Difference between revisions of "Interview Preparation topic : Pointers in C"

Revision as of 01:08, 11 December 2016

What is a Pointer?

A pointer is a variable which contains the address in memory of another 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.

Lets Look at the following example:

1. include <stdio.h>

int main ()

{
  int  temp;
  int temp1;
  printf("Address of temp variable: %x\n", &temp );
  printf("Address of temp1 variable: %x\n", &temp1 );
  return 0;

}

When the following code is compiled and executed:
Address of temp variable: edf53400
Address of temp1 variable: adf9a5f6

To declare a pointer to a variable do:

type *pointer;

Where type can be int, double, float, char and pointer can be any variable.

Consider the following example:

1. include <stdio.h>

int main () {

  int  temp= 100;                                     /* Stores the value 20 in variable temp */
  int  *ptr;                                          /* Declares a pointer variable of type int */
  ip = &temp;                                         /*store address of temp in pointer variable, so now the address of temp will be in ptr */
  printf("Address of temp variable: %x\n", &temp);    /* print the address of variable temp */
  printf("Address stored in ptr variable: %x\n",ptr ); /* prints the address stored in variable ptr */
  printf("Value of *ptr variable: %d\n", *ptr );       /* prints the value stored at address ptr, this is also known as dereferencing of a pointer */
  return 0;

}

When we compile and execute the following code:
Address of temp variable: a54fae34
Address stored in ptr variable: a54fae34
Value of *ptr variable:100

Interview Questions on pointers :

1) What is a NULL pointer?
-> A pointer pointing to nothing is called as a NULL Pointer Eg: int *x=NULL;

2) What is a dangling pointer>
-> Dangling Pointer is a pointer that doesn’t point to a valid memory location. Initially the pointer holds a valid address but later the held address is released or freed.

3) What is a void pointer?
. -> In C General Purpose Pointer is called as void Pointer. It does not have any data type associated with it and can store address of any type of variable

  for eg void *ptr;    // ptr is declared as Void pointer
         char cnum;
         int inum;
         float fnum;
         ptr = &cnum;  // ptr has address of character data
         ptr = &inum;  // ptr has address of integer data
         ptr = &fnum;  // ptr has address of float data

4) What is the difference between *ptr++ and ++*ptr? -> ptr++ means

  Increments the Pointer not Value Pointed by It
  ++*ptr means 
  Increments the Value being Pointed to by ptr

5) What is the difference between constant to pointer and pointer to constant? -> No Pointer to Constant Constant Pointers

     1	*ptr = 20 Statement is Invalid in Pointer to              1)*ptr = 20 is Absolutely Valid in Constant Pointers i.e Assigning Value is Perfectly legal
         Constant i.e Assigning Value is Illegal	
     2	 ptr ++  Statement  is Valid in Pointer to Constant       2)ptr ++  Statement  is invalid in Constant Pointers