What is the difference between declaration and definition of a variable/function?
Ans: Declaration of a variable/function simply declares that the variable/function exists somewhere in the
program but the memory is not allocated for them. But the declaration of a variable/function serves an
important role. And that is the type of the variable/function. Therefore, when a variable is declared, the
program knows the data type of that variable. In case of function declaration, the program knows what are the
arguments to that functions, their data types, the order of arguments and the return type of the function. So
thats all about declaration.
Coming to the definition, when we define a variable/function, apart from the role of declaration, it also
allocates memory for that variable/function. Therefore, we can think of definition as a super set of
declaration. (or declaration as a subset of definition). From this explanation, it should be obvious that a
variable/function can be declared any number of times but it can be defined only once. (Remember the basic
principle that you cant have two locations of the same variable/function).
// This is only declaration. y is not allocated memory by this statement
extern int y;
// This is both declaration and definition, memory to x is allocated by this statement.
int x;
What are different storage class specifiers in C?
Ans: auto, register, static, extern
What is scope of a variable? How are variables scoped in C?
Ans: Scope of a variable is the part of the program where the variable may directly be accessible. In C, all
identifiers are lexically (or statically) scoped. See this for more details.
How will you print Hello World without semicolon?
Ans:
int main(void)
{
if(printf("Hello World"))
{}
}
See print Geeks for Geeks without using a semicolon for answer.
When should we use pointers in a C program?
1. To get address of a variable
2. For achieving pass by reference in C: Pointers allow different functions to share and modify their local
variables.
3. To pass large structures so that complete copy of the structure can be avoided.
C
4. To implement linked data structures like linked lists and binary trees.
What is NULL pointer?
Ans: NULL is used to indicate that the pointer doesnt point to a valid location. Ideally, we should initialize
pointers as NULL if we dont know their value at the time of declaration. Also, we should make a pointer
NULL when memory pointed by it is deallocated in the middle of a program.
What is Dangling pointer?
Ans: Dangling Pointer is a pointer that doesnt point to a valid memory location. Dangling pointers arise when
an object is deleted or deallocated, without modifying the value of the pointer, so that the pointer still points to
the memory location of the deallocated memory. Following are examples.
// EXAMPLE 1
int *ptr = (int *)malloc(sizeof(int));
.............
.............
free(ptr);
// ptr is a dangling pointer now and operations like
following are invalid
*ptr = 10; // or printf("%d", *ptr);
// EXAMPLE 2
int *ptr = NULL
{
intx = 10;
ptr = &x;
}
// x goes out of scope and memory allocated to x
is free now.
// So ptr is a dangling pointer now.
What is memory leak? Why it should be avoided?
Ans: Memory leak occurs when programmers create a memory in heap and forget to delete it. Memory leaks
are particularly serious issues for programs like daemons and servers which by definition never terminate.
/* Function with memory leak */
#include <stdlib.h>
voidf()
{
int*ptr = (int*)
malloc(sizeof(int));
/* Do some work */
return; /* Return without
freeing ptr*/
}
What are local static variables? What is their use?
Ans: A local static variable is a variable whose lifetime doesnt end with a function call where it is declared. It
extends for the lifetime of complete program. All calls to the function share the same copy of local static
variables. Static variables can be used to count the number of times a function is called. Also, static variables
get the default value as 0. For example, the following program prints 0 1
#include <stdio.h>
voidfun()
{
// static variables get the default
value as 0.
staticintx;
printf("%d ", x);
x = x + 1;
}
intmain()
{
fun();
fun();
return0;
}
// Output: 0 1
What are static functions? What is their use?
Ans: In C, functions are global by default. The static keyword before a function name makes it static.
Unlike global functions in C, access to static functions is restricted to the file where they are declared.
Therefore, when we want to restrict access to functions, we make them static. Another reason for making
functions static can be reuse of the same function name in other files. See this for examples and more details.
What are main characteristics of C language?
Ans: C is a procedural language. The main features of C language include low-level access to memory, simple
set of keywords, and clean style. These features make it suitable for system programming like operating
system or compiler development.
What is difference between i++ and ++i?
1) The expression i++ returns the old value and then increments i. The expression ++i increments the value
and returns new value.
2) Precedence of postfix ++ is higher than that of prefix ++.
3) Associativity of postfix ++ is left to right and associativity of prefix ++ is right to left.
4) In C++, ++i can be used as l-value, but i++ cannot be. In C, they both cannot be used as l-value.
See Difference between ++*p, *p++ and *++p for more details.
What is l-value?
Ans: l-value or location value refers to an expression that can be used on left side of assignment operator. For
example in expression a = 3, a is l-value and 3 is r-value.
l-values are of two types:
nonmodifiable l-value represent a l-value that can not be modified. const variables are nonmodifiable l-
value.
modifiable l-value represent a l-value that can be modified.
What is the difference between array and pointer?
See Array vs Pointer
How to write your own sizeof operator?
Ans:
#define my_sizeof(type) (char *)(&type+1)-
(char*)(&type)
See Implement your own sizeof for more details.
How will you print numbers from 1 to 100 without using loop?
Ans: We can use recursion for this purpose.
/* Prints numbers from 1
to n */
voidprintNos(unsigned
intn)
{
if(n >0)
{
printNos(n-1);
printf("%d ", n);
}
}
What is volatile keyword?
Ans: The volatile keyword is intended to prevent the compiler from applying any optimizations on objects
that can change in ways that cannot be determined by the compiler.
Objects declared as volatile are omitted from optimization because their values can be changed by code
outside the scope of current code at any time. See Understanding volatile qualifier in C for more details.
Can a variable be both const and volatile?
Ans: Yes, the const. means that the variable cannot be assigned a new value. The value can be changed by
other code or pointer. For example the following program works fine.
int main(void)
{
constvolatile int local = 10;
int *ptr = (int *) &local;
printf("Initial value of local : %d
\n", local);
*ptr = 100;
printf("Modified value of local: %d
\n", local);
return0;
}
