1. What is C?

C is a general-purpose, procedural computer programming language supporting structured programming, lexical variable scope, and recursion. It's often called a "middle-level" language because it combines elements of high-level languages with the functionalism of assembly language.

2. What are the key features of C?

Procedural
Strongly Typed
Portable
Low-level access to memory (via Pointers)
Compact and Efficient
Large Standard Library

3. What's the difference between `main()` and `void main()`?

In Standard C, main() should return an int. int main() is the standard, while void main() is non-standard and should be avoided.

4. What is a "Pointer" in C?

A variable that stores the memory address of another variable. Pointers are one of C's most powerful (and dangerous) features.

5. What are the different data types in C?

Basic: int, char, float, double.
Derived: array, pointer, structure, union.
Enumerated: enum.
Void: void.

6. What is the difference between `NULL` and `0` in the context of pointers?

NULL is a macro (usually defined as (void*)0) representing a null pointer, while 0 is an integer literal. In many contexts, they are interchangeable for pointer initialization.

7. What is "Static" variable in C?

A variable whose lifetime is the entire duration of the program. It retains its value between function calls.

Inside function: Local scope, persistent value.
Outside function: Global scope, restricted to the source file.

8. What is "Register" variable?

A hint to the compiler to store the variable in a CPU register instead of RAM for faster access. (The compiler may ignore this hint).

9. What is "Volatile" variable?

Indicates that a variable's value can change unexpectedly (e.g., by an external hardware device). The compiler will not optimize access to it.

10. What is "Const" variable?

A variable whose value cannot be modified after initialization.

11. What is the difference between `malloc()` and `calloc()`?

malloc(): Allocates a block of memory and doesn't initialize it (contains garbage).
calloc(): Allocates multiple blocks of memory and initializes them to zero.

12. What is `realloc()`?

Used to change the size of a previously allocated memory block.

13. What is `free()`?

Used to release the memory that was previously allocated by malloc, calloc, or realloc.

14. What is a "Memory Leak" in C?

Occurs when memory is allocated (via malloc) but never released (via free), eventually exhausting the available memory.

15. What is a "Dangling Pointer"?

A pointer that points to a memory location that has been freed. Accessing it leads to undefined behavior.

16. What is a "Wild Pointer"?

An uninitialized pointer that points to an arbitrary memory location.

17. What is a "Structure" in C?

A user-defined data type that allows you to group different data types (members) into a single unit.

18. What is a "Union" in C?

Similar to a structure, but all members share the same memory location. The size of the union is the size of its largest member.

19. What is the difference between `Structure` and `Union`?

Structure: Every member has its own memory; can store multiple values.
Union: All members share the same memory; can store only one value at a time.

20. What is a "Typedef"?

Used to create an alias for an existing data type (e.g., typedef unsigned long int ulong).

21. What is "Macro" and how is it defined?

A piece of code that is replaced by the preprocessor before compilation (e.g., #define PI 3.14).

22. What is the difference between a "Macro" and a "Function"?

Macro: Faster (no function call overhead); no type checking; pre-compiled.
Function: Slower; has type checking; compiled.

23. What is "Recursion"?

The process of a function calling itself. It requires a base case to prevent infinite loops.

24. What is "Call by Value" vs "Call by Reference"?

Value: A copy of the value is passed; changes inside the function don't affect the caller.
Reference: The address (pointer) is passed; changes inside the function DO affect the caller.

25. What is the "Sizeof" operator?

Returns the size (in bytes) of a variable or data type.

26. What is an "Array"?

A collection of elements of the same data type stored in contiguous memory locations.

27. What is "String" in C?

A sequence of characters terminated by a null character (\0).

28. What is the difference between `char str[] = "hello"` and `char *str = "hello"`?

char str[]: An array that can be modified.
char *str: A pointer to a string literal (usually stored in read-only memory); cannot be modified.

29. What is "lvalue" and "rvalue"?

lvalue: An expression that refers to a memory location (e.g., a variable name).
rvalue: A value that is stored in some memory location (e.g., a literal).

30. What is "Storage Class" in C?

Determines the visibility, lifetime, and initial value of a variable (auto, extern, static, register).

31. What is the purpose of the `extern` keyword?

Used to declare a global variable that is defined in another source file.

32. What is "Bit Field" in C?

A way to specify the exact number of bits used by a structure member (useful for low-level hardware programming).

33. What is "Function Pointer"?

A pointer that points to the beginning of a function's code in memory.

34. What is a "Callback Function" in C?

A function that is passed as a pointer to another function, which then invokes it.

35. What is "File Handling" in C?

Using functions from the stdio.h library (e.g., fopen, fclose, fread, fwrite) to interact with disk files.

36. What is "Binary File" vs "Text File"?

Text: Stored as a sequence of characters; readable by humans.
Binary: Stored as a sequence of bytes; not directly human-readable.

37. What is "Pre-processor" in C?

The stage of compilation that handles directives starting with # (e.g., #include, #define).

38. What is the difference between `#include <file>` and `#include "file"`?

<file>: Searches in standard system directories.
"file": Searches first in the current directory, then in system directories.

39. What is "Conditional Compilation"?

Using directives like #ifdef, #ifndef, and #endif to compile specific parts of the code only if certain conditions are met.

40. What is "Type Casting" in C?

Manual conversion from one data type to another (e.g., (float)10 / 3).

41. What is "Operator Precedence" and "Associativity"?

Precedence: The order in which different operators are evaluated.
Associativity: The order in which operators of the same precedence are evaluated (left-to-right or right-to-left).

42. What is "Ternary Operator" (`?:`)?

A shorthand way of writing an if-else statement.

43. What is "Enum" in C?

A user-defined data type that consists of a set of named integer constants.

44. What is "Command Line Arguments"?

Arguments passed to the main() function from the terminal (int argc, char *argv[]).

45. What is the difference between `printf()` and `puts()`?

printf(): Versatile; supports formatting; doesn't automatically add a newline.
puts(): Simple; only prints strings; always adds a newline.

46. What is "Dangling Else" problem?

An ambiguity in nested if-else structures where it's unclear which if the else belongs to. In C, it always belongs to the closest preceding if.

47. What is "Short Circuit Evaluation"?

A feature in logical operators (&& and ||) where the second condition is only evaluated if the first one doesn't determine the entire result.

48. What is "Header File" (.h)?

A file containing function declarations, macro definitions, and type declarations to be shared across multiple source files.

49. What is the "Stack" vs "Heap" memory?

Stack: Automatically managed; used for local variables; small and fast.
Heap: Manually managed (via malloc); used for dynamic allocation; larger and slower.

50. What is "Infinite Loop" and how to stop it?

A loop that never ends (e.g., while(1)). It can be stopped using a break statement inside or by terminating the program externally.

C Interview Questions

Table of Contents

What is C?

What are the key features of C?

What's the difference between <code>main()</code> and <code>void main()</code>?

What is a "Pointer" in C?

What are the different data types in C?

What is the difference between <code>NULL</code> and <code>0</code> in the context of pointers?

What is "Static" variable in C?

What is "Register" variable?

What is "Volatile" variable?

What is "Const" variable?

What is the difference between <code>malloc()</code> and <code>calloc()</code>?

What is <code>realloc()</code>?

What is <code>free()</code>?

What is a "Memory Leak" in C?

What is a "Dangling Pointer"?

What is a "Wild Pointer"?

What is a "Structure" in C?

What is a "Union" in C?

What is the difference between <code>Structure</code> and <code>Union</code>?

What is a "Typedef"?

What is "Macro" and how is it defined?

What is the difference between a "Macro" and a "Function"?

What is "Recursion"?

What is "Call by Value" vs "Call by Reference"?

What is the "Sizeof" operator?

What is an "Array"?

What is "String" in C?

What is the difference between <code>char str[] = "hello"</code> and <code>char *str = "hello"</code>?

What is "lvalue" and "rvalue"?

What is "Storage Class" in C?

What is the purpose of the <code>extern</code> keyword?

What is "Bit Field" in C?

What is "Function Pointer"?

What is a "Callback Function" in C?

What is "File Handling" in C?

What is "Binary File" vs "Text File"?

What is "Pre-processor" in C?

What is the difference between <code>#include &#x3C;file></code> and <code>#include "file"</code>?

What is "Conditional Compilation"?

What is "Type Casting" in C?

What is "Operator Precedence" and "Associativity"?

What is "Ternary Operator" (<code>?:</code>)?

What is "Enum" in C?

What is "Command Line Arguments"?

What is the difference between <code>printf()</code> and <code>puts()</code>?

What is "Dangling Else" problem?

What is "Short Circuit Evaluation"?

What is "Header File" (.h)?

What is the "Stack" vs "Heap" memory?

What is "Infinite Loop" and how to stop it?

What is the difference between <code>#include <file></code> and <code>#include "file"</code>?