Pointers in C

A pointer is a variable that stores the address of another variable. Unlike other variables that hold values of a certain type, pointer holds the address of a variable.

For example, an integer variable holds (or you can say stores) an integer value, however an integer pointer holds the address of a integer variable.

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.

Pointer Declaration :

Syntax:(type) *var-name;

Example:


 int *ip; /* pointer to an integer */ 

 double *dp; /* pointer to a double */

 float *fp; /* pointer to a float */ 

 char *ch /* pointer to a character */

Pointer Use Using Code:

#include <stdio.h>

int main()

{

    int x = 10;

    int* px = &x;

    printf("Direct access, x = %d\n",x);

    printf("Indirect access, *px = %d\n",*px);

    *px = 20;

    printf("Direct access, x = %d\n",x);

    printf("Indirect access, *px = %d\n",*px);

    /* examining memory address of x and px */

    printf("The memory address of x is %p\n",&x);

    printf("The value of the pointer px is %p\n",px);

    printf("The memory address of the pointer px is %p\n",&px);

    return 0;

}

C output:

Direct access, x = 10
Indirect access, *px = 10
Direct access, x = 20
Indirect access, *px = 20
The memory address of x is 0x7fffc97820cc
The value of the pointer px is 0x7fffc97820cc

The memory address of the pointer px is 0x7fffc97820d0

C Compiler to run above code:https://goo.gl/bzuHcQ

NULL Pointers :

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

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

In 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 */

Dereferencing:

Dereferencing is the act of referring to where the pointer points at, instead of the memory address. We are already using dereferencing in arrays - but we just didn't know it yet. The brackets operator - [0] for example, accesses the first item of the array. And since arrays are actually pointers, accessing the first item in the array is the same as dereferencing a pointer. Dereferencing a pointer is done using the asterisk operator *

/* define a local variable a */
int a = 1;

/* define a pointer variable, and point it to a using the & operator */
int * pointer_to_a = &a;

printf("The value a is %d\n", a);
printf("The value of a is also %d\n", *pointer_to_a);

C Compiler to run above code:https://goo.gl/bzuHcQ