C Interview Questions

C Interview Questions with Answers

This page has common C interview questions, with short answers and code examples. The questions go from the basics to pointers and memory management, which almost every C interview includes.

Each answer links to the tutorial chapter where you can learn more. At the end of the page, you can test yourself with an interview quiz.

C basics
Data types and operators
Conditions and loops
Arrays and strings
Pointers
Functions
Memory management
Structures and more

C Basics

1. What does #include do?

Answer:

#include tells the preprocessor to copy the contents of a header file into your program, before it is compiled.
Use angle brackets for the standard library, and double quotes for your own header files:

#include <stdio.h>      // A standard library header
#include "myfunctions.h"  // Your own header file

Learn more: C Organize Code

2. What does main() return, and why?

Answer:

main() returns an int to the operating system when the program ends.
0 means that the program finished with no errors. Any other value means that something went wrong.

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

Learn more: C Syntax

3. What is the difference between a declaration and a definition?

Answer:

A declaration tells the compiler that something exists, and what type it has.
A definition creates it. For a function, the definition is the body with the code:

int add(int a, int b);    // Declaration

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

Learn more: C Function Declaration

Data Types and Operators

4. What are the basic data types in C?

Answer:

int for whole numbers, float and double for decimal numbers, and char for single characters.
The size of each type depends on the system. On most computers, int is 4 bytes, float is 4 bytes, double is 8 bytes and char is 1 byte.

Learn more: C Data Types

5. What is the difference between float and double?

Answer:

double is more precise. It stores about 15 digits correctly, while float stores about 6-7 digits.
double usually uses 8 bytes, and float uses 4. For most calculations, double is the safer choice.

Learn more: C Numbers

6. What is the difference between signed and unsigned?

Answer:

A signed type can store negative and positive numbers. An unsigned type can only store zero and positive numbers.
Because it does not need room for negative numbers, an unsigned type can store bigger positive numbers. For example, a 4-byte unsigned int goes from 0 to 4294967295.

Learn more: C Extended Types

7. What does sizeof do?

Answer:

sizeof gives the size of a type or a variable, in bytes.
It is an operator, not a function, and the size is usually known already when the program is compiled.

int numbers[5];
printf("%zu\n", sizeof(int));      // 4 (on most systems)
printf("%zu\n", sizeof(numbers));  // 20 (5 x 4 bytes)

Learn more: C Memory Size

8. What is the difference between i++ and ++i?

Answer:

Both add 1 to i. The difference shows when you use the value in the same statement.
i++ (postfix) uses the current value first, and then adds 1. ++i (prefix) adds 1 first, and then uses the new value:

int i = 5;
int a = i++;  // a is 5, i is 6

int j = 5;
int b = ++j;  // b is 6, j is 6

Learn more: C Arithmetic Operators

9. What does this code print?

printf("%d\n", 7 / 2);

Answer:

It prints 3, not 3.5. When you divide two integers, the result is also an integer, and the decimals are removed.
To get 3.5, use at least one decimal number: 7.0 / 2, and print it with %f.

Learn more: C Arithmetic Operators

10. What is the difference between = and ==?

Answer:

= gives a variable a value. == checks if two values are equal.
Mixing them up is a common bug. if (x = 5) changes x to 5, and the condition is always true.

Learn more: C Comparison Operators

11. What is the difference between const and #define?

Answer:

const creates a variable that cannot be changed. It has a type, and the compiler checks how you use it.
#define creates a macro. The preprocessor replaces the name with the value everywhere in the code, before compiling. It has no type.

const int MAX_USERS = 100;
#define MAX_SCORE 100

Learn more: C Constants and C Macros

Conditions and Loops

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

Answer:

A while loop checks the condition first, so it may not run at all.
A do...while loop runs the code first, and checks the condition after. So it always runs at least one time.

Learn more: C Do/While Loop

13. What is the difference between break and continue?

Answer:

break stops the loop completely.
continue skips the rest of the current round, and goes on with the next one.

Learn more: C Break and Continue

14. What happens if you forget break in a switch?

Answer:

The program goes on and runs the code in the next case too. This is called fall-through:

int day = 1;

switch (day) {
  case 1:
    printf("Monday\n");
  case 2:
    printf("Tuesday\n");
    break;
}
// Prints both Monday and Tuesday

Learn more: C Switch

Arrays and Strings

15. How are strings stored in C?

Answer:

C has no string type. A string is an array of characters, which ends with the null character \0.
That is why "Hello" uses 6 characters of memory, not 5.

Learn more: C The Null Character

16. What is the difference between sizeof and strlen()?

Answer:

sizeof gives the size of the whole array, in bytes. strlen() counts the characters in the string, until the \0:

char name[20] = "Liam";
printf("%zu\n", sizeof(name));  // 20
printf("%zu\n", strlen(name));  // 4

Learn more: C String Functions

17. Why can you not compare strings with ==?

Answer:

== compares the addresses of the two strings in memory, not the characters in them.
Use strcmp() instead. It returns 0 when the two strings are equal:

char a[] = "Hello";
char b[] = "Hello";

if (strcmp(a, b) == 0) {
  printf("The strings are equal\n");
}

Learn more: C String Functions

18. How do you find the number of elements in an array?

Answer:

Divide the size of the whole array by the size of one element:

int numbers[] = {10, 20, 30, 40};
int length = sizeof(numbers) / sizeof(numbers[0]);  // 4

This does not work inside a function that gets the array as a parameter, because the function only gets a pointer to the array. That is why you often pass the length as an extra parameter.

Learn more: C Array Size

19. What happens if you use an index outside an array?

Answer:

C does not check the index. The program reads or writes memory that does not belong to the array.
This is undefined behavior: the program may print a wrong value, change other variables, or crash. Writing outside an array is called a buffer overflow.

Learn more: C Memory Mistakes

Pointers

20. What is a pointer?

Answer:

A pointer is a variable that stores the memory address of another variable.
& gives the address of a variable, and * gives the value at an address (this is called dereferencing):

int age = 20;
int *ptr = &age;   // ptr stores the address of age

printf("%d\n", *ptr);  // 20

Learn more: C Pointers

21. What is a NULL pointer, and why should you check for it?

Answer:

A NULL pointer does not point to anything.
Functions like malloc() and fopen() return NULL when they fail. Using a NULL pointer to read or change a value crashes the program, so always check first:

int *ptr = malloc(sizeof(*ptr));

if (ptr == NULL) {
  printf("Not enough memory\n");
  return 1;
}

Learn more: C NULL

22. How are arrays and pointers related?

Answer:

In most cases, the name of an array works as a pointer to its first element. numbers[i] is the same as *(numbers + i).
But an array is not a pointer: sizeof gives the size of the whole array, and you cannot give an array a new address.

Learn more: C Pointers and Arrays

23. What is pointer arithmetic?

Answer:

When you add 1 to a pointer, it moves to the next element, not to the next byte. For an int pointer, that is usually 4 bytes further:

int numbers[3] = {10, 20, 30};
int *ptr = numbers;

printf("%d\n", *(ptr + 1));  // 20

Learn more: C Pointer Arithmetic

24. What is a void pointer?

Answer:

A void * pointer can store the address of any type. For example, malloc() returns a void *.
You cannot use * on a void * directly, because C does not know what type it points to. Store it in a pointer of the right type first.

25. What is a function pointer?

Answer:

A function pointer stores the address of a function, so you can call the function through the pointer.
It is often used for callbacks, where you pass a function to another function (like qsort()):

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

int (*operation)(int, int) = add;
printf("%d\n", operation(2, 3));  // 5

Learn more: C Function Pointers

Functions

26. What is the difference between pass by value and pass by reference?

Answer:

C always passes a copy of the value to a function (pass by value). If the function changes the copy, the original variable does not change.
To let a function change a variable, pass its address instead. This is called pass by reference:

void swap(int *a, int *b) {
  int temp = *a;
  *a = *b;
  *b = temp;
}

int x = 1;
int y = 2;
swap(&x, &y);  // Now x is 2 and y is 1

Learn more: C Pass By Reference

27. What is recursion?

Answer:

Recursion is when a function calls itself.
A recursive function needs a base case, a condition where it stops calling itself. Without it, the function never stops, and the program crashes:

int factorial(int n) {
  if (n <= 1) {
    return 1;  // Base case
  }
  return n * factorial(n - 1);
}

Learn more: C Recursion

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

Answer:

A local variable is created inside a function. It can only be used there, and it is removed when the function ends.
A global variable is created outside all functions. It can be used everywhere in the file, and it exists while the program runs.

Learn more: C Scope

29. What does the static keyword do?

Answer:

For a local variable, static keeps the value between function calls:

void count() {
  static int calls = 0;
  calls++;
  printf("%d\n", calls);
}
// Calling count() three times prints 1, 2 and 3

For a global variable or a function, static means that it can only be used in the file where it is created.

Learn more: C Storage Classes

Memory Management

30. What is the difference between malloc() and calloc()?

Answer:

malloc() takes the number of bytes to allocate. The memory is not cleared, so it can contain random values.
calloc() takes the number of elements and the size of each element, and sets all the memory to zero:

int *a = malloc(5 * sizeof(int));  // 5 ints, random values
int *b = calloc(5, sizeof(int));   // 5 ints, all set to 0

Learn more: C Allocate Memory

31. What does realloc() do?

Answer:

realloc() changes the size of memory that you allocated before. It keeps the old values, but it may move them to a new address.
If it fails, it returns NULL, and the old memory is not freed. So store the result in a new pointer first, and check it for NULL.

Learn more: C Reallocate Memory

32. What is a memory leak?

Answer:

A memory leak happens when you allocate memory, but never free it. For example, when the pointer to the memory is lost before you call free().
The program then uses more and more memory, which can make it slow, or make it run out of memory.

Learn more: C Deallocate Memory

33. What is a dangling pointer?

Answer:

A dangling pointer points to memory that is no longer valid, for example after free(), or to a local variable of a function that has ended.
To avoid it, set the pointer to NULL after you free the memory:

free(ptr);
ptr = NULL;

Learn more: C Memory Mistakes

34. What is undefined behavior?

Answer:

Undefined behavior is code where the C language does not say what should happen. The result cannot be predicted: the program may seem to work, print wrong values, or crash.
Examples are using an index outside an array, using memory after free(), and dividing an integer by zero.

Learn more: C Memory Mistakes

Structures and More

35. What is the difference between a struct and a union?

Answer:

In a struct, every member has its own memory, so you can use all of them at the same time.
In a union, all members share the same memory. The union is as big as its biggest member, and only one member can hold a value at a time.

Learn more: C Unions

36. What is the difference between . and ->?

Answer:

Use . to access a member of a struct variable. Use -> to access a member through a pointer to a struct:

struct Person p1 = {"Liam", 20};
struct Person *ptr = &p1;

printf("%d\n", p1.age);    // 20
printf("%d\n", ptr->age);  // 20, the same as (*ptr).age

Learn more: C Structs and Pointers

37. What is typedef used for?

Answer:

typedef gives a type a new name. It is often used to make struct types shorter to write:

typedef struct {
  int x;
  int y;
} Point;

Point p1 = {3, 4};  // No need to write struct

Learn more: C typedef

38. What are header guards?

Answer:

Header guards stop a header file from being included more than once in the same file, which would cause errors:

#ifndef MYFUNCTIONS_H
#define MYFUNCTIONS_H

int add(int a, int b);

#endif

Learn more: C Organize Code

39. What is the difference between a macro and a function?

Answer:

A macro is text that the preprocessor replaces before compiling. It has no type checking, and it can give unexpected results.
A function is real code, and the compiler checks the types of its parameters:

#define SQUARE(x) x * x

printf("%d\n", SQUARE(2 + 3));  // 11, not 25: 2 + 3 * 2 + 3

To avoid this, put parentheses around the parameters in a macro: ((x) * (x)).

Learn more: C Macros

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