Top 25 Interview Questions on C Language for 2025: A Complete Guide
The C programming language continues to be one of the most relevant and powerful languages in software development. Despite the rise of modern programming languages, C remains a vital skill for developers and is widely tested in technical interviews, especially for roles involving low-level programming, systems programming, and embedded systems.
As we move into 2025, it’s crucial to stay up-to-date with the latest trends in C programming and its real-world applications. To help you prepare for your next interview, we’ve compiled the top 50 C language interview questions for 2025. Whether you’re a beginner or an experienced developer, these questions cover a broad range of topics and will ensure you are fully prepared for any interview scenario.
1. What is C language and why is it important?
Answer: C is a procedural programming language developed by Dennis Ritchie in the 1970s. It’s widely used for system programming, developing operating systems, and embedded systems because of its efficiency, portability, and ability to interact directly with hardware.
2. What is the difference between C and C++?
Answer: C is a procedural programming language, while C++ is a multi-paradigm language that supports both procedural and object-oriented programming. C does not support classes, objects, and inheritance, which are integral to C++. C++ also includes features like templates and exception handling.
3. What are the fundamental data types in C?
Answer: The fundamental data types in C are:
int: Integer data type.
char: Character type, stores a single character.
float: Stores single-precision floating-point numbers.
double: Stores double-precision floating-point numbers.
void: Represents an incomplete type, typically used for functions that don’t return a value.
4. What is a pointer in C?
Answer: A pointer is a variable that stores the memory address of another variable. It allows direct manipulation of memory and facilitates dynamic memory allocation, among other things.
5. How do you differentiate between a pointer and a reference in C?
Answer: C does not have references like C++. However, pointers and references in C++ are often compared. Pointers can be null, can be reassigned, and can point to different memory locations, while references in C++ are always bound to a variable and cannot be null.
6. What is a void pointer in C?
Answer: A void pointer is a special type of pointer that can point to any data type but cannot be dereferenced directly without casting it to another pointer type.
7. What is a memory leak and how can it be prevented in C?
Answer: A memory leak occurs when dynamically allocated memory is not freed, leading to unused memory being consumed. To prevent memory leaks, always ensure that memory allocated with functions like malloc() or calloc() is freed using free().
8. How does the sizeof() operator work in C?
Answer: The sizeof() operator returns the size, in bytes, of a variable or data type. It is evaluated at compile time, and the result is a constant value.
9. What is the difference between a structure and a union in C?
Answer: A struct allows multiple members to store values simultaneously, while a union allows different data types to share the same memory location. The size of a union is equal to the size of its largest member, while the size of a struct is the sum of the sizes of its members.
10. What are function pointers in C?
Answer: A function pointer is a pointer that points to a function instead of a variable. It can be used to call functions dynamically, especially in cases like callback functions or implementing a table of functions.
11. What are the different storage classes in C?
Answer: The storage classes in C are:
auto: Default for local variables, automatically created and destroyed when the function is entered and exited.
register: Requests the compiler to store the variable in a CPU register.
static: Retains the variable’s value between function calls, initialized only once.
extern: Used to declare variables that are defined outside the current file or function.
12. What is recursion in C?
Answer: Recursion occurs when a function calls itself to solve smaller instances of a problem. It is often used in algorithms like factorials, Fibonacci numbers, and tree traversals.
13. What is the preprocessor in C and how does it work?
Answer: The preprocessor is a part of the compiler that processes code before it is compiled. It handles directives like #include, #define, and #if, allowing for file inclusion, macro expansion, and conditional compilation.
14. What are macros in C and how do they differ from functions?
Answer: Macros are preprocessor directives that allow code substitution before compilation. Unlike functions, macros don’t have type checking, and they are expanded inline, which can lead to issues like repeated evaluation of parameters.
15. What is a static variable in C?
Answer: A static variable retains its value between function calls. It is initialized only once and does not lose its value when the function exits.
16. What is the difference between ++i and i++ in C?
Answer: ++i is a pre-increment operator that increments the value of i before its value is used in an expression. i++ is a post-increment operator that increments the value of i after its current value is used.
17. What are command-line arguments in C?
Answer: Command-line arguments are input values passed to a C program when it is executed from the command line. They are accessible via the parameters argc (argument count) and argv (argument vector) in the main() function.
18. What is the return statement in C?
Answer: The return statement is used to exit from a function and optionally pass a value back to the calling function. It is essential for terminating a function and controlling program flow.
19. What is a null pointer?
Answer: A null pointer is a pointer that does not point to any memory address. It is often used to signify that a pointer is not yet assigned to a valid memory location.
20. What is dynamic memory allocation in C?
Answer: Dynamic memory allocation in C is the process of allocating memory during runtime using functions like malloc(), calloc(), realloc(), and freeing it with free().
21. What is the difference between malloc() and calloc()?
Answer: malloc() allocates uninitialized memory, while calloc() allocates memory and initializes it to zero. Both functions return a pointer to the allocated memory.
22. How does the free() function work in C?
Answer: The free() function deallocates memory previously allocated by malloc(), calloc(), or realloc(). After calling free(), the pointer becomes invalid and should not be dereferenced.
23. What are bitwise operators in C?
Answer: Bitwise operators perform operations at the bit level:
&: AND
|: OR
^: XOR
~: NOT
<<: Left shift
>>: Right shift
24. What is the difference between == and = in C?
Answer: == is a comparison operator that checks if two values are equal, while = is an assignment operator used to assign a value to a variable.
25. What is an array in C?
Answer: An array in C is a collection of variables of the same data type, stored in contiguous memory locations. Arrays are accessed using an index starting from 0.
Conclusion
By preparing for these top 50 C language interview questions for 2025, you’ll not only gain a deeper understanding of core C concepts but also sharpen your skills for the ever-evolving tech landscape. Remember to practice coding and apply the concepts in real-world projects to reinforce your learning.
Stay confident and good luck with your interviews in 2025!