C Interview Question and Answers (Part-A)

Following is the list of some commonly asked questions in C programming language interviews along with their answers and relevant code:

What is the difference between malloc and calloc?

• malloc: It allocates a specified number of bytes of memory but does not initialize the memory. The initial values are unpredictable.
• calloc: It allocates a specified number of blocks of memory, each of a specified size, and initializes all bytes to zero.

   // Example using malloc
   int *arr = (int *)malloc(5 * sizeof(int));

   // Example using calloc
   int *arr = (int *)calloc(5, sizeof(int));

What is a pointer in C?

A pointer is a variable that stores the memory address of another variable. It allows indirect access to the value of the variable it points to.

   int x = 10;
   int *ptr = &x; // ptr stores the address of variable x

What is the output of the following code?

   #include <stdio.h>
   int main() {
       char str[] = "Hello";
       printf("%c\n", str[5]);
       return 0;
   }

The code tries to access the 6th character of the string “Hello,” which is the null character '\0'. The output will be an empty line.

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

• ++i: Pre-increment increments the value of i before using it in an expression.
• i++: post-increment increments the value of i after using it in an expression.

   int i = 5;
   int a = ++i; // a = 6, i = 6
   int b = i++; // b = 6, i = 7

Explain the concept of recursion. Provide an example.

Recursion is a programming technique where a function calls itself to solve a problem. It requires a base case to terminate the recursion.

   int factorial(int n) {
       if (n == 0 || n == 1)
           return 1;
       else
           return n * factorial(n - 1);
   }

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

Explain the const keyword in C.

• const is used to define constants or make variables read-only. A const variable cannot be modified once it’s assigned a value.

   const int x = 10; // x is a constant with a value of 10
   int y = 20;
   const int *ptr = &y; // ptr is a pointer to a constant int (can't modify y)
   int const *ptr = &y; // Same as above, pointer to a constant int
   int *const ptr = &y; // ptr is a constant pointer to int (can't point to another address)

What is the output of the following code?

   #include <stdio.h>
   int main() {
       int x = 10;
       int *ptr = &x;
       *ptr = 20;
       printf("%d\n", x);
       return 0;
   }

The output will be 20. *ptr is dereferencing the pointer and updating the value of x.

What is the sizeof operator used for?

• The sizeof operator returns the size in bytes of a data type or a variable.

   int x = 10;
   printf("Size of int: %zu\n", sizeof(int));
   printf("Size of x: %zu\n", sizeof(x));

Please check Part-B for more question and answers.