C Programming Interview Questions (2025)

As we step into 2025, C programming continues to be a cornerstone language in both academic learning and industry applications. Its low-level access to memory and system resources, coupled with its efficiency, make it an essential language in fields like system programming, embedded systems, and performance-critical applications. Whether you’re a fresh graduate or a seasoned developer, preparing for a C programming interview requires a solid understanding of fundamental concepts as well as practical problem-solving skills.

In this blog post, we’ll walk through some of the top C programming interview questions that you should be prepared for in 2025. These questions cover a range of topics from basic syntax to more advanced memory management and optimization techniques.

1. What are the key differences between malloc() and calloc()?

Both malloc() and calloc() are used to allocate memory dynamically, but there are crucial differences between them:

• malloc() (Memory Allocation): Allocates a block of memory of a specified size but does not initialize it. The content of the allocated memory is indeterminate.

  Syntax:

  void *malloc(size_t size);
  

• calloc() (Contiguous Allocation): Allocates memory for an array of elements and initializes the memory to zero.

  Syntax:

  void *calloc(size_t num, size_t size);
  

Example:

int *arr1 = malloc(10 * sizeof(int)); // Uninitialized memory for 10 integers
int *arr2 = calloc(10, sizeof(int));  // Zero-initialized memory for 10 integers

2. Explain the difference between ++i and i++.

Both ++i and i++ are increment operators, but they differ in terms of when the increment happens in relation to the expression they are used in.

• ++i: Pre-increment. The value of i is incremented before it is used in the expression.
• i++: Post-increment. The value of i is used first, and then it is incremented.

Example:

int i = 5;
printf("%d", ++i);  // Outputs 6 (increments first)
printf("%d", i++);  // Outputs 6, then i becomes 7 after this statement

3. What is a pointer in C, and how does it work?

A pointer in C is a variable that stores the memory address of another variable. Pointers are essential for dynamic memory management, passing large data structures to functions, and manipulating memory directly.

Syntax:

type *pointer_name;

You can access or modify the value stored at the memory address using the dereference operator (*), while the address of a variable is obtained using the address-of operator (&).

Example:

int num = 10;
int *ptr = #  // ptr points to the address of num
printf("%d", *ptr); // Dereferencing ptr gives the value of num (10)

4. What is a segmentation fault, and how can you avoid it?

A segmentation fault (segfault) occurs when a program attempts to access memory it is not allowed to, such as accessing null or uninitialized pointers, or reading/writing beyond array bounds.

Common causes:

• Dereferencing a null pointer.
• Accessing memory outside the allocated range.
• Buffer overflows or writing to read-only memory.

Prevention:

• Always initialize pointers before use.
• Perform bounds checking for arrays.
• Ensure proper memory allocation and deallocation.

Example:

int *ptr = NULL;
*ptr = 10;  // This will cause a segmentation fault

5. Explain the difference between struct and union in C.

Both struct and union are used to store multiple data types in a single variable, but they differ in how memory is allocated.

• struct: Each member has its own memory space. The total size of the struct is the sum of the sizes of its members.
• union: All members share the same memory space, and the total size is determined by the largest member.

Example:

struct MyStruct {
    int x;
    float y;
};

union MyUnion {
    int x;
    float y;
};

6. What is the significance of the static keyword in C?

The static keyword in C has two primary uses:

1. Inside a function: A static variable retains its value between function calls, i.e., it is initialized only once and keeps its value throughout the program’s execution.
2. At the global level: It restricts the visibility of the variable or function to the file in which it is declared, making it inaccessible from other files.

Example:

void counter() {
    static int count = 0; // Retains value between calls
    count++;
    printf("%d\n", count);
}

int main() {
    counter(); // Outputs 1
    counter(); // Outputs 2
    counter(); // Outputs 3
    return 0;
}

7. What are function pointers in C, and how are they used?

Function pointers are pointers that point to functions instead of variables. They allow for dynamic function calls and can be useful in cases like implementing callback functions or designing state machines.

Syntax:

return_type (*function_ptr)(parameter_list);

Example:

#include <stdio.h>

void greet() {
    printf("Hello, World!\n");
}

int main() {
    void (*func_ptr)() = greet;
    func_ptr(); // Calls the greet function
    return 0;
}

8. What is the difference between == and = in C?

• ==: The equality operator, used to compare two values or expressions for equality.
• =: The assignment operator, used to assign a value to a variable.

Example:

int x = 10;  // Assignment
if (x == 10) {  // Comparison
    printf("x is 10\n");
}

9. What is the const keyword used for in C?

The const keyword is used to define constant variables whose values cannot be modified once they are initialized. It can be applied to both variables and pointers.

Example:

const int x = 10;
x = 20;  // Error: cannot modify a constant variable

const int *ptr = &x;  // Pointer to constant integer (the integer cannot be changed through the pointer)

10. What is recursion in C? Provide an example.

Recursion is a programming technique where a function calls itself directly or indirectly to solve a problem. It is particularly useful for problems that can be broken down into smaller subproblems.

Example:

#include <stdio.h>

int factorial(int n) {
    if (n == 0) return 1;  // Base case
    return n * factorial(n - 1);  // Recursive call
}

int main() {
    int num = 5;
    printf("Factorial of %d is %d\n", num, factorial(num));
    return 0;
}

Conclusion

Mastering the fundamental concepts of C programming is critical to performing well in technical interviews. By understanding key concepts such as memory allocation, pointers, recursion, and function pointers, you will be able to handle the most commonly asked interview questions in 2025 and beyond. Additionally, practicing coding problems and reviewing complex topics will sharpen your problem-solving abilities, making you better equipped to tackle real-world programming challenges.

Good luck with your C programming journey and your upcoming interviews!