C Interview Questions

Dive into the fundamentals and complexities of the C programming language with 'C Interview Questions.' This blog is your go-to resource for a wide array of questions and answers tailored for C programming interviews. Whether you're a student, a budding programmer, or a seasoned developer, this guide provides valuable insights into key concepts, common challenges, and best practices in C, equipping you with the knowledge and confidence needed to excel in technical interviews and coding tests.

1. What is C?

C is a general-purpose programming language that was developed at Bell Labs in 1972. It is widely used for system programming, developing embedded systems, and various applications.

2. What is the difference between C and C++?

C is a procedural programming language, while C++ is an extension of C that supports object-oriented programming. C++ includes features like classes and polymorphism that are not present in C.

3. What are the features of the C language?

Some features of the C language are- 

  1. It is Simple And Efficient. 
  2. C language is portable or Machine Independent.
  3. C is a mid-level Programming Language. 
  4. It is a structured Programming Language.
  5. It has a function-rich library. 
  6. Dynamic Memory Management.
  7. C is super fast.
  8. We can use pointers in C.
  9. It is extensible.

4. What is a token?

The individual elements of a program are called Tokens. There are following 6 types of tokens are available in C:

5. What is the use of printf() and scanf() functions? Also explain format specifiers?

Some datatype format specifiers for both printing and scanning purposes are as follows:

6. What's the value of the expression 5["abxdef"]?

The answer is 'f'.

Explanation: The string mentioned "abxdef" is an array, and the  expression is equal to "abxdef"[5]. Why is the inside-out expression equivalent?  Because a[b] is equivalent to *(a + b) which is equivalent to *(b + a) which is equivalent to b[a].

7. What is a built-in function in C?

The most commonly used built-in functions in C are scanf(), printf(), strcpy, strlwr, strcmp, strlen, strcat, and many more.

Built-function is also known as library functions that are provided by the system to make the life of a developer easy by assisting them to do certain commonly used predefined tasks. For example, if you need to print output or your program into the terminal, we use printf() in C.

8. In C, What is the #line used for?

In C, #line is used as a preprocessor to re-set the line number in the code, which takes a parameter as line number. Here is an example for the same. 

#include <stdio.h>\t\t\t\t\t\t\t\t/*line 1*/\t\t\t\t\t\t\t\t\t\t\t\t/*line 2*/\tint main(){\t\t\t\t\t\t\t\t\t/*line 3*/\t\t\t\t\t\t\t\t\t\t\t\t/*line 4*/\tprintf("Hello world");\t\t\t\t\t/*line 5*/\t//print current line\t\t\t\t\t\t/*line 6*/\tprintf("Line: %d",__LINE__);\t\t\t\t/*line 7*/\t//reset the line number by 36\t\t\t\t/*line 8*/\t#line 36\t/*reseting*/\t//print current line\t\t\t\t\t\t/*line 36*/\tprintf("Line: %d",__LINE__);\t\t\t\t/*line 37*/\tprintf("Bye bye!!!");\t\t\t\t\t\t/*line 39*/\t\t\t\t\t\t\t\t\t\t\t\t/*line 40*/\treturn 0;\t\t\t\t\t\t\t\t\t/*line 41*/}\t\t\t\t\t\t\t\t\t\t\t\t/*line 42*/

9. What is recursion in C?

When a function in C calls a copy of itself, this is known as recursion. To put it another way, when a function calls itself, this technique is called Recursion. Also, this function is known as recursive function.

Syntax of Recursive Function:

void do_recursion(){ ... .. ... do_recursion();... .. ...]} int main() {... .. ...do_recursion();... .. ...}

10. Why doesn’t C support function overloading?

After you compile the C source, the symbol names need to be intact in the object code. If we introduce function overloading in our source, we should also provide name mangling as a preventive measure to avoid function name clashes. Also, as C is not a strictly typed language many things(ex: data types) are convertible to each other in C. Therefore, the complexity of overload resolution can introduce confusion in a language such as C.

When you compile a C source, symbol names will remain intact. If you introduce function overloading, you should provide a name mangling technique to prevent name clashes. Consequently, like C++, you'll have machine-generated symbol names in the compiled binary.

Additionally, C does not feature strict typing. Many things are implicitly convertible to each other in C. The complexity of overload resolution rules could introduce confusion in such kind of language

11. What is the difference between global int and static int declaration?

The difference between this is in scope. A truly global variable has a global scope and is visible everywhere in your program. 

#include <stdio.h>  int my_global_var = 0;  int main(void)  {   printf("%d", my_global_var);   return 0; }

global_temp is a global variable that is visible to everything in your program, although to make it visible in other modules, you'd need an ”extern int global_temp; ” in other source files if you have a multi-file project.

A static variable has a local scope but its variables are not allocated in the stack segment of the memory. It can have less than global scope, although - like global variables - it resides in the .bss segment of your compiled binary. 

#include <stdio.h>  int myfunc(int val)  {     static int my_static_var = 0;      my_static_var += val;     return my_static_var; }  int main(void)  {    int myval;     myval = myfunc(1);    printf("first call %d", myval);     myval = myfunc(10);     printf("second call %d", myval);     return 0; }

12. Difference between const char* p and char const* p?

Since const char and char const are the same, it's the same.

13. What is pointer to pointer in C?

In C, a pointer can also be used to store the address of another pointer. A double pointer or pointer to pointer is such a pointer. The address of a variable is stored in the first pointer, whereas the address of the first pointer is stored in the second pointer.

The syntax of declaring a double pointer is given below:

int **p; // pointer to a pointer which is pointing to an integer

14. What is typecasting in C?

Typecasting is the process to convert a variable from one datatype to another.  If we want to store the large type value to an int type, then we will convert the data type into another data type explicitly.

Syntax: (data_type)expression;

For Example:

int x;for(x=97; x<=122; x++){   printf("%c", (char)x);   /*Explicit casting from int to char*/}

15. What are the advantages of Macro over function?

Macro on a high-level copy-paste, its definitions to places wherever it is called. Due to which it saves a lot of time, as no time is spent while passing the control to a new function and the control is always with the callee function. However, one downside is the size of the compiled binary is large but once compiled the program comparatively runs faster.

16. What are Enumerations?

Enumeration, also known as Enum in C, is a user-defined data type. It consists of constant integrals or integers that have names assigned to them by the user. Because the integer values are named with enum in C, the whole program is simple to learn, understand, and maintain by the same or even different programmer.

17. When should we use the register storage specifier?

If a variable is used frequently, it should be declared with the register storage specifier, and the compiler may allocate a CPU register for its storage to speed up variable lookup.

18. Specify different types of decision control statements?

All statements written in a program are executed from top to bottom one by one. Control statements are used to execute/transfer the control from one part of the program to another depending on the condition.

19. What is an r-value and l-value?

20. What is the difference between struct and union in C?

A struct is a group of complex data structures stored in a block of memory where each member on the block gets a separate memory location to make them accessible at once

Whereas in the union, all the member variables are stored at the same location on the memory as a result to which while assigning a value to a member variable will change the value of all other members. 

/* struct & union definations*/struct bar {\tint a;\t// we can use a & b both simultaneously\tchar b;}\tbar;union foo {\tint a;\t// we can't use both a and b simultaneously\tchar b;}\tfoo;/* using struc and union variables*/struct bar y;y.a = 3;\t// OK to usey.b = 'c'; // OK to useunion foo x;x.a = 3; // OKx.b = 'c'; // NOl this affects the value of x.a!

21. What is call by reference in functions?

When we caller function makes a function call bypassing the addresses of actual parameters being passed, then this is called call by reference. In incall by reference, the operation performed on formal parameters affects the value of actual parameters because all the operations performed on the value stored in the address of actual parameters.

22. What is a memory leak? How to avoid it?

When we assign a variable it takes space of our RAM (either heap or RAM)dependent on the size of data type, however, if a programmer uses a memory available on the heap and forgets to a delta it, at some point all the memory available on the ram will be occupied with no memory left this can lead to a memory leak. 

int main(){    char * ptr = malloc(sizeof(int));        /* Do some work */    /*Not freeing the allocated memory*/    return 0;}

To avoid memory leaks, you can trace all your memory allocations and think forward, where you want to destroy (in a good sense) that memory and place delete there. Another way is to use C++ smart pointer in C linking it to GNU compilers.

23. What is Dynamic memory allocation in C? Name the dynamic allocation functions.

C is a language known for its low-level control over the memory allocation of variables in DMA there are two major standard library malloc() and free. The malloc() function takes a single input parameter which tells the size of the memory requested It returns a pointer to the allocated memory. If the allocation fails, it returns NULL. The prototype for the standard library function is like this:

void *malloc(size_t size);
The free() function takes the pointer returned by malloc() and de-allocates the memory. No indication of success or failure is returned. The function prototype is like this: 

void free(void *pointer);
There are 4 library functions provided by C defined under <stdlib.h> header file to facilitate dynamic memory allocation in C programming. They are:

24. What is typedef?

typedef is a C keyword, used to define alias/synonyms for an existing type in C language. In most cases, we use typedef's to simplify the existing type declaration syntax. Or to provide specific descriptive names to a type. 

typedef <existing-type> <new-type-identifiers>;

typedef provides an alias name to the existing complex type definition. With typedef, you can simply create an alias for any type. Whether it is a simple integer to complex function pointer or structure declaration, typedef will shorten your code.

25. Why is it usually a bad idea to use gets()? Suggest a workaround.

The standard input library gets() reads user input till it encounters a new line character. However, it does not check on the size of the variable being provided by the user is under the maximum size of the data type which makes the system vulnerable to buffer overflow and the input being written into memory where it isn’t supposed to.

We, therefore, use gets() to achieve the same with a restricted range of input

Bonus: It remained an official part of the language up to the 1999 ISO C standard, but it was officially removed by the 2011 standard. Most C implementations still support it, but at least GCC issues a warning for any code that uses it.

26. What is the difference between #include "..." and #include <...>?

In practice, the difference is in the location where the preprocessor searches for the included file.

For #include <filename> the C preprocessor looks for the filename in the predefined list of system directories first and then to the directories told by the user(we can use -I option to add directories to the mentioned predefined list). 

For #include "filename" the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename> form. This method is normally used to include programmer-defined header files.

27. What are dangling pointers? How are dangling pointers different from memory leaks?

The dangling pointer points to a memory that has already been freed. The storage is no longer allocated. Trying to access it might cause a Segmentation fault. A common way to end up with a dangling pointer: 

#include<stdio.h>#include<string.h>char *func(){    char str[10];    strcpy(str,"Hello!");    return(str);}

You are returning an address that was a local variable, which would have gone out of scope by the time control was returned to the calling function. (Undefined behavior) 

*c = malloc(5izeof(int));free(c);*c = 3; //writing to freed location!

In the figure shown above writing to a memory that has been freed is an example of the dangling pointer, which makes the program crash.

A memory leak is something where the memory allocated is not freed which causes the program to use an undefined amount of memory from the ram making it unavailable for every other running program(or daemon) which causes the programs to crash. There are various tools like O profile testing which is useful to detect memory leaks on your programs. 

void function(){\tchar *leak = malloc (10);\t//leak assigned but not freed}

28. What is the difference between ‘g’ and “g” in C?

In C double-quotes variables are identified as a string whereas single-quoted variables are identified as the character. Another major difference being the string (double-quoted) variables end with a null terminator that makes it a 2 character array.

29. What is a near pointer and a far pointer in C?

30. Which structure is used to link the program and the operating system?

The file structure is used to link the operating system and a program. The header file "stdio.h" (standard input/output header file) defines the file. It contains information about the file being used like its current size and its memory location. It contains a character pointer that points to the character which is currently being opened. When you open a file, it establishes a link between the program and the operating system about which file is to be accessed.

31. Suppose a global variable and local variable have the same name. Is it possible to access a global variable from a block where local variables are defined?

No. This isn’t possible in C. It’s always the most local variable that gets preference.

32. Which is better #define or enum?

33. How can you remove duplicates in an array?

The following program will help you to remove duplicates from an array.

#include <stdio.h>int main() {    int n, a[100], b[100], calc = 0, i, j,count;    printf("Enter no. of elements in array: ");    scanf("%d", &n);    printf("Enter %d integers: ", n);    for (i = 0; i < n; i++)        scanf("%d", &a[i]);              for (i = 0; i<n; i++) {        for (j = 0; j<calc; j++) {           if(a[i] == b[j])           break;          }        if (j== calc) {             b[calc] = a[i];             calc++;         }     }          printf("Array obtained after removing duplicate elements: ");     for (i = 0; i<calc; i++) {             printf("%d ", b[i]);     }     return 0;}

34. Can we compile a program without a main() function?

Yes, we can compile a program without main() function Using Macro.

E.g.

#include<studio.h>#define abc mainint abc (){printf("Hello World ");return 0;}

35. Write a program to get the higher and lower nibble of a byte without using shift operator?

#include<stdio.h>struct full_byte{char first : 4;char second : 4;};union A{char x;struct full_byte by;};main(){char c = 100;union A a;a.x = c;printf("the two nibbles are: %d and %d", a.by.first, a.by.second);}

36. How do you override a defined macro?

To override a defined macro we can use #ifdef and #undef preprocessors as follows:

If macro A is defined, it will be undefined using undef and then defined again using define.

37. Write a C program to check if it is a palindrome number or not using a recursive method.

#include <stdio.h>#include <conio.h>int reverse(int num);int isPalindrome(int num);int main(){    int num;    printf("Enter a number: ");    scanf("%d", &num);if(isPalindrome(num) == 1){ printf("the given number is a palindrome");    }    else    {        printf("the given number is not a palindrome number");   }    return 0;}int isPalindrome(int num){    if(num == reverse(num))    {        return 1;    }    return 0;}int reverse(int num){int rem;    static int sum=0;    if(num!=0){            rem=num%10;            sum=sum*10+rem;            reverse(num/10);    }        else             return sum;    return sum;}

38. C program to check the given number format is in binary or not.

#include<stdio.h>#include<conio.h>int main() {\tint j,num;\tprintf("Please enter a number :");\tscanf("%d",&num);    while(num>0)    {        j=num%10;        if( j!=0 && j!=1 )        {           printf("num is not binary");           break;        }        num=num/10;        if(num==0)        {            printf("num is binary");        }    }\tgetch();}

39. C Program to find a sum of digits of a number using recursion.

#include<stdio.h>#include<conio.h>int sumOfDigits(int num)    {            static int sum = 0;            int rem;            sum = sum + (num%10);            rem = num/10;            if(rem > 0)            {                sumOfDigits(rem);            }            return sum;    }int main() {\tint j,num;\tprintf("Please enter a number :");\tscanf("%d",&num);    printf("sum of digits of the number = %d ",sumOfDigits(num));    getch();}

40. What is the use of a semicolon (;) at the end of every program statement?

It is majorly related to how the compiler reads( or parses) the entire code and breaks it into a set of instructions(or statements), to which semicolon in C acts as a boundary between two sets of instructions.

41. How to call a function before main()?

To call a function before the main(), pragma startup directive should be used. E.g.-

#pragma startup funvoid fun(){printf("In fun");}main(){printf("In main");}

The output of the above program will be -

In funIn main

This pragma directive, on the other hand, is compiler-dependent. This is not supported by gcc. As a result, it will ignore the startup directive and produce no error. But the output, in that case, will be - 

In main

42. Differentiate between the macros and the functions.

The differences between macros and functions can be explained as follows:

Macros Functions
It is preprocessed rather than compiled. It is compiled not preprocessed.
It is preprocessed rather than compiled. Function checks for compilation errors.
Code length is increased. Code length remains the same.
Macros are faster in execution. Functions are a bit slower in execution.
Macros are useful when a small piece of code is used multiple times in a program. Functions are helpful when a large piece of code is repeated a number of times.

43. When is the "void" keyword used in a function

The keyword “void” is a data type that literally represents no data at all. The most obvious use of this is a function that returns nothing: 

void PrintHello() { \tprintf("Hello"); \treturn;\t\t// the function does "return", but no value is returned }

Here we’ve declared a function, and all functions have a return type. In this case, we’ve said the return type is “void”, and that means, “no data at all” is returned. 
The other use for the void keyword is a void pointer. A void pointer points to the memory location where the data type is undefined at the time of variable definition. Even you can define a function of return type void* or void pointer meaning “at compile time we don’t know what it will return” Let’s see an example of that. 

void MyMemCopy(void* dst, const void* src, int numBytes) { \tchar* dst_c = reinterpret_cast<char*>(dst); \tconst char* src_c = reinterpret_cast<const char*>(src); \tfor (int i = 0; i < numBytes; ++i) \t\tdst_c[i] = src_c[i]; }

44. What is dynamic data structure?

A dynamic data structure (DDS) refers to an organization or collection of data in memory that has the flexibility to grow or shrink in size, enabling a programmer to control exactly how much memory is utilized. Dynamic data structures change in size by having unused memory allocated or de-allocated from the heap as needed. 

Dynamic data structures play a key role in programming languages like C, C++, and Java because they provide the programmer with the flexibility to adjust the memory consumption of software programs.

45. Add Two Numbers Without Using the Addition Operator

For the sum of two numbers, we use the addition (+) operator. In these tricky C programs, we will write a C program to add two numbers without using the addition operator. 

#include<stdio.h>#include<stdlib.h>int main(){  int x, y;  printf("Enter two number: ");  scanf("%d %d",&x,&y);  // method 1  printf("%d", x-(-y));  // method 2  printf("%d", -(-x-y));  // method 3  printf("%d", abs(-x-y));  // method 4  printf("%d", x-(~y)-1);  return 0;}

46. Subtract Two Number Without Using Subtraction Operator

#include<stdio.h>#include<stdlib.h>int main(){   int x, y;   printf("Enter two number: ");   scanf("%d %d",&x,&y);   printf("%d", x+(~y)+1);   return 0;}

The bitwise complement operator is used in this program. The bitwise complement of number ~y=-(y+1). So, expression will become x+(-(y+1))+1=x-y-1+1=x-y

47. Multiply an Integer Number by 2 Without Using Multiplication Operator

#include<stdio.h>int main(){   int x;   printf("Enter a number: ");   scanf("%d",&x);   printf("%d", x<<1);   return 0;}

The left shift operator shifts all bits towards the left by a certain number of specified bits. The expression x<<1 always returns x*2. Note that the shift operator doesn’t work on floating-point values.

For multiple of x by 4, use x<<2. Similarly x<<3 multiply x by 8. For multiple of the number x by 2^n, use x<<n.

48. Check whether the number is EVEN or ODD, without using any arithmetic or relational operators

#include<stdio.h>int main(){   int x;   printf("Enter a number: ");   scanf("%d", &x);   (x&1)?printf("Odd"):printf("Even");   return 0;}

The bitwise and(&) operator can be used to quickly check the number is odd or even.

49. Program to find n’th Fibonacci number

Fibonacci sequence is characterized by the fact that every number after the first two is the sum of the two preceding ones. For example, consider below sequence

0, 1, 1, 2, 3, 5, 8, 13, 21, 34, . .. and so on

Where in F{n} = F{n-1} + F{n-2} with base values F(0) = 0 and <code>F(1) = 1

Below is naive implementation for finding the nth member of the Fibonacci sequence

// Function to find the nth Fibonacci numberint fib(int n){    if (n <= 1) {        return n;    }     return fib(n - 1) + fib(n - 2);} int main(){    int n = 8;     printf("nth Fibonacci number is %d", fib(8));     return 0;}

50. Code Snippet: Hello World in C

            
#include <stdio.h>

int main() {
    printf("Hello, World!");
    return 0;
}

51. Online Resources: