C Programming Interview Questions and Answers

Question 1

What does the & operator give when used as &x?

Accepted Answer

The address of x. &x is the memory address of x, which is why scanf needs &x to write into it.

Question 2

Which header file declares printf() and scanf()?

Accepted Answer

stdio.h. stdio.h is the standard input/output header.

Question 3

What character ends every C string?

Accepted Answer

'\0' (the null character). C strings are char arrays terminated by '\0'; strlen counts up to it.

Question 4

Which function allocates memory on the heap?

Accepted Answer

malloc(). malloc returns a pointer to a heap block (or NULL); release it with free().

Question 5

What happens if you forget to free() memory you malloc'd?

Accepted Answer

A memory leak. The memory stays reserved until the program ends.

Question 6

Which format specifier prints a double with printf?

Accepted Answer

%f. %f (or %lf) prints floating-point values; scanf needs %lf for a double.

Question 7

What is a pointer?

Accepted Answer

A variable that stores a memory address. int *p = &x; stores the address of x in p.

Question 8

What is the difference between a struct and a union?

Accepted Answer

A union's members share the same memory. A struct gives each member its own space; a union overlaps them, so its size is that of the largest member.

Question 9

Which loop always runs its body at least once?

Accepted Answer

do-while. do { } while (cond); checks the condition after the body.

Question 10

What is the value of an uninitialised local int in C?

Accepted Answer

Unpredictable (garbage). Local variables are not initialised automatically; globals and statics start at 0.

Question 11

Which keyword keeps a local variable's value between function calls?

Accepted Answer

static. A static local is created once and keeps its value across calls.

Question 12

What does strcmp(a, b) return when the strings are equal?

Accepted Answer

0. strcmp returns 0 for equal strings, negative if a < b and positive if a > b.

Question 13

Why can't you compare two strings with == in C?

Accepted Answer

It compares addresses, not contents. Array names decay to pointers; use strcmp to compare the text.

Question 14

What is a dangling pointer?

Accepted Answer

A pointer to memory that has already been freed. Using it is undefined behaviour; set pointers to NULL after free().

Question 15

How are arrays passed to functions in C?

Accepted Answer

As a pointer to the first element. So the function can change the caller's array, and sizeof inside the function gives the pointer size.

Question 16

What does the preprocessor directive #define PI 3.14 do?

Accepted Answer

Replaces PI with 3.14 before compilation. Macros are text substitutions done by the preprocessor.

Question 17

Which operator accesses a struct member through a pointer?

Accepted Answer

->. p->x is shorthand for (*p).x.

Question 18

What is the size of a char in C?

Accepted Answer

1 byte. sizeof(char) is always 1 by definition.

Question 19

What does recursion need so it doesn't run forever?

Accepted Answer

A base case. Without a base case the calls never stop and the stack overflows.

Question 20

What does calloc() do differently from malloc()?

Accepted Answer

It sets the allocated memory to zero. calloc(n, size) allocates n * size bytes and zeroes them.

Question 21

What does this code print?
int a = 5, b = 2;
printf("%d
", a / b);

Accepted Answer

2. Integer division of two ints drops the fraction.

Question 22

What does this code print?
int a = 5, b = 2;
printf("%.2f
", (double) a / b);

Accepted Answer

2.50. Casting a to double makes the division happen in floating point.

Question 23

What does this code print?
int x = 5;
int y = ++x;
printf("%d %d
", y, x);

Accepted Answer

6 6. ++x increments first, so y gets 6 and x is 6.

Question 24

What does this code print?
#include <stdio.h>

void set(int *p) { *p = 20; }

int main() {
    int x = 10;
    set(&x);
    printf("%d
", x);
    return 0;
}

Accepted Answer

20. The function receives the address of x and changes x through the pointer.

Question 25

What does this code print?
#include <stdio.h>

void set(int v) { v = 20; }

int main() {
    int x = 10;
    set(x);
    printf("%d
", x);
    return 0;
}

Accepted Answer

10. C passes a copy of the value; the function changes only its copy.

Question 26

What does this code print?
char s[] = "hello";
printf("%zu
", strlen(s));

Accepted Answer

5. strlen counts the characters before '\0': h, e, l, l, o.

Question 27

What does this code print?
char s[] = "hello";
printf("%zu
", sizeof(s));

Accepted Answer

6. sizeof includes the terminating '\0': 5 letters + 1.

Question 28

What does this code print?
int arr[] = {10, 20, 30, 40};
int *p = arr;
printf("%d
", *(p + 2));

Accepted Answer

30. p points to arr[0]; p + 2 points to arr[2].

Question 29

What does this code print?
int n = 2;
switch (n) {
    case 1: printf("One ");
    case 2: printf("Two ");
    case 3: printf("Three
"); break;
    default: printf("Default
");
}

Accepted Answer

Two Three. Without break, case 2 falls through into case 3.

Question 30

What does this code print?
#include <stdio.h>

int next(void) {
    static int count = 0;
    return ++count;
}

int main() {
    int a = next(), b = next(), c = next();
    printf("%d %d %d
", a, b, c);
    return 0;
}

Accepted Answer

1 2 3. A static local keeps its value between calls.

Question 31

What does this code print?
for (int i = 0; i < 3; i++) printf(i < 2 ? "%d " : "%d
", i);

Accepted Answer

0 1 2. The loop runs while i < 3.

Question 32

What does this code print?
char ch = 'A';
printf("%c
", ch + 1);

Accepted Answer

B. 'A' + 1 is 66, which %c prints as 'B'.

Question 33

What does this code print?
printf("%d
", 5 > 3);

Accepted Answer

1. In C, the comparison 5 > 3 evaluates to the int 1 (true).

Question 34

What does this code print?
char a[] = "cat", b[] = "cat";
printf(strcmp(a, b) == 0 ? "Equal
" : "Not equal
");

Accepted Answer

Equal. strcmp returns 0 when the two strings have the same characters.

Question 35

What does this code print?
int x = 10 % 7;
printf("%d
", x);

Accepted Answer

3. x = 10 % 7 = 3.

Question 36

What does this code print?
#include <stdio.h>

void swap(int *a, int *b) { int t = *a; *a = *b; *b = t; }

int main() {
    int x = 1, y = 2;
    swap(&x, &y);
    printf("%d %d
", x, y);
    return 0;
}

Accepted Answer

2 1. swap receives addresses and exchanges the values through the pointers.

Question 37

What does this code print?
#include <stdio.h>

int fact(int n) { return n <= 1 ? 1 : n * fact(n - 1); }

int main() {
    printf("%d
", fact(4));
    return 0;
}

Accepted Answer

24. fact(4) = 4 × 3 × 2 × 1 = 24.

Question 38

What does this code print?
#include <stdio.h>

struct Student { char name[20]; int marks; };

int main() {
    struct Student s = {"Kavin", 90};
    printf("%s %d
", s.name, s.marks);
    return 0;
}

Accepted Answer

Kavin 90. Members of a struct are accessed with the dot operator.

Question 39

What is the return type of main() in standard C?

Accepted Answer

int. Return 0 for success.

Question 40

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

Accepted Answer

++i increments then uses the value; i++ uses the value then increments. Matters only when the expression's value is used.

Question 41

What does the sizeof operator return?

Accepted Answer

The size of a type or variable in bytes. sizeof(int) is usually 4.

Question 42

What is a NULL pointer?

Accepted Answer

A pointer that points to nothing. Always check before dereferencing.

Question 43

What is a void pointer?

Accepted Answer

A generic pointer that can hold the address of any type. malloc returns void *.

Question 44

What is a function pointer?

Accepted Answer

A pointer that stores the address of a function. Used for callbacks, e.g. qsort's comparator.

Question 45

What does the extern keyword do?

Accepted Answer

Declares a variable defined in another file. Used in header files for shared globals.

Question 46

What is the scope of a static global variable?

Accepted Answer

Only the file it is defined in. static limits linkage to the current file.

Question 47

What does the const keyword in const int *p mean?

Accepted Answer

The value pointed to can't be changed through p. int *const p makes the pointer itself constant.

Question 48

What is the difference between malloc and calloc?

Accepted Answer

calloc zero-initialises memory; malloc doesn't. calloc(n, size) also takes the count separately.

Question 49

What does realloc do?

Accepted Answer

Changes the size of previously allocated memory. It may move the block; use the returned pointer.

Question 50

What is a segmentation fault?

Accepted Answer

Accessing memory the program isn't allowed to. Often caused by NULL or dangling pointers, or out-of-bounds arrays.

Question 51

What is a buffer overflow?

Accepted Answer

Writing past the end of an array. A common security bug; gets() is dangerous for this reason.

Question 52

Which function safely reads a line with a size limit?

Accepted Answer

fgets. gets has no limit and was removed from the standard.

Question 53

What does the -> operator do?

Accepted Answer

Accesses a struct member through a pointer. p->x equals (*p).x.

Question 54

What is typedef used for?

Accepted Answer

Giving a new name to an existing type. typedef struct Node Node;

Question 55

What is an enum in C?

Accepted Answer

A set of named integer constants. enum Color { RED, GREEN, BLUE }; RED is 0.

Question 56

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

Accepted Answer

<> searches system directories; "" searches the current directory first. Use quotes for your own headers.

Question 57

What is a macro?

Accepted Answer

A preprocessor text substitution defined with #define. Macros have no type checking; inline functions are safer.

Question 58

What is the output stream for error messages in C?

Accepted Answer

stderr. fprintf(stderr, "...").

Question 59

What does the break statement do in a loop?

Accepted Answer

Exits the loop immediately. continue skips to the next iteration.

Question 60

What is an infinite loop?

Accepted Answer

A loop whose condition never becomes false. for (;;) or while (1).

Question 61

What is the purpose of the volatile keyword in C?

Accepted Answer

Tells the compiler a variable may change outside the program's control. Used for hardware registers and signal handlers.

Question 62

What is the range of an unsigned char?

Accepted Answer

0 to 255. 8 bits, no sign.

Question 63

What does the bitwise operator & do on 6 & 3?

Accepted Answer

2. 110 & 011 = 010.

Question 64

What does 1 << 3 evaluate to?

Accepted Answer

8. Shifting left by 3 multiplies by 2³.

C Programming Interview Questions and Answers

Q1. What does the & operator give when used as &x?

Q2. Which header file declares printf() and scanf()?

Q3. What character ends every C string?

Q4. Which function allocates memory on the heap?

Q5. What happens if you forget to free() memory you malloc'd?

Q6. Which format specifier prints a double with printf?

Q7. What is a pointer?

Q8. What is the difference between a struct and a union?

Q9. Which loop always runs its body at least once?

Q10. What is the value of an uninitialised local int in C?

Q11. Which keyword keeps a local variable's value between function calls?

Q12. What does strcmp(a, b) return when the strings are equal?

Q13. Why can't you compare two strings with == in C?

Q14. What is a dangling pointer?

Q15. How are arrays passed to functions in C?

Q16. What does the preprocessor directive #define PI 3.14 do?

Q17. Which operator accesses a struct member through a pointer?

Q18. What is the size of a char in C?

Q19. What does recursion need so it doesn't run forever?

Q20. What does calloc() do differently from malloc()?

Q21. What does this code print?

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Q39. What is the return type of main() in standard C?

Q40. What is the difference between ++i and i++?

Q41. What does the sizeof operator return?

Q42. What is a NULL pointer?

Q43. What is a void pointer?

Q44. What is a function pointer?

Q45. What does the extern keyword do?

Q46. What is the scope of a static global variable?

Q47. What does the const keyword in const int *p mean?

Q48. What is the difference between malloc and calloc?

Q49. What does realloc do?

Q50. What is a segmentation fault?

Q51. What is a buffer overflow?

Q52. Which function safely reads a line with a size limit?

Q53. What does the -> operator do?

Q54. What is typedef used for?

Q55. What is an enum in C?

Q56. What is the difference between #include <file.h> and #include "file.h"?

Q57. What is a macro?

Q58. What is the output stream for error messages in C?

Q59. What does the break statement do in a loop?

Q60. What is an infinite loop?

Q61. What is the purpose of the volatile keyword in C?

Q62. What is the range of an unsigned char?

Q63. What does the bitwise operator & do on 6 & 3?

Q64. What does 1 << 3 evaluate to?

Q65. What does the ternary operator a > b ? a : b give?

Q66. What is recursion's main risk in C?

Q67. What does the storage class 'register' suggest?

Q68. What is the difference between a declaration and a definition?

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