Malenaadu: Frequently asked C interview questions with answers

Introduction:

contains 50 frequently asked C interview questions with answers. A must have

learning resource for interview takers.

1. What does a static variable mean?

A static variable is a special variable that is stored in the data segment unlike the

default automatic variable that is stored in stack. A static variable can be initialised

by using keyword static before variable name.

For Example:

static int a = 5;

A static variable behaves in a different manner depending upon whether it is a global

variable or a local variable. A static global variable is same as an ordinary global

variable except that it cannot be accessed by other files in the same program /

project even with the use of keyword extern. A static local variable is different from

local variable. It is initialised only once no matter how many times that function in

which it resides is called. It may be used as a count variable.

Example:

void count(void) {

static int count1 = 0;

int count2 = 0;

count1++;

count2++;

printf("\nValue of count1 is %d Value of count2 is %d", count1,

count2);

}

//In Main function:

main() {

count();

}

Output would be:

Value of count1 is 1 Value of count2 is 1

Value of count1 is 2 Value of count2 is 1

Value of count1 is 3 Value of count2 is 1

2. What is a pointer?

A pointer is a special variable in C language meant just to store address of any other

variable or function. Pointer variables unlike ordinary variables cannot be operated

with all the arithmetic operations such as '*','%' operators. It follows a special

arithmetic called as pointer arithmetic.

A pointer is declared as:

int *ap;

int a = 5;

In the above two statements an integer a was declared and initialized to 5. A pointer

to an integer with name ap was declared.

Next before ap is used

ap=&a;

This operation would initialize the declared pointer to int. The pointer ap is now said

to point to a.

Operations on a pointer:

Dereferencing operator ' * ':

This operator gives the value at the address pointed by the pointer . For example

after the above C statements if we give

printf("%d",*ap);

Actual value of a that is 5 would be printed. That is because ap points to a.

Addition operator ' + ': Pointer arithmetic is different from ordinary arithmetic.

Expression ap=ap+1; Would not increment the value of ap by one byte but would

increment it by the number of bytes of the data type it is pointing to. Here ap is

pointing to an integer variable hence ap is incremented by 2 or 4 bytes depending

upon the compiler.

3. What is a structure?

A structure is a collection of pre-defined data types to create a user-defined data

type. Let us say we need to create records of students. Each student has three

fields:

int roll_number;

char name[30];

int total_marks;

This concept would be particularly useful in grouping data types. You could declare a

structure student as:

struct student {

int roll_number;

char name[30];

int total_marks;

} student1, student2;

The above snippet of code would declare a structure by name student and it

initializes two objects student1, student2. Now these objects and their fields could

be accessed by saying student1.roll_number for accesing roll number field of

student1 object Similarly student2.name for accessing name field of student2

object.

4. How to print below pattern?

2 3

4 5 6

7 8 9 10

Program:

#include

main() {

int i, j, ctr = 1;

for (i = 1; i < 5; i++) {

for (j = 1; j <= i; j++)

printf("%2d ", ctr++);

printf("\n");

}

Explanation:

There are two loops, a loop inside another one. Outer loop iterates 5 times. Inner

loop iterates as many times as current value of i. So for first time outer loop is

executed, inner loop is executed once. Second time the outer loop is entered, inner

loop is executed twice and so on. And every time the program enters inner loop,

value of variable ctr is printed and is incremented by 1. %2d ensures that the

number is printed in two spaces for proper alignment.

5. How to swap two numbers using bitwise operators?

Program:

#include

main() {

int i = 65;

int k = 120;

printf("\n value of i=%d k=%d before swapping", i, k);

i = i ^ k;

k = i ^ k;

i = i ^ k;

printf("\n value of i=%d k=%d after swapping", i, k);

}

Explanation:

i = 65; binary equivalent of 65 is 0100 0001

k = 120; binary equivalent of 120 is 0111 1000

i = i^k;

i...0100 0001

k...0111 1000

---------

val of i = 0011 1001

---------

k = i^k

i...0011 1001

k...0111 1000

---------

val of k = 0100 0001 binary equivalent of this is 65

---------(that is the initial value of i)

i = i^k

i...0011 1001

k...0100 0001

---------

val of i = 0111 1000 binary equivalent of this is 120

---------(that is the initial value of k)

6. What is recursion? Write a program using recursion (factorial)?

Recursion: A function is called 'recursive' if a statement within the body of a

function calls the same function. It is also called 'circular definition'. Recursion is

thus a process of defining something in terms of itself.

Program: To calculate the factorial value using recursion.

#include

int fact(int n);

main() {

int x, i;

printf("Enter a value for x: \n");

scanf("%d", &x);

i = fact(x);

printf("\nFactorial of %d is %d", x, i);

}

int fact(int n) {

/* n=0 indicates a terminating condition */

if (n <= 0) {

return (1);

} else {

/* function calling itself */

return (n * fact(n - 1));

/*n*fact(n-1) is a recursive expression */

}

Output:

Enter a value for x:

Factorial of 4 is 24

Explanation:

fact(n) = n * fact(n-1)

If n=4

fact(4) = 4 * fact(3) there is a call to fact(3)

fact(3) = 3 * fact(2)

fact(2) = 2 * fact(1)

fact(1) = 1 * fact(0)

fact(0) = 1

fact(1) = 1 * 1 = 1

fact(2) = 2 * 1 = 2

fact(3) = 3 * 2 = 6

Thus fact(4) = 4 * 6 = 24

Terminating condition(n <= 0 here;) is a must for a recursive program. Otherwise

the program enters into an infinite loop.

7. To which numbering system, can the binary number 1101100100111100 be

easily converted to?

1101100100111100 can be easily converted to hexadecimal numbering system.

Hexa-decimal integer constants consist of combination of digits from 0 to 9 and

alphabets 'A' to 'F'. The alphabets represent numbers 10 to 15 respectively. Hexa-

decimal numbers are preceeded by '0x'.

1101,1001,0011,1100

1101 = D

1001 = 9

0011 = 3

1100 = C

1101,1001,0011,1100 = 0xD93C

Thus the given binary number 1101100100111100 in hexadecimal form is 0xD93C

8. What are the differences between structures and unions?

Structures and Unions are used to store members of different data types.

STRUCTURE

UNION

a)Declaration:

struct

{

data type member1;

data type member2;

};

union

{

data type member1;

data type member2;

};

b)Every structure member is

allocated memory when a structure

variable is defined.

b)The memory equivalent to the largest item is

allocated commonly for all members.

Example:

union emp1 {

struct emp {

char name[5];

int age;

float sal;

};

int age;

float sal;

};

union emp1 e2;

struct emp e1;

Memory allocated to a union is equal to size of

the largest member. In this case, float is the

largest-sized data type. Hence memory allocated

to this union is 4 bytes.

Memory allocated for structure is

1+2+4=7 bytes. 1 byte for name,

2 bytes for age and 4 bytes for sal.

c)All structure variables can be

initialized at a time

c)Only one union member can be initialized at a

time

struct st {

union un {

int a;

float b;

};

int a;

float b;

};

struct st s = { .a=4, .b=10.5

};

Structure is used when all members

are to be independently used in a

program.

union un un1 = { .a=10 };

Union is used when members of it are not

required to be accessed at the same time.

9. What are the advantages of using unions?

Union is a collection of data items of different data types. It can hold data of only

one member at a time though it has members of different data types. If a union has

two members of different data types, they are allocated the same memory. The

memory allocated is equal to maximum size of the members. The data is interpreted

in bytes depending on which member is being accessed.

Example:

union pen {

char name;

float point;

};

Here name and point are union members. Out of these two variables, 'point' is

larger variable which is of float data type and it would need 4 bytes of memory.

Therefore 4 bytes space is allocated for both the variables. Both the variables have

the same memory location. They are accessed according to their type. Union is

efficient when members of it are not required to be accessed at the same time.

10. What is scope & storage allocation of global and extern variables? Explain

with an example

Extern variables: belong to the External storage class and are stored in the main

memory. extern is used when we have to refer a function or variable that is

implemented in other file in the same project. The scope of the extern variables is

Global.

Example:

//program in file f1.c

#include

extern int x;

main() {

printf("value of x %d", x);

}

//program in file f2.c

int x = 3;

Here, the program written in file f1.c has the main function and reference to variable

x. The file f2.c has the declaration of variable x. The compiler should know the

datatype of x and this is done by extern definition.

Global variables: are variables which are declared above the main( ) function.

These variables are accessible throughout the program. They can be accessed by all

the functions in the program. Their default value is zero.

Example:

#include

int x = 0;/* variable x is a global variable */

/* It can be accessed throughout the program */

void increment(void) {

x = x + 1;

printf("\n value of x: %d", x);

}

main() {

printf("\n value of x: %d", x);

increment();

}

11. What is scope & storage allocation of static, local and register variables?

Explain with an example.

registers. The scope of the register variables is local to the block in which the

variables are defined. The variables which are used for more number of times in a

program are declared as register variables for faster access.

Example: loop counter variables.

Static variables: Memory is allocated at the beginning of the program execution

and it is reallocated only after the program terminates. The scope of the static

variables is local to the block in which the variables are defined.

Example:

void decrement

{

static int a=5;

a--;

printf("%d", a);

}

Here 'a' is initialized only once. Every time this function is called, 'a' does not get

initialized. so output would be 4 3 2 etc.,

Local variables: are variables which are declared within any function or a block.

They can be accessed only by function or block in which they are declared. Their

default value is a garbage value.

12. What is Pass by Value? Write a C program showing this concept.

Pass by Value: In this method, the value of each of the actual arguments in the

calling function is copied into corresponding formal arguments of the called function.

In pass by value, the changes made to formal arguments in the called function have

no effect on the values of actual arguments in the calling function.

Example:

#include

swap(int x, int y) {

int t;

t = x;

x = y;

y = t;

}

main() {

int m = 10, n = 20;

printf("\n Before executing swap m=%d n=%d", m, n);

swap(m, n);

printf("\n After executing swap m=%d n=%d", m, n);

}

Output:

Before executing swap m=10 n=20

After executing swap m=10 n=20

Explanation:

In the main function, value of variables m, n are not changed though they are

passed to function 'swap'. Swap function has a copy of m, n and hence it can not

manipulate the actual value of arguments passed to it.

13. What is Pass by Reference? Write a C program showing this concept. Pass

by Reference: In this method, the addresses of actual arguments in the calling

function are copied into formal arguments of the called function. This means that

using these addresses, we would have an access to the actual arguments and hence

we would be able to manipulate them. Pass by reference is accomplished using

pointers.

Example:

#include

/* function definition */

void swap(int *x, int *y) {

int t;

t = *x; /* assign the value at address x to t */

*x = *y; /* put the value at y into x */

*y = t; /* put the value at to y */

}

main() {

int m = 10, n = 20;

printf("\n Before executing swap m=%d n=%d", m, n);

swap(&m, &n);

printf("\n After executing swap m=%d n=%d", m, n);

}

Output:

Before executing swap m=10 n=20

After executing swap m=20 n=10

Explanation:

In the main function, address of variables m, n are sent as arguments to the

function 'swap'. As swap function has the access to address of the arguments,

manipulation of passed arguments inside swap function would be directly reflected

in the values of m, n.

14. What is an Enumeration?

Enumeration is a data type. We can create our own data type and define values that

the variable can take. This can help in making program more readable. enum

definition is similar to that of a structure.

Example: consider light_status as a data type. It can have two possible values - on

or off.

enum light_status

{

on, off;

};

enum light_status bulb1, bulb2;

/* bulb1, bulb2 are the variables */

Declaration of enum has two parts:

a) First part declares the data type and specifies the possible values, called

'enumerators'.

b) Second part declares the variables of this data type.

We can give values to these variables:

bulb1 = on;

bulb2 = off;

15. What is the use of typedef?

typedef declaration helps to make source code of a C program more readable. Its

purpose is to redefine the name of an existing variable type. It provides a short and

meaningful way to call a data type. typedef is useful when the name of the data

type is long. Use of typedef can reduce length and complexity of data types.

Note: Usually uppercase letters are used to make it clear that we are dealing with

our own data type.

Example:

struct employee {

char name[20];

int age;

};

struct employee e;

The above declaration of the structure would be easy to use when renamed using

typedef as:

struct employee {

char name[20];

int age;

};

typedef struct employee EMP;

EMP e1, e2;

16. What are register variables? What are advantages of using register

variables?

value. Scope of a register variable is local to the block in which it is defined. Lifetime

is till control remains within the block in which the register variable is defined.

Variable stored in a CPU register can always be accessed faster than the one that is

stored in memory. Therefore, if a variable is used at many places in a program, it is

better to declare its storage class as register

Example:

Variables for loop counters can be declared as register. Note that register keyword

may be ignored by some compilers.

17. What are storage memory, default value, scope and life of Automatic and

1. Automatic storage class:

Storage : main memory.

Default value : garbage value.

Scope : local to the block in which the variable is defined.

Lifetime : till control remains within the block.

2. Register storage class:

Storage : cpu registers.

Default value : garbage value.

Scope : local to the block in which the variable is defined.

Lifetime : till control remains within the block.

18. What are storage memory, default value, scope and life of Static and

External storage class?

1. Static storage class :

Storage : main memory

Default value : zero

Scope : local to the block in which the variable is defined

lifetime : till the value of the variable persists between different

function calls.

2. External storage class:

Storage : main memory

Default value : zero

Scope : global

Lifetime : as long as the program execution doesn't come to an end.

19. What are the advantages of using pointers in a program?

Pointers are special variables which store address of some other variables.

Syntax: datatype *ptr;

Here * indicates that ptr is a pointer variable which represents value stored at a

particular address.

Example: int *p;

'p' is a pointer variable pointing to address location where an integer type is stored.

Advantages:

3. Pointers allow us to pass values to functions using call by reference. This is

useful when large sized arrays are passed as arguments to functions. A

function can return more than one value by using call by reference.

4. Dynamic allocation of memory is possible with the help of pointers.

5. We can resize data structures. For instance, if an array's memory is fixed, it

cannot be resized. But in case of an array whose memory is created out of

malloc can be resized.

6. Pointers point to physical memory and allow quicker access to data.

20. Which bitwise operator is suitable for checking whether a particular bit is

ON or OFF?

Bitwise AND operator.

Example: Suppose in byte that has a value 10101101 . We wish to check whether

bit number 3 is ON (1) or OFF (0) . Since we want to check the bit number 3, the

second operand for AND operation we choose is binary 00001000, which is equal to

8 in decimal.

Explanation:

ANDing operation :

10101101 original bit pattern

00001000 AND mask

---------

00001000 resulting bit pattern

The resulting value we get in this case is 8, i.e. the value of the second operand.

The result turned out to be a 8 since the third bit of operand was ON. Had it been

OFF, the bit number 3 in the resulting bit pattern would have evaluated to 0 and

complete bit pattern would have been 00000000. Thus depending upon the bit

number to be checked in the first operand we decide the second operand, and on

ANDing these two operands the result decides whether the bit was ON or OFF.

21. Which bit wise operator is suitable for turning OFF a particular bit in a

number?

Bitwise AND operator (&), one's complement operator(~)

Example: To unset the 4th bit of byte_data or to turn off a particular bit in a

number.

Explanation:

consider,

char byte_data= 0b00010111;

byte_data= (byte_data)&(~(1<<4));

EXPLANATION:

1 can be represented in binary as 0b00000001

(1<<4)

<< is a left bit shift operator, it shits the bit 1 by 4 places towards

left.

(1<<4) becomes 0b00010000

And ~ is the one's complement operator in C language.

So ~(1<<4) = complement of 0b00010000

= 0b11101111

replacing value of byte_data and ~(1<<4) in

(byte_data)&(~(1<<4));

we get

(0b00010111) & (0b11101111)

Perform AND operation to below bytes.

00010111

11101111

-----------

00000111

-----------

Thus the 4th bit is unset.

22. What do the 'c' and ' v ' in argc and argv stand for? Explain their purpose?

In C, we can supply arguments to 'main' function. The arguments that we pass to

main ( ) at command prompt are called command line arguments. These arguments

are supplied at the time of invoking the program.

The main ( ) function can take arguments as: main(int argc, char *argv[]) { }

The first argument argc is known as 'argument counter'. It represents the number

of arguments in the command line. The second argument argv is known as

'argument vector'. It is an array of char type pointers that points to the command

line arguments. Size of this array will be equal to the value of argc.

Example: at the command prompt if we give:

C:\> fruit.exe apple mango

then

argc would contain value 3

argv [0] would contain base address of string " fruit.exe" which is the command

name that invokes the program.

argv [1] would contain base address of string "apple"

argv [2] would contain base address of string "mango"

here apple and mango are the arguments passed to the program fruit.exe

Program:

#include

main(int argc, char *argv[]) {

int n;

printf("Following are the arguments entered in the command line");

for (n = 0; n < argc; n++) {

printf("\n %s", argv[n]);

}

printf("\n Number of arguments entered are\n %d", argc);

}

Output:

Following are the arguments entered in the command line

C:\testproject.exe

apple

mango

Number of arguments entered are 3

23. What are the differences between malloc() and calloc()?

Allocation of memory at the time of execution is called dynamic memory allocation.

It is done using the standard library functions malloc() and calloc(). It is defined in

"stdlib.h".

malloc(): used to allocate required number of bytes in memory at runtime. It takes

one argument, viz. size in bytes to be allocated.

Syntax:

void * malloc(size_t size);

Example:

a = (int*) malloc(4);

4 is the size (in bytes) of memory to be allocated.

calloc(): used to allocate required number of bytes in memory at runtime. It needs

two arguments viz.,

1. total number of data and

2. size of each data.

Syntax:

void * calloc(size_t nmemb, size_t size);

Example:

a = (int*) calloc(8, sizeof(int));

Here sizeof indicates the size of the data type and 8 indicates that we want to

reserve space for storing 8 integers.

Differences between malloc() and calloc() are:

1. Number of arguments differ.

2. By default, memory allocated by malloc() contains garbage values. Whereas

memory allocated by calloc() contains all zeros.

24. Where are the auto variables stored?

Main memory and CPU registers are the two memory locations where auto variables

are stored. Auto variables are defined under automatic storage class. They are

stored in main memory. Memory is allocated to an automatic variable when the

block which contains it is called and it is de-allocated at the completion of its block

execution.

Auto variables:

Storage : main memory.

Default value : garbage value.

Scope : local to the block in which the variable is defined.

Lifetime : till the control remains within the block in which the variable is

defined.

25. Out of fgets( ) and gets( ) which function is safer to use and why?

Out of functions fgets( ) and gets( ), fgets( ) is safer to use. gets( ) receives a string

from the keyboard and it is terminated only when the enter key is hit. There is no

limit for the input string. The string can be too long and may lead to buffer overflow.

Example:

gets(s) /* s is the input string */

Whereas fgets( ) reads string with a specified limit, from a file and displays it on

screen.The function fgets( ) takes three arguments.

First argument : address where the string is stored.

Second argument : maximum length of the string.

Third argument : pointer to a FILE.

Example:

fgets(s,20,fp); /* s: address of the string, 20: maximum length of string, fp: pointer

to a file */

The second argument limits the length of string to be read. Thereby it avoids

overflow of input buffer. Thus fgets( ) is preferable to gets( ).

26. How can you increase the size of a dynamically allocated array?

realloc(): This function is used to increase or decrease the size of any dynamic

memory which is allocated using malloc() or calloc() functions.

Syntax: void *realloc(void *ptr, size_t newsize);

The first argument 'ptr' is a pointer to the memory previously allocated by the

malloc or calloc functions. The second argument 'newsize' is the size in bytes, of a

new memory region to be allocated by realloc. This value can be larger or smaller

than the previously allocated memory. The realloc function adjusts the old memory

region if newsize is smaller than the size of old memory.

If the newsize is larger than the existing memory size, it increases the size by

copying the contents of old memory region to new memory region. The function

then deallocates the old memory region. realloc function is helpful in managing a

dynamic array whose size may change during execution.

Example: a program that reads input from standard input may not know the size of

data in advance. In this case, dynamically allocated array can be used so that it is

possible allocate the exact amount of memory using realloc function.

27. Write a program to check whether a given number is even or odd. Program:

#include

main() {

int a;

printf("Enter a: \n");

scanf("%d", &a);

/* logic */

if (a % 2 == 0) {

printf("The given number is EVEN");

} else {

printf("The given number is ODD");

}

Output:

Enter a: 2

The given number is EVEN

Explanation with examples:

Example 1: If entered number is an even number

Let value of 'a' entered is 4

if(a%2==0) then a is an even number, else odd.

i.e. if(4%2==0) then 4 is an even number, else odd.

To check whether 4 is even or odd, we need to calculate (4%2).

/* % (modulus) implies remainder value. */

/* Therefore if the remainder obtained when 4 is divided by 2 is 0,

then 4 is even. */

4%2==0 is true

Thus 4 is an even number.

Example 2: If entered number is an odd number.

Let value of 'a' entered is 7

if(a%2==0) then a is an even number, else odd.

i.e. if(7%2==0) then 4 is an even number, else odd.

To check whether 7 is even or odd, we need to calculate (7%2).

7%2==0 is false /* 7%2==1 condition fails and else part is executed */

Thus 7 is an odd number.

28. Write a program to find the greatest of three numbers.

Program:

#include

main() {

int a, b, c;

printf("Enter a,b,c: \n");

scanf("%d %d %d", &a, &b, &c);

if (a > b && a > c) {

printf("a is Greater than b and c");

} else if (b > a && b > c) {

printf("b is Greater than a and c");

} else if (c > a && c > b) {

printf("c is Greater than a and b");

} else {

printf("all are equal or any two values are equal");

}

Output:

Enter a,b,c: 3 5 8

c is Greater than a and b

Explanation with examples:

consider three numbers a=5,b=4,c=8

if(a>b && a>c) then a is greater than b and c

now check this condition for the three numbers 5,4,8 i.e.

if(5>4 && 5>8) /* 5>4 is true but 5>8 fails */

so the control shifts to else if condition

else if(b>a && b>c) then b is greater than a and c

now checking this condition for 5,4,8 i.e.

else if(4>5 && 4>8) /* both the conditions fail */

now the control shifts to the next else if condition

else if(c>a && c>b) then c is greater than a and b

now checking this condition for 5,4,8 i.e.

else if(8>5 && 8>4) /* both conditions are satisfied */

Thus c is greater than a and b.

29. Write a program to check whether a given number is a prime.

A prime number is a natural number that has only one and itself as factors.

Examples: 2, 3, 5, 7,… are prime numbers.

Program:

#include

main() {

int n, i, c = 0;

printf("Enter any number n: \n");

scanf("%d", &n);

/*logic*/

for (i = 1; i <= n; i++) {

if (n % i == 0) {

c++;

}

if (c == 2) {

printf("n is a Prime number");

} else {

printf("n is not a Prime number");

}

Output:

Enter any number n: 7

n is Prime

Explanation with examples:

consider a number n=5

for(i=0;i<=n;i++) /* for loop is executed untill the n value equals i

i.e. for(i=0;i<=5;i++) /* here the for loop is executed untill i is

equal to n */

1st iteration: i=1;i<=5;i++

here i is incremented i.e. i value for next iteration is 2