C Interview Questions

C Interview Questions with Answers

This page gives simple C interview questions with short answers. The questions are made for beginners, so you can use them to study for tests and job interviews.

• Core C basics and program structure
• Variables and data types
• Conditions and loops
• Arrays and strings
• Pointers and memory
• Functions, scope, and structures

Core C Basics

1. What is C used for?

Answer:

• C is a programming language used to write fast and efficient programs.
• It is often used for system software, embedded systems, and performance-critical applications.

2. What is the basic structure of a C program?

Answer:

• A typical C program has #include lines, a main() function, and C statements inside { }.

#include <stdio.h>

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

3. What does the main() function do in C?

Answer:

• main() is the starting point of every C program.
• The code inside main() runs first when the program starts.
• The return 0; line usually means "the program ended successfully".

4. What is the purpose of #include <stdio.h>?

Answer:

• #include <stdio.h> tells the compiler to include the standard input/output library.
• It lets you use functions like printf() and scanf().

5. What does printf() do?

Answer:

• printf() prints text and values to the screen.
• You can use format specifiers like %d for integers and %f for floating-point numbers.

#include <stdio.h>

int main() {
  int age = 30;
  printf("Age: %d\n", age);
  return 0;
}

Variables and Data Types

6. What is a variable in C?

Answer:

• A variable is a named place in memory where a value is stored.
• In C, you must declare the type before the variable name.

int count = 10;
float price = 19.95;
char grade = 'A';

7. What is the difference between int, float, and char?

Answer:

• int stores whole numbers (e.g. 3, -10, 200).
• float stores numbers with decimals (e.g. 3.14, -0.5).
• char stores a single character (e.g. 'A', 'B').

8. What is a constant and why would you use one?

Answer:

• A constant is a value that cannot be changed once it is set.
• Using const makes your intent clear and protects the value from being changed by mistake.

const int DAYS_IN_WEEK = 7;
// DAYS_IN_WEEK = 8; // not allowed

9. How do you read user input in C?

Answer:

• You can use scanf() to read values from the keyboard.
• You must pass the address of the variable using &.

int number;
scanf("%d", &number);

Conditions and Loops

10. What is an if statement used for?

Answer:

• An if statement lets the program make a decision.
• Code inside the if block runs only when the condition is true.

int age = 20;

if (age >= 18) {
  printf("Adult\n");
} else {
  printf("Not adult\n");
}

11. What is a loop in C?

Answer:

• A loop repeats the same block of code multiple times.
• C has for, while, and do...while loops.

for (int i = 1; i <= 5; i++) {
  printf("%d\n", i);
}

12. What is the difference between while and do...while?

Answer:

• while checks the condition before running the loop body.
• do...while runs the loop body once, then checks the condition.
• This means do...while always runs at least one time.

13. What is a switch statement?

Answer:

• A switch statement chooses one block of code to run based on the value of an expression.
• Each option is written as a case, and default handles all other values.

int day = 2;

switch (day) {
  case 1:
    printf("Monday");
    break;
  case 2:
    printf("Tuesday");
    break;
  default:
    printf("Another day");
}

Arrays and Strings

14. What is an array in C?

Answer:

• An array stores many values of the same type in one variable name.
• You access each element by its index, starting from 0.

int scores[3] = {10, 20, 30};
printf("%d\n", scores[0]); // first element

15. How are strings represented in C?

Answer:

• In C, a string is an array of characters.
• It ends with a special character '\0' (the null terminator).

char greeting[] = "Hello";
printf("%s\n", greeting);

Pointers and Memory

16. What is a pointer in C?

Answer:

• A pointer is a variable that stores the memory address of another variable.
• Pointers are used to work directly with memory and to share data with functions.

int value = 10;
int *ptr = &value;  // ptr holds the address of value

17. What do the & and * operators mean with pointers?

Answer:

• & gives the address of a variable (the "address of" operator).
• * gives the value at an address (the "dereference" operator).

int x = 5;
int *p = &x;    // &x is the address of x
printf("%d\n", *p); // *p is the value at that address (5)

18. What is a NULL pointer?

Answer:

• A NULL pointer is a pointer that does not point to any valid memory location.
• It is often used to show that the pointer is "empty" or not yet set.

int *ptr = NULL;  // pointer not pointing anywhere yet

Functions, Scope, and Structures

19. What is a function in C and why use it?

Answer:

• A function is a block of code that performs a specific task.
• Functions help you split a large program into smaller, reusable pieces.

int add(int a, int b) {
  return a + b;
}

20. What is the difference between local and global variables?

Answer:

• A local variable is declared inside a function and can be used only there.
• A global variable is declared outside all functions and can be used in the entire file (after its declaration).

21. What is a struct in C?

Answer:

• A struct (structure) is a custom type that groups related variables under one name.
• The grouped variables can have different types.

struct Person {
  char name[50];
  int age;
};

22. Why would you use a struct instead of separate variables?

Answer:

• A struct keeps related information together, making the code easier to understand.
• It is useful when you want to treat multiple pieces of data as one unit (for example, a person or a product).