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.

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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