;
}

When the above code is compiled and executed, it produces the following result:

The value of ptr is 0

On most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system. However, the memory address 0 has special significance; it signals that the pointer is not intended to point to an accessible memory location. But by convention, if a pointer contains the null (zero) value, it is assumed to point to nothing.

To check for a null pointer you can use an if statement as follows:

if(ptr)     /* succeeds if p is not null */
if(!ptr)    /* succeeds if p is null */

C Pointers in Detail:

Pointers have many but easy concepts and they are very important to C programming. There are following few important pointer concepts which should be clear to a C programmer:

Concept	Description
C - Pointer arithmetic	There are four arithmetic operators that can be used on pointers: ++, --, +, -
C - Array of pointers	You can define arrays to hold a number of pointers.
C - Pointer to pointer	C allows you to have pointer on a pointer and so on.
Passing pointers to functions in C	Passing an argument by reference or by address both enable the passed argument to be changed in the calling function by the called function.
Return pointer from functions in C	C allows a function to return a pointer to local variable, static variable and dynamically allocated memory as well.