Prepare for your next C programming interview with these 30 essential questions covering basic, intermediate, and advanced topics. Whether you’re a fresher, have 1-3 years of experience, or are a seasoned professional with 3-6 years, this guide helps you master core C concepts, pointers, memory management, and practical coding scenarios.
Basic C Interview Questions (1-10)
1. What is C and why is it called a middle-level language?
C is a procedural programming language that combines low-level memory access with high-level abstractions, allowing direct hardware manipulation while providing structured programming features.[1][6]
2. What are the basic data types in C?
C supports primary data types: int, char, float, double, and void. Derived types include arrays, pointers, structures, and unions.[3][5]
3. What is the difference between local and global variables?
Local variables are declared inside a function and have block scope, while global variables are declared outside functions and are accessible throughout the program.[5]
4. Explain the structure of a basic C program.
A C program starts with preprocessor directives (#include), followed by global declarations, the main() function containing declarations, statements, and sub-functions.[5]
#include <stdio.h>
int main() {
printf("Hello, World!\n");
return 0;
}
5. What is the purpose of the main() function?
main() is the entry point of every C program. Execution begins here, and it returns an integer value (0 for success) to the operating system.[5]
6. Differentiate between == and = operators.
= is the assignment operator that stores a value, while == is the equality operator that compares two values for equality.[1]
7. What are constants in C? How do you declare them?
Constants are fixed values that cannot change. Use the const keyword: const int MAX = 100;[3]
8. What is a preprocessor directive?
Preprocessor directives like #include, #define, and #ifdef are processed before compilation to include files, define macros, and handle conditional compilation.[2][3]
9. What does #include <stdio.h> do?
It includes the standard input/output header file, providing functions like printf(), scanf(), getchar(), and putchar().[5]
10. What is the difference between break and continue?
break exits the loop entirely, while continue skips the current iteration and proceeds to the next.[3]
Intermediate C Interview Questions (11-20)
11. What are arrays in C? How do you declare a 2D array?
Arrays store multiple values of the same type. A 2D array declaration: int matrix[3][4]; represents 3 rows and 4 columns.[1]
int arr[5] = {1, 2, 3, 4, 5};
printf("%d", arr[0]); // Outputs 1
12. Explain strings in C.
Strings are character arrays ending with ‘\0’. Declare as: char str[50] = "Hello";[1]
13. What is a pointer? Write a simple pointer example.
A pointer stores the memory address of another variable. Use * for declaration and & for address.[1][3]
int x = 10;
int *ptr = &x;
printf("%d", *ptr); // Outputs 10 (dereference)
14. What is the difference between malloc() and calloc()?
malloc() allocates uninitialized memory, while calloc() allocates memory initialized to zero.[3]
15. How do you reverse a string in C at Flipkart?
Use two pointers or a temporary array to swap characters from start and end until they meet.
#include <string.h>
void reverse(char *str) {
int len = strlen(str);
for(int i = 0; i < len/2; i++) {
char temp = str[i];
str[i] = str[len-1-i];
str[len-1-i] = temp;
}
}
16. What are storage class specifiers?
auto, register, static, extern control variable scope, lifetime, and storage location.[1]
17. Explain recursion with an example for factorial.
Recursion is a function calling itself. Base case prevents infinite calls.[3]
int factorial(int n) {
if(n <= 1) return 1;
return n * factorial(n-1);
}
18. What is typecasting? Provide an example.
Typecasting converts one data type to another: int num = (int)3.14;[3]
19. How do you find the size of a variable using sizeof?
sizeof operator returns the memory size: sizeof(int) typically returns 4 bytes.[2]
20. Implement strlen() without using library functions for Zoho interviews.
Iterate through characters until ‘\0’ is encountered.
int my_strlen(char *str) {
int len = 0;
while(str[len] != '\0') len++;
return len;
}
Advanced C Interview Questions (21-30)
21. What is a function pointer? Give an example.
Function pointer stores the address of a function: int (*fptr)(int) = myfunc;[1]
int add(int a, int b) { return a+b; }
int main() {
int (*op)(int,int) = add;
printf("%d", op(2,3)); // 5
}
22. Explain structure padding and how to avoid it.
Padding adds unused bytes for alignment. Use #pragma pack(1) or reorder members.[1]
23. What are bit fields in structures?
Bit fields allocate specific bits: struct { int flag:1; int value:7; };[3]
24. Differentiate stack and heap memory.
Stack is automatic and fast but limited; heap is dynamic via malloc/free but manual management.[2]
25. Write a program to count set bits in Paytm scenarios.
Use bitwise AND with shifting or Brian Kernighan’s algorithm.[2]
int count_set_bits(int n) {
int count = 0;
while(n) {
n &= (n-1);
count++;
}
return count;
}
26. What is a memory leak and how to prevent it?
Memory leak occurs when allocated memory isn’t freed. Always pair malloc with free.[1]
27. Implement binary search for Salesforce coding rounds.
Divide sorted array mid-point repeatedly.
int binary_search(int arr[], int size, int key) {
int left = 0, right = size-1;
while(left <= right) {
int mid = left + (right-left)/2;
if(arr[mid] == key) return mid;
if(arr[mid] < key) left = mid+1;
else right = mid-1;
}
return -1;
}
28. What is the volatile keyword?
volatile prevents compiler optimization for variables that may change unexpectedly, like hardware registers.[1]
29. Explain lvalue and rvalue for Oracle technical rounds.
lvalue has address (assignable), rvalue is a value (not assignable).[1]
30. How do you handle file operations in C for SAP applications?
Use fopen(), fread(), fwrite(), fclose(). Check file pointers for NULL.[1]
FILE *fp = fopen("file.txt", "r");
if(fp == NULL) { /* error */ }
fclose(fp);
