Advanced c interview questions and answers

Advanced c interview questions with explanation

1

Do you memory representation of float data type in c?

Answer

Explanation:

Float numbers are stored in exponential form i.e.

(Mantissa)*10^ (Exponent)

Here * indicates multiplication and ^ indicates power.

In memory only Mantissa and Exponent is stored not *, 10 and ^.

Total size of float data type: 32 bit

Those bits are used in following manner:

Exponent bit: 8

Mantissa bit: 24

Mantissa is signed number, so 24 bit are used as:

Mantissa_sign bit: 1

Mantisaa_data bit: 23

For only mantissa:

Mantissa_sign bit will zero if number is positive and Mantissa_sign bit will one if number is negative.

Exponent is also signed number, So 8 bit are used as:

Exponent_sign bit: 1

Exponent_data bit: 7

Following figure illustrate how floating point number is stored in memory.

Five important rules:

Rule 1: To find the mantissa and exponent, we convert data into scientific form.

Rule 2: Before the storing of exponent, 127 is added to exponent.

Rule 3: Exponent is stored in memory in first byte from right to left side.

Rule 4: If exponent will negative number it will be stored in 2’s complement form.

Rule 5: Mantissa is stored in the memory in second byte onward from right to left side. Example:

Memory representation of:

         float a = -10.3f;

For this you have to follow following steps:

step1: convert the number (10.3) into binary form
Binary value of 10.3 is: 1010.0100110011001100110011001100110011…

step2: convert the above binary number in the scientific form. Scientific form of 1010.0100110011001100110011001100110011…=

1.01001001100110011001100 11001100110011…*10^3

Note: First digit i.e. 1, decimal point symbol, base of power i.e. 10, power symbol ^ and multiplication symbol * are not stored in the memory.

Step3: find exponent and mantissa and signed bit
Mantissa_data bit in binary = 0100100 11001100 11001101   (Only first 23 bit from left side) 

Mantissa_sign bit: 1 (Since it is a negative number)

Exponent in decimal: 3

Question:

Why we have taken right most bit of mantissa_data bit one instead of zero?

Step 5: Add 127 in the exponent and convert in the binary number form.

(Why 127? since size of exponent_data bit is 7 and maximum possible number in seven bit will 1111111 in binary or 127 in decimal)

Exponent= 127+3=130
Binary value of 130 in eight bit: 1000001 0
Exponent_data bit: 1000001 (Take first seven bit from left side)

Exponent_sign bit: 0 (Take rightmost bit)

Step 6: Now store the Mantissa_data bit, Mantissa_sign bit, Exponent_data bit and Exponent_sign bit at appropriate location as shown in the following figure. 

Note: Mantissa_data bits are stored from left to right while Exponent_data bits are stored from right to left.

How to check above memory representation is correct?

Answer:

We will take one char pointer and visit each byte of a float number and observe the output.

#include<stdio.h>

int main(){

    int i;

    float f=-10.3f;

    char *p=(char *)&f;

    for(i=0;i<4;i++)

         printf("%d   ",*p++);

    return 0;

}

Output: -51 -52 36 -63

Explanation:

Binary value of -51 in eight bit: 11001101

Binary value of -52 in eight bit: 11001100

Binary value of 36 in eight bit:  00100100

Binary value of -63 in eight bit: 11000001

This is exactly same as which we have represented in memory in the above figure.

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2

Write a c program which changes the position of cursor?

Answer

Explanation:

#include<dos.h>

#include<stdio.h>

int main(){

union REGS i,o;

i.h.ah=2;   //positioning the cursor 

i.h.bh=0;

i.h.dh=30;   

i.h.dl=45;

int86(0x10,&i,&o);

printf("World");

return 0;

}

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3

Do you know pragma directives in c?

Answer

Explanation:

Pragma is implementation specific directive i.e each pragma directive has different implementation rule and use. There are many type of pragma directive and varies from one compiler to another compiler .If compiler does not recognize particular pragma the it simply ignore that pragma statement without showing any error or warning message and execute the whole program assuming this pragma statement is not present.

For example suppose there is any pragma directive is #pragma world.

#include<stdio.h>

#pragma world

int main(){

    printf("C is powerful language ");

    return 0;

}

Output: C is powerful language

Explanation:

Since #pragma world is unknown for Turbo c 3.0 compilers so it will ignore this directive without showing any error or warning message and execute the whole program assuming #pragma world statement is not present.

List of pragma directives in turbo c 3.0:

1. #pragma startup

2. #pragma exit

3. #pragma warn

4. #pragma option

5. #pragma inline

6. #pragma argsused

7. #pragma hdrfile

8. #pragma hdrstop

9. #pragma saveregs

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4

What is array of pointers in c?

Answer

Explanation:

Array whose content is address of another variable is known as array pointers.  For example:

#include<stdio.h>

int main(){

float a=0.0f,b=1.0f,c=2.0f;

    float * arr[]={&a,&b,&c};

    b=a+c;

    printf("%f",arr[1]);

    return 0;        

}

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5

Can you declare such function which return type is pointer to an enum?

Answer

Explanation:

#include <stdio.h>

typedef enum color{a,b,c,d,e}co;

enum color eee(){

    static co x;

    x=b+c/2;

    return x;

}

int main(){

    int num;

    num=eee();

    printf("%#d",num);

    return 0;

}

Output: 2

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6

Have you worked in text video memory in c?

Answer

Explanation:

Segment number 0XB is known as text video memory. This segment is divided into 25 rows and 80 columns which creates 80*25=2000 cells. Size of each cell is two byte. Each cell is divided into two parts each of size one byte.

(a) Text byte: First byte stores character information. It stores character as ASCII code.

(b) Color byte: Second byte stores color information of text byte character.

In other word we can say each even byte stores character and each odd byte stores color. Simple example:

#include<stdio.h>

int main(){

int i;

char far *ptr=(char *)0XB8000000;

*ptr='A';

*(ptr+1)=4;
return 0;

} 

Output: It will display character A in the red color as shown following screen dump:

Color scheme:

Color byte of size 8 bit stores the color information in the following manner. First four bits stores color information of character.

0000 0001: Blue color (1)

0000 0010: Green color (2)

0000 0100: Red color (4)

0000 1000: To increase the intensity of color. (8)

Note: Any other number will generate mixture of above four basic colors.

Next four bits stores color information of background of character.

0001 0000: Blue color (16)

0010 0000: Green color (32)

0100 0000: Red color (64)

1000 0000: To increase the intensity of color. (128)

Note: Any other number will generate after mixing of above four basic colors. Examples:

(1)What will be output of following c program?

#include<stdio.h>

int main(){

int i;

char far *ptr=(char *)0XB8000000;

*ptr='A';

*(ptr+1)=1;

*(ptr+2)='B';

*(ptr+3)=2;

*(ptr+4)='C';

*(ptr+5)=4;
return 0;

}

Output:

(2)What will be output of following c program?

#include<stdio.h>

int main(){

int i;

char far *ptr=(char *)0XB8000000;

*ptr='W';

*(ptr+1)=1;

*(ptr+2)='O';

*(ptr+3)=2;

*(ptr+4)='R';

*(ptr+5)=4;

*(ptr+6)='L';

*(ptr+7)=1;

*(ptr+8)='D';

*(ptr+9)=2;
return 0;

}

Output:

(3)What will be output of following c program?

#include<stdio.h>

//Mixture of basic color

int main(){

int i;

char far *ptr=(char *)0XB8000000;

*ptr='W';

*(ptr+1)=3;

*(ptr+2)='O';

*(ptr+3)=5;

*(ptr+4)='R';

*(ptr+5)=6;

*(ptr+6)='L';

*(ptr+7)=7;

*(ptr+8)='D';

*(ptr+9)=3;

return 0;

}

Output:

(4)What will be output of following c program?

#include<stdio.h>

//To increase the intensity of color.

int main(){

    int i;

char far *ptr=(char *)0XB8000000;

*ptr='P';

*(ptr+1)=1+8;

*(ptr+2)='O';

*(ptr+3)=2+8;

*(ptr+4)='I';

*(ptr+5)=3+8;

*(ptr+6)='N';

*(ptr+7)=4+8;

*(ptr+8)='T';

*(ptr+9)=5+8;

*(ptr+10)='E';

*(ptr+11)=6+8;

*(ptr+12)='R';

*(ptr+13)=7+8;

return 0;

}

Output:

(5)What will be output of following c program?

#include<stdio.h>

// for background color

int main(){

int i;

char far *ptr=(char *)0XB8000000;

*ptr='M';

*(ptr+1)=4+32;

*(ptr+2)='A';

*(ptr+3)=4+32;

*(ptr+4)='N';

*(ptr+5)=4+32;

*(ptr+6)='I';

*(ptr+7)=4+16;

*(ptr+8)='S';

*(ptr+9)=4+16;

*(ptr+10)='H';

*(ptr+11)=4+16;

return 0;

}

Output:

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7

Can you declare pointer to structure in c?

Answer

Explanation:

A pointer which is pointing to a structure is known as pointer to structure. Examples of pointers to structure:

What will be output if you will execute following code?

#include<stdio.h>

struct address{

char *name;

char street[10];

int pin;
}cus={"A.Kumar","H-2",456003},*p=&cus;

int main(){

printf("%s %s",p->name,(*p).street);
return 0;

}

Output: A.Kumar H-2

Explanation:

p is pointer to structure address.-> and (*). Both are same thing. These operators are used to access data member of structure by using structure’s pointer.

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8

What is pointer to array of character in c?

Answer

Explanation:

A pointer to such an array which contents is character constants is known as pointer to array of character constant.

What will be output if you will execute following code?

#include<stdio.h>

char display(char (*)[]);

int main(){

char c;

char character[]={65,66,67,68};

char (*ptr)[]=&character;

c=display(ptr);

printf("%c",c);

return 0;

}

char display(char (*s)[]){

**s+=2;

return **s;

}

Output: C

Explanation: Here function display is passing pointer to array of characters and returning char data type.

**s+=2

=>**s=**s+2

=>**ptr=**ptr+2 //s=ptr

=>**&character= **&character+2 //ptr=&character

=>*character=*character+2 //from rule *&p =p

=>character[0]=character[0]+2 //from rule *(p+i)=p[i]

=>character [0] =67

**s=character [0] =67

Note: ASCII value of ‘C’ is 67

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9

Can you explain pointer to two dimensional arrays in c by an example?

Answer

Explanation:

Examples of pointers to 2 dimensional arrays:

#include<stdio.h>

void main(){

long array[][3]={7l,14l,21l,28l,35l,42l};

long int (*ptr)[2][3]=&array;

printf("%li ",-0[1[0[ptr]]]);
return 0;

}

Output: -28

Explanation:

-0[1[0[ptr]]]

=-1[0[ptr]][0] //From rule array[i]=i[array]

=-0[ptr][1][0]

=-ptr [0] [1] [0]

=-*ptr [0] [1] //From rule array[i]=*(array+i)

=-*(&array) [0] [1]

=-(&array) [0] [1][0]

=-(*&array)[1][0] //From rule *&p=p

=-array[1][0]

array[1][0] means 1*(3)+ 0 = 3^rd element of array starting from zero i.e. 28

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10

What is data segment in c?

Answer

Explanation:

All the segments are used for specific purpose. Like segment number 15 is used for ROM, segment number 14 is used for BIOS etc.  

We will discuss about how to access text video memory, graphics video memory in the pointer and union chapters of 255 bits color graphics programming. Segment number eight has special name which is known as data segment. This segment has been divided into four parts. This is very important for c programming

1. Stack area

All automatic variables and constants are stored into stack area. Automatic variables and constants in c:

1. All the local variables of default storage class.

2. Variables of storage calls auto.

3. Integer constants, character constants, string constants, float constants etc in any expression.

4. Function parameters and function return value.

Variables in the stack area are always deleted when program control reaches it out of scope. Due to this stack area is also called temporary memory area. For example:

What will be output of following c code?

#include<stdio.h>

int main(){

    int i;

    for(i=0;i<3;i++){

         int a=5;

         printf("%d",a);

    }

    return 0;

}

Output: 5 5 5

Explanation: Since variable a is automatic variable, it will store in the stack area. Scope of variable a is within for loop. So after each iteration variable a will be deleted from stack and in each iteration variable a will initialize.  This two concepts are only for Turbo C 3.0 It follows LIFO data structure. That in the stack area of memory data is stored in last in first out. For example:

What will be output of flowing c code. (Turbo c 3.0)?

#include<stdio.h>

int main(){

    int a =5, b = 6, c = 7,d =8;

    printf("%d %d %d");

    return 0;

}

Output: 8 7 6

Explanation:

Default storage class of variables a, b, c and d is auto .Since it automatic variable it will be sorted in the stack area of memory. It will store in the stack as

Stack always follows LIFO data structure. In the printf function, name of variables is not written explicitly. So default output will content of stack which will be in the LIFO order i.e. 8 7 6.

It has two part one for initialize variable another for non-initialize variable. All initialize variables are more nearer than not initialized variable and vice versa. For example:

What will be output of following program (Turbo c 3.0)?

#include<stdio.h>

int main(){

    int a =5, b, c =7;

    printf("%d %d %d");

    return 0;

}

Output: 7 5 garbage value

Explanation:

Automatic variable a and c has initialized while b has not initialized. Initialize variables are more nearer than uninitialized variable .They will be stored in the stack. So due to LIFO first output will be 7 then 6 (since a is more nearer than b with respect to c) then any garbage value will be output which is present in the stack.

Note: Default storage class of any local variable is auto.

2. Data area:

All static and extern variable are stored in the data area. It is permanent memory spaAutomatic variable a and c has initialized while b has not initialized. Initialize variables are more nearer than uninitialized variable .They will be stored in the stack. So due to LIFO first output will be 7 then 6 (since a is more nearer than b with respect to c) then any garbage value will be output which is present in the stack.

Note: Default storage class of any local variable is auto.ce and variable will store in the memory unless and until program end. For example:

#include<stdio.h>

int main(){

    int i;

    for(i=0;i<3;i++){

         static int a=5;

         printf("%d",a);

    }

    return 0;

}

Output: 5 6 7

3. Heap area:

This memory area is use to allocate memory dynamically. In c we can allocate the memory space dynamically by using function malloc and calloc. It always allocates memory in the heap area. Its size is variable and depends upon free space in the memory.

4. Code area:

Function pointer can only access code area. Size of this area is always fixed and it is read only memory area.

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11

Can you write a c program to check a given number is odd or even without using any conditional operators?

Answer

Explanation:

#include <stdio.h>

int fun();

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

    int num;

    scanf("%d",&num);

    if(num&1)

         printf("odd");

    else

         printf("even");

    return 0;

}

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12

Write a c program to find out generic root of a number.

Answer

Explanation:

#include <stdio.h>

int fun();

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

    int num,x;

    scanf("%d",&num);

    printf("%d",(x=num%9)?x:9);

    return 0;

}

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13

Can you write a function which executes after the main function in c?

Answer

Explanation:

#include <stdio.h>

#include<stdlib.h>

void fun();

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

   atexit(fun);

   printf("cquestion");

   return 0;

}

void fun(){

  printf("bank");

}

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14

What is a multilevel pointer in c?

Answer

Explanation:

A pointer is pointer to another pointer which can be pointer to others pointers and so on is known as multilevel pointers. We can have any level of pointers. Examples of multilevel pointers in c:

(1) What will be output if you will execute following code?

#include<stdio.h>

int main(){

int s=2,*r=&s,**q=&r,***p=&q;

printf("%d",p[0][0][0]);
return 0;

}

Output: 2

Explanation: 

As we know p[i] =*(p+i)

So,

P[0][0][0]=*(p[0][0]+0)=**p[0]=***p

Another rule is: *&i=i

So,

***p=*** (&q) =**q=** (&r) =*r=*(&s) =s=2

(2) What will be output if you will execute following code?

#include<stdio.h>

#define int int*

int main(){

    int *p,q;

p=(int *)5;

q=10;

printf("%d",q+p);
return 0;

}

Output: 25

Explanation: If you will see intermediate file you will find following code:

#include<stdio.h>

void main(){

int **p,q;

p=(int **)5;

q=10;

printf("%d",q+p);
return 0;

}

Explanations:

Here q pointer and p is a number.

In c, Address + number = Address

So, New address = old address + number * Size of data type to which pointer is pointing.

= 5 + 10 * sizeof (*int)

= 5+10*2 = 25.

Note: We are assuming default pointer is near. Actually it depends upon memory model.

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15

What is function recursion in c programming? Can you write any program using function?

Answer

Explanation:

Calling of same function from its function body is known as function recursion. It is alternative of loop. Any c program which is possible using loop it must be possible using function recursion. Simple example:

Find the sum of all even numbers from 0 to 20 using function recursion. Program:

#include<stdio.h>

int main(){

int total;

total=sum(2);

printf("%d",total);

return 0;

}

int sum(int i){

static int even=0;

if(i<=20){

even=even+i;

sum(i+2); //calling same function

}

return even;

}

Output:

It is very difficult to understand the execution as well as to write the function recursion program.

How to write function recursion program in easier way:

I have already told it is very difficult to write function recursion program directly. If any person is writing such program directly he may be memorized that program. I am telling a very nice trick: how to write function recursion program in easier way?

As we know any c program which possible using loop it must be possible using function recursion.

Steps for how to write function recursion program:

Step1: First of all write same program using while loop and function. (Except main function)

Step 2: In that function make all local variable static.

Step 3: Replace while keyword by if.

Step 4: The increment or decrement of variable which is used for condition checking, replace with function call and pass the parameter of that incremented or decremented variable. Now understand by example:

Find the sum of all even numbers from 0 to 20 using function recursion.

Step 1: Write the same program using while loop and function. Here function is sum.

#include<stdio.h>

int main(){

int total;

total=sum(2);

printf("%d",total);

return 0;        

}

int sum(int i){

int even=0;

while(i<=20){

even=even+i;

i=i+2;

}

return even;

}

Step 2: Make local variable even as static variable

int sum(int i){

static int even=0;

while(i<=20){

even=even+i;

i=i+2;

}

return even;

}

Step 3: Replace while keyword by if keyword.

int sum(int i){

int even=0;

if(i<=20){

even=even+i;

i=i+2;

}

return even;

}

Step 4: Since here variable i has used in condition checking. So replace the statement i=i+2 by sum (i+2).

int sum(int i){

int even=0;

if(i<=20){

even=even+i;

sum(i+2);

}

return even;

}

Following only three simple change you can write any difficult function recursion program.

#include<stdio.h>

int main(){

int total;

total=sum(2);

printf("%d",total);

return 0;        

}

int sum(int i){

int even=0;

if(i<=20){

even=even+i;

sum(i+2);

}

return even;

}

One more example:

Write a program to find a factorial of given number using function recursion.

Step 1: write same program using while loop and function.

#include<stdio.h>

int main(){

int fact,num;

scanf("%d",&num);

fact=factorial(num);

printf("%d",fact);

return 0;        

}

int factorial(int num){

int fact=1; //make it static

while(num>0){ //replace while by if

fact=fact*num;

num--; // replace by function call as factorial(num-1);

}

return fact;

}

After following step1, step 2, step 3:

#include<stdio.h>

int main(){

int fact,num;

scanf("%d",&num);

fact=factorial(num);

printf("%d",fact);

return 0;        

}

int factorial(int num){

static int fact=1;

if(num>0){

fact=fact*num;

factorial (num-1);

}

return fact;

}

Note1: In step 3 while calling function don’t pass the parameter using unary operator like factorial (num--);

Note2: write the function in such a way parameter of calling function must be used in conditional statement. For example in the above program:

int factorial(int num)

num is function parameter and num is also used in the conditional if statement.

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