C language Interview Questions and Answers

C language Interview Questions and Answers

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 CHAPTER 1: VARIABLES &CONTROL FLOW

1. What is the difference between declaring a variable and defining a variable?


Declaration of a variable in C hints the compiler about the type and size of the variable in compile time. Similarly,
declaration of a function hints about type and size of function parameters. No space is reserved in memory for
any variable in case of declaration.
Example: int a;
Here variable ‘a’ is declared of data type ‘int’
Defining a variable means declaring it and also allocating space to hold it.
We can say “Definition = Declaration + Space reservation”.
Example: int a = 10;
Here variable “a” is described as an int to the compiler and memory is allocated to hold value 10.


2. What is a static variable?


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 initialized by using keyword static before variable name.
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 initialized only once no matter how many times that function in which it resides is called. It may be
used as a count variable.
Example:
#include <stdio.h>
//program in file f1.c
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);
}/
*Main function*/
int main(){
count();
count();
count();
return 0;
}
Output:
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


3. What is a register variable?


Register variables are stored in the CPU registers. Its default value is a garbage 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:
register int x=5;
Variables for loop counters can be declared as register. Note that register keyword may be ignored by some
compilers.


4. Where is an 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.


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


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:
/***************
Index: f1.c
****************/
#include <stdio.h>
extern int x;
int main() {
printf(“value of x %d”, x);
return 0;
}
Index: 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 <stdio.h>
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);
} int main(){
printf(“n value of x: %d”, x);
increment();
return 0;
}


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


Register variables: belong to the register storage class and are stored in the CPU 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.
register int y=6;
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:
#include <stdio.h>
void decrement(){
static int a=5;
a–;
printf(“Value of a:%dn”, a);
} int main(){
decrement();
return 0;
}
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.


7. What are storage memory, default value, scope and life of Automatic and Register storage class?


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.


8. 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.


9. What is the difference between ‘break’ and ‘continue’ statements?


Differences between ‘break’ and ‘continue’ statements
break continue
1. break is a keyword used to terminate the loop or
exit from the block. The control jumps to next
statement after the loop or block.
1. continue is a keyword used for skipping the current
iteration and go to next iteration of the loop
2.Syntax:
{
Statement 1;
Statement 2;
Statement n;
break;
}
2.Syntax:
{
Statement 1;
continue;
Statement 2;
}
3. break can be used with for, while, do- while, and
switch statements. When break is used in nested
loops i.e. within the inner most loop then only the
innermost loop is terminated.
3. This statement when occurs in a loop does not
terminate it but skips the statements after this continue
statement. The control goes to the next iteration. continue
can be used with for, while and do-while.
4. Example:
i = 1, j = 0;
while(i<=5)
{
i=i+1;
if(i== 2)
4. Example:
i = 1, j = 0;
while(i<=5)
{
i=i+1;
if(i== 2)


break;
j=j+1;
}
continue;
j=j+1;
}


10. What is the difference between ‘for’ and ‘while’ loops?


for loop: When it is desired to do initialization, condition check and increment/decrement in a single statement of
an iterative loop, it is recommended to use ‘for’ loop.
Syntax:
for(initialization;condition;increment/decrement)
{/
/block of statements
increment or decrement
}
Program: Program to illustrate for loop
#include<stdio.h>
int main() {
int i;
for (i = 1; i <= 5; i++) {
//print the number
printf(“n %d”, i);
}
return 0;
}
Output:
12345
Explanation:
The loop repeats for 5 times and prints value of ‘i’ each time. ‘i’ increases by 1 for every cycle of loop.
while loop: When it is not necessary to do initialization, condition check and increment/decrement in a single
statement of an iterative loop, while loop could be used. In while loop statement, only condition statement is
present.
Syntax:
#include<stdio.h>
int main() {
int i = 0, flag = 0;
int a[10] = { 0, 1, 4, 6, 89, 54, 78, 25, 635, 500 };
//This loop is repeated until the condition is false.


while (flag == 0) {
if (a[i] == 54) {
//as element is found, flag = 1,the loop terminates
flag = 1;
}
else {
i++;
}
}
printf(“Element found at %d th location”, i);
return 0;
}
Output:
Element found at 5th location
Explanation:
Here flag is initialized to zero. ‘while’ loop repeats until the value of flag is zero, increments i by 1. ‘if’ condition
checks whether number 54 is found. If found, value of flag is set to 1 and ‘while’ loop terminates.


 CHAPTER 2: OPERATORS, CONSTANTS & STRUCTURES

1. 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.


2. Which bitwise 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)); 1 can be represented in binary as 0b00000001 = (1<<4) << is a left bit shift operator, it shifts 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. 


 3. What is equivalent of multiplying an unsigned int by 2:
left shift of number by 1 or right shift of number by 1? Left shifting of an unsigned integer is equivalent to multiplying an unsigned int by 2. 


Eg1: 
14<<1; 
Consider a number 14—–00001110 (8+4+2)is its binary equivalent left shift it by 1————–00011100(16+8+4) which is 28. 


Eg2: 
1<<1; 
consider the number as 1—00000001(0+0+1). left shift that by 1————00000010(0+2+0) which is 2.
 left shift by 1 bit of a number=2*number left shift by 1 bit of 2*number=2*2*number left shift by n bits of number=(2^n)*number 


Program: Program to illustrate left shift and right shift operations. 


#include
int main(void)
{
int x=10,y=10;
printf(“left shift of 10 is %d n”,x<<1); printf(“right shift of 10 is %d n”,y>>1);
return 0;
}


Output:
left shift of 10 is 20
right shift of 10 is 5
Explanation:
Left shift (by 1 position) multiplies a number by two. Right shift divides a number by 2.
4. What is an Enumeration Constant?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;

5. 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 accesing name field of student2 object.

6. What are the differences between a structure and a union?

Structures and Unions are used to store members of different data types.
STRUCTURE UNION
a)Declaration:
struct
{
data type member1;
data type member2;
};
a)Declaration:
union
{
data type member1;
data type member2;
};
b)Every structure member is allocated memory when
a structure variable is defined.
Example:
b)The memory equivalent to the largest item is allocated
commonly for all members.
Example:

struct emp {
char name[5];
int age;
float sal;
};
struct emp e1;
Memory allocated for structure is 1+2+4=7 bytes. 1
byte for name, 2 bytes for age and 4 bytes for sal.
union emp1 {
char name[5];
int age;
float sal;
};
union emp1 e2;
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.
c)All structure variables can be initialized at a time
struct st {
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.
c)Only one union member can be initialized at a time
union un {
int a;
float b;
};
union un un1 = { .a=10 };
Union is used when members of it are not required to be
accessed at the same time.

7. What are the advantages of 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.

8. How can typedef be to define a type of structure?

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;

9. Write a program that returns 3 numbers from a function using a structure.

A function in C can return only one value. If we want the function to return multiple values, we need to create a
structure variable, which has three integer members and return this structure.
Program: Program with a function to return 3 values
#include
//sample structure which has three integer variables.
struct sample {
int a, b, c;
};
//this is function which returns three values.
struct sample return3val() {
struct sample s1;
s1.a = 10;
s1.b = 20;
s1.c = 30;
//return structure s1, which means return s1.a ,s1.b and s1.c
return s1;
}
int main() {
struct sample accept3val;
//three values returned are accepted by structure accept3val.
accept3val = return3val();
//prints the values
printf(” n %d”, accept3val.a);
printf(“n %d”, accept3val.b);
printf(” n %d”, accept3val.c);
return 0;
}
Output:
10
20
30.
Explanation:
In this program, we use C structure to return multiple values from a function. Here we have a structure holding
three int variables and a function which returns it. ‘return3val’ is a function which assigns 10, 20, 30 to its integer
variables and returns this structure. In this program, ‘accept3val’ is a structure used to accept the values
returned by the function. It accepts those values and shows the output.

10. In code snippet below:

struct Date {
int yr;
int day;
int month;
} date1,date2;
date1.yr = 2004;
date1.day = 4;
date1.month = 12;
Write a function that assigns values to date2. Arguments to the function must be pointers to the
structure, Date and integer variables date, month, year.
Date is a structure with three int variables as members. set_date(..) is a function used to assign values to the
structure variable.
Program: Program to illustrate a function that assigns value to the structure.
#include
#include
//declare structure Date
struct Date {
int yr;
int day;
int month;
} date1, date2;
//declare function to assign date to structure variable
void set_date(struct Date *dte, int dt, int mnt, int year) {
dte->day = dt;
dte->yr = year;
dte->month = mnt;
}
int main(void) {
date1.yr = 2004;
date1.day = 4;
//assigning values one by one
date1.month = 12;
//assigning values in a single statement
set_date(&date2, 05, 12, 2008);
//prints both dates in date/month/year format
printf(“n %d %d %d “, date1.day, date1.month, date1.yr);
printf(“n %d %d %d “, date2.day, date2.month, date2.yr);
return 0;
}
Output:
4 12 2004
5 12 2008
Explanation:
Two variables of type Date are created and named ‘date1’, ‘date2’. ‘date2’ is assigned by using the function
set_date(..). Address of ‘date2’ is passed to set_date function.


 CHAPTER 3:  FUNCTIONS

1. What is the purpose of main() function?


In C, program execution starts from the main() function. Every C program must contain a main() function. The
main function may contain any number of statements. These statements are executed sequentially in the order
which they are written.
The main function can in-turn call other functions. When main calls a function, it passes the execution control to
that function. The function returns control to main when a return statement is executed or when end of function is
reached.
In C, the function prototype of the ‘main’ is one of the following:
int main(); //main with no arguments
int main(int argc, char *argv[]); //main with arguments
The parameters argc and argv respectively give the number and value of the program’s command-line
arguments.
Example:
#include
/* program section begins here */
int main() {
// opening brace – program execution starts here
printf(“Welcome to the world of C”);
return 0;
}/
/ closing brace – program terminates here
Output:
Welcome to the world of C
2. Explain command line arguments of main function?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
int 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 aren %dn”, argc); return 0; } Output: Following are the arguments entered in the command line C:testproject.exe apple mango Number of arguments entered are 3 3. What are header files? Are functions declared or defined in header files ?
 Functions and macros are declared in header files. Header files would be included in source files by the compiler at the time of compilation. Header files are included in source code using #include directive.#include includes all the declarations
present in the header file ‘some.h’.
A header file may contain declarations of sub-routines, functions, macros and also variables which we may want
to use in our program. Header files help in reduction of repetitive code.
Syntax of include directive:
#include //includes the header file stdio.h, standard input output header into the source code
Functions can be declared as well as defined in header files. But it is recommended only to declare functions
and not to define in the header files. When we include a header file in our program we actually are including all
the functions, macros and variables declared in it.
In case of pre-defined C standard library header files ex(stdio.h), the functions calls are replaced by equivalent
binary code present in the pre-compiled libraries. Code for C standard functions are linked and then the program
is executed. Header files with custom names can also be created.


Program: Custom header files example
/****************
Index: restaurant.h
****************/
int billAll(int food_cost, int tax, int tip);
/****************
Index: restaurant.c
****************/
#include
int billAll(int food_cost, int tax, int tip) {
int result;
result = food_cost + tax + tip;
printf(“Total bill is %dn”,result);
return result;
}
/****************
Index: main.c
****************/
#include
#include”restaurant.h”
int main() {
int food_cost, tax, tip;
food_cost = 50;
tax = 10;
tip = 5;
billAll(food_cost,tax,tip);
return 0;
}

4. What are the differences between formal arguments and actual arguments of a function?

Argument: An argument is an expression which is passed to a function by its caller (or macro by its invoker) in
order for the function(or macro) to perform its task. It is an expression in the comma-separated list bound by the
parentheses in a function call expression.
Actual arguments:
The arguments that are passed in a function call are called actual arguments. These arguments are defined in
the calling function.
Formal arguments:
The formal arguments are the parameters/arguments in a function declaration. The scope of formal arguments is
local to the function definition in which they are used. Formal arguments belong to the called function. Formal
arguments are a copy of the actual arguments. A change in formal arguments would not be reflected in the
actual arguments.
Example:
#include
void sum(int i, int j, int k);
/* calling function */
int main() {
int a = 5;
// actual arguments
sum(3, 2 * a, a);
return 0;
}
/* called function */
/* formal arguments*/
void sum(int i, int j, int k) {
int s;
s = i + j + k;
printf(“sum is %d”, s);
}
Here 3,2*a,a are actual arguments and i,j,k are formal arguments.

5. What is pass by value in functions?

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
void swap(int x, int y) {
int t;
t = x;
x = y;
y = t;
} int main() {
int m = 10, n = 20;
printf(“Before executing swap m=%d n=%dn”, m, n);
swap(m, n);
printf(“After executing swap m=%d n=%dn”, m, n);
return 0;
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.

6. What is pass by reference in functions?

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. C does not support Call by reference.
But it can be simulated 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 */
} int main() {
int m = 10, n = 20;
printf(“Before executing swap m=%d n=%dn”, m, n);
swap(&m, &n);
printf(“After executing swap m=%d n=%dn”, m, n);
return 0;
}

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.

7. What are the differences between getchar() and scanf() functions for reading strings?

Differences between getchar and scanf functions for reading strings:
scanf getchar
1. Entering of each character should be followed
by return key. 1. Need not type return key.
2. Continuous stream of characters cannot be
directly supplied using scanf function.
2. Continuous stream of characters can be directly supplied
using getchar function
3. Scanf function can be used to provide data at
execution time irrespective of its type(int, char,
float).
Example:
#include
int main() {
char a[10];
printf(“Enter a: n”);
scanf(“%s”,a);
return 0;
}
3. getchar() function is used only with character type.
Example:
#include
int main() {
char a;
printf(“Enter any character: n”);
a = getchar();
printf(“Character entered:%c n”,a);
return 0;
}
4. scanf() returns the number of items read
successfully. A return value 0 indicates that no
fields were read. EOF(end of file) is returned in
case of an error or if end-of-file/end-of-string
character is encountered.
4. getchar() returns the character entered as the value of the
function. It returns EOF in case of an error. It is recommeded
to use getchar instead of scanf.

8. Out of the functions fgets() and gets(), which one 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( ).

9. What is the difference between the functions strdup() and strcpy()?

strcpy function: copies a source string to a destination defined by user. In strcpy function both source and
destination strings are passed as arguments. User should make sure that destination has enough space to
accommodate the string to be copied.
‘strcpy’ sounds like short form of “string copy”.
Syntax:
strcpy(char *destination, const char *source);
Source string is the string to be copied and destination string is string into which source string is copied. If
successful, strcpy subroutine returns the address of the copied string. Otherwise, a null pointer is returned.
Example Program:
#include
#include
int main() {
char myname[10];
//copy contents to myname
strcpy(myname, “abcdefghi”);
//print the string
puts(myname);
return 0;
}
Output:
abcdefghi
Explanation:
If the string to be copied has more than 10 letters, strcpy cannot copy this string into the string ‘myname’. This is
because string ‘myname’ is declared to be of size 10 characters only.
In the above program, string “nodalo” is copied in myname and is printed on output screen.
strdup function: duplicates a string to a location that will be decided by the function itself. Function will copy the
contents of string to certain memory location and returns the address to that location. ‘strdup’ sounds like short
form of “string duplicate”
Syntax:
strdup (const char *s);
strdup returns a pointer to a character or base address of an array. Function returns address of the memory
location where the string has been copied. In case free space could not be created then it returns a null pointer.
Both strcpy and strdup functions are present in header file
Program: Program to illustrate strdup().
#include
#include
#include
int main() {
char myname[] = “abcdefghi”;

//name is pointer variable which can store the address of memory location of string
char* name;
//contents of myname are copied in a memory address and are assigned to name
name = strdup(myname);
//prints the contents of ‘name’
puts(name);
//prints the contents of ‘myname’
puts(myname);
//memory allocated to ‘name’ is now freed
free(name);
return 0;
}
Output:
abcdefghi
abcdefghi 
Explanation:
String myname consists of “
abcdefghi ” stored in it. Contents of myname are copied in a memory
address and memory is assigned to name. At the end of the program, memory can be freed using free(name);


 CHAPTER 4: POINTERS

1.     What is a pointer in C?
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.
ap=ap+1;
Above expression would not increment the value of ap by one, 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.
2.     What are the advantages of using pointers?
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:
1. 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.
2. Dynamic allocation of memory is possible with the help of pointers.
3. 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.
3.     Pointers point to physical memory and allow quicker access to data.
3. 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.
4.     How to use realloc() to dynamically increase size of an already 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.
5. What is the equivalent pointer expression for referring an element a[i][j][k][l], in a four
dimensional array?
Consider a multidimensional array a[w][x][y][z].
In this array, a[i] gives address of a[i][0][0][0] and a[i]+j gives the address of a[i][j][0][0]
Similarly, a[i][j] gives address of a[i][j][0][0] and a[i][j]+k gives the address of a[i][j][k][0]
a[i][j][k] gives address of a[i][j][k][0] and a[i][j][k]+l gives address of a[i][j][k][l]
Hence a[i][j][k][l] can be accessed using pointers as *(a[i][j][k]+l)
where * stands for value at address and a[i][j][k]+l gives the address location of a[i][j][k][l].
Program: Example program to illustrate pointer denotation of multi-dimensional arrays.
#include<stdio.h>
#include<string.h>
int main() {
int a[3][3][3][3];
//it gives address of a[0][0][0][0] .
printf(” n address of array a is %u”, a);
printf(“n address of a[2][0][0][0] is %u ,given by a[2], %u given by a+2”,
a[2], a + 2);
printf(“n address of a[2][2][0][0] is %u ,given by a[2][2], %u given by a[2]+2”,
a[2][2], a[2] + 2);
printf(“n address of a[2][2][1][0] is %u ,given by a[2][2][1] , %u given by a[2][2]+1”,
a[2][2][1], a[2][2] + 1);
return 0;
}
Output:
address of array a is 65340
address of a[2][0][0][0] is 65448, given by a[2] , 65448 given by a+2
address of a[2][2][0][0] is 65484, given by a[2][2] ,65484 given by a[2]+2
address of a[2][2][1][0] is 65490, given by a[2][2][1] , 65490 given by a[2][2]+1
Explanation:
This output may differ from computer to computer as the address locations are not same for every computer.
 
 
6. Declare an array of three function pointers where each function receives two integers and
returns float.
Declaration:
float (*fn[3])(int, int);
Program: Illustrates the usage of above declaration
#include<stdio.h>
float (*fn[3])(int, int);
float add(int, int);
int main() {
int x, y, z, j;
for (j = 0; j < 3; j++){
fn[j] = &add;
}
x = fn[0](10, 20);
y = fn[1](100, 200);
z = fn[2](1000, 2000);
printf(“sum1 is: %d n”, x);
printf(“sum2 is: %d n”, y);
printf(“sum3 is: %d n”, z);
return 0;
}f
loat add(int x, int y) {
float f = x + y;
return f;
}
Output:
sum1 is: 30
sum2 is: 300
sum3 is: 3000
Explanation:
Here ‘fn[3]’ is an array of function pointers. Each element of the array can store the address of function ‘float
add(int, int)’.
fn[0]=fn[1]=fn[2]=&add
Wherever this address is encountered add(int, int) function is called.
7. Explain the variable assignment in the declaration
int *(*p[10])(char *, char *);
It is an array of function pointers that returns an integer pointer. Each function has two arguments which in turn
are pointers to character type variable. p[0], p[1],….., p[9] are function pointers.
return type : integer pointer.
p[10] : array of function pointers
char * : arguments passed to the function
Program: Example program to explain function pointers.
#include<stdio.h>
#include<stdlib.h>
int *(*p[10])(char *, char *);
//average function which returns pointer to integer whose value is average of ascii value of characters passed by
pointers
int *average(char *, char *);
//function which returns pointer to integer whose value is sum of ascii value of characters passed by pointers
int *sum(char *, char *);
int retrn;
int main(void) {
int i;
for (i = 0; i < 5; i++) {
//p[0] to p[4] are pointers to average function.
p[i] = &(average);
}
for (i = 5; i < 10; i++) {
//p[5] to p[9] are pointers to sum function
p[i] = &(sum);
}
char str[10] = “nodalo.com”;
int *intstr[10];
for (i = 0; i < 9; i++) {
//upto p[4] average function is called, from p[5] sum is called.
intstr[i] = p[i](&str[i], &str[i + 1]);
if (i < 5) {
//prints the average of ascii of both characters
printf(” n average of %c and %c is %d”,
str[i], str[i + 1],*intstr[i]);
}
else {
//prints the sum of ascii of both characters.
printf(” n sum of %c and %c is %d”,
str[i], str[i + 1], *intstr[i]);
}
}
return 0;
}/
/function average is defined here
int *average(char *arg1, char *arg2) {
retrn = (*arg1 + *arg2) / 2;
return (&retrn);
}/
/function sum is defined here
int *sum(char *arg1, char *arg2) {
retrn = (*arg1 + *arg2);
return (&retrn);
}
Output:
average of n and o is 110
average of o and d is 105
average of d and a is 98 average of d and a is 98
average of a and l is 102
average of l and o is 109
sum of o and . is 157
sum of . and c is 145
sum of c and o is 210
sum of o and m is 220
Explanation:
In this program p[10] is an array of function pointers. First five elements of p[10] point to the function: int
*average(char *arg1,char *arg2). Next five elements point to the function int *sum(char *arg1,char *arg2). They
return pointer to an integer and accept pointer to char as arguments.
Function average:
int *average(char *arg1,char *arg2) This function finds the average of the two values of the addresses passed to
it as arguments and returns address of the average value as an integer pointer.
Function sum:
int *sum(char *arg1,char *arg2) This function finds the sum of the two values of the addresses passed to it as
arguments and returns address of the sum value as an integer pointer.
8. What is the value of
sizeof(a) /sizeof(char *)
in a code snippet:
char *a[4]={“abc”,”def”,”ghij”,”klmno”};
Explanation:
Here a[4] is an array which holds the address of strings. Strings are character arrays themselves.
Memory required to store an address is 4 bits. So memory required to store 4 addresses is equal to 4*4=16 bits.
char *; is a pointer variable which stores the address of a char variable.
So sizeof(char *) is 4 bits. Therefore sizeof(a) /sizeof(char *) = 16/4 = 4 bytes.
9. (i) What are the differences between the C statements below:
char *str = “Hello”;
char arr[] = “Hello”;
(ii) Whether following statements get complied or not? Explain each statement.
arr++;
*(arr + 1) = ‘s’;
printf(“%s”,arr);
(i) char *str=”Hello”;
“Hello” is an anonymous string present in the memory. ‘str’ is a pointer variable that holds the address of this
string.
char arr[]=”Hello”;
This statement assigns space for six characters: ‘H’ ‘e’ ‘l’ ‘l’ ‘o’ ‘�’ . ‘arr’ is the variable name assigned to this
array of characters.
str[4] and arr[4] also have different meanings.
str[4]: adds 4 to the value of ‘str’ and points to the address same as value of str + 4.
arr[4]: points to the fourth element in array named ‘arr’.
(ii) ‘arr’ is variable name of an array. A variable name can not be incremented or decremented. Hence arr++ is an
invalid statement and would result in a compilation error.
*(arr+1)=’s’;
‘arr’ is the name of a character array that holds string “Hello”. Usually, name of an array points to its base
address. Hence value of arr is same as &arr[0].
arr+1 is address of the next element: &arr[1]
Character ‘s’ is assigned to the second element in array ‘arr’, thereby string changes from “Hello” to “Hsllo”.
printf(“%s”,arr );
This statement prints the string stored in character array ‘arr’.


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