C Programming Interview Questions and Answers for Experienced Developers

list of 25 important C programming interview questions and answers tailored for experienced candidates. These cover advanced concepts, memory management, pointers, and best practices.

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1. What is the difference between malloc(), calloc(), and realloc()?

Answer:

• malloc(): Allocates memory of a given size (uninitialized).
• calloc(): Allocates memory for an array of elements and initializes them to zero.
• realloc(): Resizes previously allocated memory (can expand or shrink).

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2. Explain the difference between const char* p and char* const p.

Answer:

• const char* p: Pointer to a constant character (value cannot be modified).
• char* const p: Constant pointer to a character (pointer cannot point elsewhere).

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3. What is a dangling pointer? How can it be avoided?

Answer:
A dangling pointer points to a deallocated memory location.
Avoid by:

• Setting pointers to NULL after free().
• Avoiding returning local variable addresses.
• Using dynamic memory carefully.

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4. How does volatile keyword work in C?

Answer:
volatile tells the compiler that a variable may change unexpectedly (e.g., by hardware or interrupts), preventing compiler optimizations.

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5. What is the use of restrict keyword in C?

Answer:
restrict indicates that a pointer is the only way to access the memory it points to, enabling compiler optimizations.

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6. Explain the difference between #include <file.h> and #include "file.h".

Answer:

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

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7. What is a memory leak? How can it be detected and prevented?

Answer:
A memory leak occurs when dynamically allocated memory is not freed.
Detection: Tools like Valgrind, AddressSanitizer.
Prevention: Always free() allocated memory, use smart pointers (in C++), or RAII techniques.

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8. What is the purpose of typedef in C?

Answer:
typedef creates an alias for existing types, improving readability.
Example:

typedef unsigned int uint;

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9. How does sizeof() work for pointers and arrays?

Answer:

• For a pointer: Returns the size of the pointer (e.g., sizeof(int*) → 4 or 8 bytes).
• For an array: Returns the total size of the array (e.g., sizeof(arr) for int arr[10] → 40 bytes).

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10. What is a function pointer? Give an example.

Answer:
A function pointer stores the address of a function.
Example:

int (*funcPtr)(int, int) = &add; // Points to function 'add'

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11. Explain the difference between strcpy() and strncpy().

Answer:

• strcpy() copies until \0 (unsafe if destination is smaller).
• strncpy() copies up to n characters (safer but may not null-terminate).

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12. What is the stack and heap? How are they different?

Answer:

• Stack: Stores local variables, automatically managed (LIFO), faster but limited size.
• Heap: Dynamic memory allocation (malloc, free), larger but slower and manual management.

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13. What is a segmentation fault? Common causes?

Answer:
A segfault occurs when accessing invalid memory.
Causes:

• Dereferencing NULL or wild pointers.
• Buffer overflow.
• Stack overflow.

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14. How do you implement a linked list in C?

Answer:

struct Node {
    int data;
    struct Node* next;
};

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

Answer:

• ++i (pre-increment): Increments first, then uses the value.
• i++ (post-increment): Uses the value first, then increments.

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16. Explain static variables in C.

Answer:

• Static local variables: Retain value between function calls.
• Static global variables: Limited to the file scope (not accessible externally).

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17. What is #pragma once? How is it different from include guards?

Answer:

• #pragma once: Non-standard but efficient (prevents multiple inclusions).
• Include guards: Standard (#ifndef HEADER_H), works everywhere.

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18. How does qsort() work in C?

Answer:

void qsort(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *));

It sorts an array using a comparator function.

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19. What is the difference between struct and union?

Answer:

• Struct: Allocates memory for all members (size = sum of members).
• Union: Allocates memory for the largest member (size = largest member).

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20. How do you use setjmp() and longjmp()?

Answer:
They provide non-local jumps (like exception handling).

jmp_buf env;
if (setjmp(env) { /* Error handling */ }
else { longjmp(env, 1); }

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21. What is inline function in C?

Answer:
inline suggests the compiler to embed the function code at the call site (avoiding function call overhead).

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22. Explain the restrict keyword with an example.

Answer:

void copy(int *restrict dest, int *restrict src, int n) { ... }

Ensures dest and src do not overlap, allowing optimizations.

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23. What is the purpose of assert() in C?

Answer:
assert() checks if a condition is true; if not, it aborts the program (used for debugging).

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24. How do you reverse a string in C?

Answer:

void reverse(char *str) {
    int n = strlen(str);
    for (int i = 0; i < n/2; i++) {
        char temp = str[i];
        str[i] = str[n-i-1];
        str[n-i-1] = temp;
    }
}

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25. What are variadic functions? Give an example.

Answer:
Functions that take a variable number of arguments (e.g., printf).
Example:

#include <stdarg.h>
int sum(int count, ...) {
    va_list args;
    va_start(args, count);
    int total = 0;
    for (int i = 0; i < count; i++) {
        total += va_arg(args, int);
    }
    va_end(args);
    return total;
}

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These questions cover memory management, pointers, compiler directives, and best practices, making them ideal for experienced C programmers.

Here are section-wise titles for your article to make it well-structured and SEO-friendly:

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1. Memory Management & Dynamic Allocation

1. What is the difference between malloc(), calloc(), and realloc()?
2. What is a memory leak? How can it be detected and prevented?
3. Explain the difference between the stack and the heap.

2. Pointers & Advanced Concepts

4. What is a dangling pointer? How can it be avoided?
5. Explain the difference between const char* p and char* const p.
6. What is the use of the restrict keyword in C?
7. How does sizeof() work for pointers and arrays?
8. What is a function pointer? Give an example.

3. Compiler Directives & Preprocessor

9. Explain the difference between #include <file.h> and #include "file.h".
10. What is #pragma once? How is it different from include guards?
11. What is the purpose of typedef in C?

4. Storage Classes & Variable Modifiers

12. Explain static variables in C.
13. How does the volatile keyword work in C?
14. What is an inline function in C?

5. Strings & Arrays

15. Explain the difference between strcpy() and strncpy().
16. How do you reverse a string in C?
17. How does qsort() work in C?

6. Structures, Unions & Data Types

18. What is the difference between a struct and a union?
19. How do you implement a linked list in C?

7. Control Flow & Error Handling

20. What is the difference between ++i and i++?
21. How do you use setjmp() and longjmp()?
22. What is the purpose of assert() in C?

8. Advanced Functions & Utilities

23. What are variadic functions? Give an example.
24. Explain the restrict keyword with an example.

9. Common Pitfalls & Debugging

25. What is a segmentation fault? What are its common causes?

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