INTERMEDIATE-LEVEL C INTERVIEW QUESTIONS
1. What is C language and why is it widely used?
C is a procedural, structured programming language used for system-level programming (OS kernels, compilers).
It is popular because it is fast, portable, and close to hardware.
2. What is a compiler?
A compiler converts human-readable C code → machine code.
3. Difference between variables and constants?
| Variables | Constants |
|---|---|
| Value can change | Value cannot change |
| Declared with type | Declared with const |
4. What are data types in C? Explain primary types.
Primary types: int, float, double, char, void.
They define the type & size of stored data.
5. What is the use of printf() and scanf()?
printf() → prints outputscanf() → takes input
6. What is a keyword? List any five.
Reserved words used by compiler.
Examples: int, return, if, while, switch.
7. Difference between = and ==?
= → assignment== → comparison
8. What is an operator? Types of operators?
Symbols that perform operations.
Types: Arithmetic, Logical, Relational, Bitwise, Assignment.
9. What is type casting?
Converting one data type into another.
Example:
float x = (float)5;
10. What are loops?
Used to repeat code.
Types: for, while, do-while.
11. Difference between break and continue.
break → exits loopcontinue → skips iteration, continues loop
12. What is an array?
A collection of elements of same type in continuous memory.
13. How do you declare an array?
14. What is a string in C?
A string is a char array terminated with \0.
15. What does sizeof() do?
Returns memory size of variable/type.
16. What are functions? Why used?
Functions are reusable code blocks for modularity.
17. Difference between call by value and call by reference.
Call by value → copy passed
Call by reference → address passed using pointer
18. What are header files?
Files that contain declarations, e.g., stdio.h.
19. What is the purpose of main()?
Entry point of every C program.
20. How do comments work in C?
Single-line: //
Multi-line: /* ... */
21. Why semicolons are required in C?
Mark the end of a statement.
22. What is an infinite loop?
A loop that runs forever.
Example:
23. What is the return statement?
Ends function & optionally returns value.
24. What is a pointer?
Variable that stores memory address of another variable.
25. What is a null pointer?
A pointer that points to nothing (value = NULL).
INTERMEDIATE-LEVEL C INTERVIEW QUESTIONS
26. What are storage classes?
Define the scope, lifetime, and visibility of variables.
Types: auto, static, extern, register.
27. Difference between auto, static, extern?
auto: Local (default)static: Retains value between function callsextern: Declares global variable from another file
28. What is recursion?
Function calling itself.
Used for factorial, Fibonacci, tree traversal.
29. What is pointer arithmetic?
Operations allowed: ++, --, +, -.
Pointer moves based on data size.
30. Array of pointers vs pointer to array.
Array of pointers:
int *arr[5];Pointer to array:
int (*ptr)[5];
31. What is a structure?
User-defined datatype to group mixed data types.
32. Difference between structure and union.
In a union, only one member can keep value at a time.
Structures store all members simultaneously.
33. What is dynamic memory allocation?
Allocating memory at runtime using:malloc(), calloc(), realloc(), free().
34. Explain malloc(), calloc(), realloc(), free().
malloc()→ uninitialized memorycalloc()→ zero-initialized memoryrealloc()→ resize memoryfree()→ release memory
35. What is dangling pointer?
Pointer pointing to freed memory.
36. What is segmentation fault?
Accessing unauthorized memory.
37. What is a function pointer?
Pointer pointing to a function.
Example:
38. Explain pass-by-reference using pointers.
Instead of value, address is passed → function modifies original variable.
39. What are macros?
Preprocessor replacements defined using #define.
40. What is preprocessor?
Handles directives like #include, #define, #ifdef.
41. What is typedef?
Creates alias names for types.
42. Explain file handling in C.
Use of functions:fopen(), fclose(), fprintf(), fscanf(), fgets().
43. Difference between text mode and binary mode.
Text mode: characters
Binary mode: raw bytes
44. What is pointer-to-pointer?
A pointer that stores address of another pointer.
Example:
45. Why can’t we return arrays from a function?
Arrays cannot be returned by value; instead return pointer.
46. Difference between const char*, char const*, char* const?
const char*→ constant datachar const*→ constant data (same as above)char* const→ constant pointer
47. What is an enum?
User-defined numeric constants.
48. Can we use break in nested loops?
Yes, but it breaks only one loop.
49. Use of volatile keyword?
Prevents compiler optimization for variables that change unexpectedly (e.g., hardware registers).
50. What is a memory leak?
Allocated memory not released via free().
ADVANCED-LEVEL C INTERVIEW QUESTIONS
51. Explain stack vs heap memory.
| Stack | Heap |
|---|---|
| Automatic memory | Dynamic memory |
| Fast | Slower |
| Managed by compiler | Managed by programmer |
| Limited size | Large size |
52. How does the C compiler translate code?
Steps:
Preprocessing
Compilation
Assembly
Linking
53. What is undefined behavior?
Code that the C standard does not define.
Example:
54. What are sequence points?
Points where all previous operations complete (e.g., end of expression, &&, ||).
55. Inline function vs macro?
Inline → type-safe, checked by compiler
Macro → text substitution, no type checking
56. What is pointer aliasing?
Multiple pointers referencing same memory.
57. Memory alignment and padding in structures?
Compiler adds padding to align data in memory for faster access.
58. Little endian vs big endian?
Little → LSB stored first
Big → MSB stored first
59. How does function calling work internally?
Stack frame stores:
return address
parameters
local variables
saved registers
60. Explain bitwise operations.
Operations on bits: AND, OR, XOR, NOT, shifts.
61. What are bit fields?
Allow storing multiple tiny values within a single struct.
62. Deep copy vs shallow copy?
Shallow copy → copies pointer, not data
Deep copy → copies actual data
63. Static vs dynamic binding?
C supports only static binding (compile-time).
64. What is race condition?
Two threads accessing shared data without synchronization.
65. What are setjmp() and longjmp()?
Used for non-local jumps (error handling).
66. Why is volatile important in embedded systems?
Used for hardware-mapped registers that change asynchronously.
67. How to prevent buffer overflow?
Use safe functions:fgets(), snprintf(), bounds checking.
68. Difference between fork() and threads?
fork() → new process
Thread → lightweight execution inside same process
69. How does memcpy() work internally?
Copies memory byte-by-byte using pointer iteration.
70. How does linker resolve external symbols?
Matches undefined symbols with definitions across object files/libraries.
71. Stack overflow vs heap overflow?
Stack overflow → too many function calls
Heap overflow → buffer overflow in dynamic memory
72. How are const pointers stored?
Const affects pointer or data, but storage is same in memory.
73. What is memory fragmentation?
Free memory split into small pieces → allocation failure.
74. How does C handle function overloading?
C does NOT support function overloading.
75. Explain the entire compilation process.
Preprocessor expands macros
Compiler converts code → assembly
Assembler → object file
Linker → final executable