C Interview Questions

C Interview Questions – Prepare for C Programming Interviews

In this tutorial, we will cover some of the most commonly asked C programming interview questions. Whether you are a beginner or an experienced programmer, these questions will help you brush up on key concepts and prepare for your next interview.

Question 1: What is the difference between C and C++?

Answer: C is a procedural programming language, while C++ is an object-oriented programming language. C focuses on functions and procedures, whereas C++ supports classes and objects for data abstraction, encapsulation, inheritance, and polymorphism.

Question 2: What is a pointer in C? How does it work?

Answer: A pointer is a variable that stores the memory address of another variable. In C, pointers are used to dynamically allocate memory and pass values by reference. To declare a pointer, we use the `*` symbol.

Example: Pointer Usage

#include <stdio.h>

int main() {
    int num = 10;
    int *ptr = &num;  // Pointer to num
    
    printf("Value of num: %d\n", *ptr);  // Dereferencing pointer to get value
    
    return 0;
}

Question 3: What is the use of the `static` keyword in C?

Answer: The `static` keyword in C is used to maintain the value of a variable between function calls. It can be used for both local and global variables. When applied to a local variable, it preserves its value even after the function has returned.

Question 4: What is the difference between `malloc()` and `calloc()`?

Answer: Both `malloc()` and `calloc()` are used to dynamically allocate memory. The difference is that `malloc()` allocates a block of memory of a given size without initializing it, while `calloc()` allocates memory for an array of elements and initializes all bytes to zero.

Example: Using `malloc()` and `calloc()`

#include <stdio.h>
#include <stdlib.h>

int main() {
    int *arr_malloc, *arr_calloc;
    int i;
    
    // Using malloc
    arr_malloc = (int *)malloc(5 * sizeof(int));
    for (i = 0; i < 5; i++) {
        arr_malloc[i] = i + 1;  // Initialize manually
    }

    // Using calloc
    arr_calloc = (int *)calloc(5, sizeof(int));  // Automatically initialized to 0
    
    printf("Array allocated with malloc: ");
    for (i = 0; i < 5; i++) {
        printf("%d ", arr_malloc[i]);
    }
    printf("\n");
    
    printf("Array allocated with calloc: ");
    for (i = 0; i < 5; i++) {
        printf("%d ", arr_calloc[i]);
    }
    printf("\n");

    free(arr_malloc);  // Don't forget to free the memory
    free(arr_calloc);  // Don't forget to free the memory
    
    return 0;
}

Question 5: Explain the concept of a memory leak in C.

Answer: A memory leak occurs when a program allocates memory dynamically (using `malloc()` or `calloc()`), but fails to release it (using `free()`) after it is no longer needed. This causes the program to consume more and more memory, which can eventually lead to a crash or slowdown.

Question 6: What is the difference between `==` and `=` in C?

Answer: `==` is the equality operator, used to compare two values, while `=` is the assignment operator, used to assign a value to a variable.

Example: `==` vs `=`

#include <stdio.h>

int main() {
    int x = 5;
    int y = 10;
    
    if (x == y) {
        printf("x and y are equal.\n");
    } else {
        printf("x and y are not equal.\n");
    }
    
    x = 20;  // Assigning a new value to x
    
    printf("New value of x: %d\n", x);
    
    return 0;
}

Question 7: What is the purpose of `#define` in C?

Answer: The `#define` directive is used to define constants and macros in C. It allows you to create symbolic names for constant values, making the code more readable and easier to maintain.

Question 8: What is the difference between `struct` and `union`?

Answer: A `struct` is a user-defined data type in C that allows grouping of different types of variables. Each member of a `struct` has its own memory location. On the other hand, a `union` is similar to a `struct` but allows storing different data types in the same memory location. Only one member of a `union` can hold a value at any given time.

Question 9: What are function pointers in C?

Answer: A function pointer is a pointer that points to the address of a function. It allows functions to be passed as arguments, enabling dynamic behavior.

Example: Function Pointer

#include <stdio.h>

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

int main() {
    void (*function_ptr)() = &say_hello;  // Function pointer
    
    // Calling function using pointer
    function_ptr();
    
    return 0;
}

Question 10: What is the use of `void` pointer in C?

Answer: A `void` pointer is a special type of pointer in C that can point to any data type. It is used for creating generic functions, where the type of the data is unknown at the time of writing the function.

Conclusion

These are some of the most frequently asked C programming interview questions. Prepare well by practicing these concepts and examples, and you’ll be ready for your next C programming interview!