Viva Questions

Here you'll find a curated list of popular C language viva questions, with succinct answers, explanation tables, and short practical code snippets.

• int main(): Standard and recommended; returns a value to the OS.
• void main(): Not standard; used in older compilers like Turbo C.
• Always use: int main()

• In modern C, the compiler automatically adds return 0, so the program works as expected.

• Supports low-level features like memory access via pointers.
• Supports high-level features like functions and loops.

c Run in compiler
#include <stdio.h>
int main() {
    printf("%d", printf("Hello"));
    return 0;
}

text
Hello5

• printf("Hello") prints 'Hello' and returns 5 (the string's length), so the outer printf prints 5.

c Run in compiler
#include <stdio.h>
int main() {
    int a = 5;
    printf("%d %d %d", a, a++, ++a);
    return 0;
}

text
Undefined behavior

• The variable a is modified more than once between sequence points. Output depends on the compiler and is not predictable.

• A pointer pointing to memory that has been freed or deleted.

c Run in compiler
#include <stdio.h>
int main() {
    int x = 10;
    if (x = 5)
        printf("Yes");
    else
        printf("No");
    return 0;
}

text
Yes

• x = 5 is assignment, not comparison (==). Assignment returns 5 (true), so 'Yes' is printed.

Common Mistake: Use == for comparison.

Correct Version:

c Run in compiler
#include <stdio.h>
int main() {
    int x = 10;
    if (x == 5)
        printf("Yes");
    else
        printf("No");
    return 0;
}

• Tip: Always use == in if when comparing values.

• Depends on the system/compiler. Usually 4 bytes.

c Run in compiler
#include <stdio.h>
int main() {
    char ch = 'A';
    printf("%d", ch);
    return 0;
}

text
65

• ASCII value of 'A'.

• Yes, but it is dangerous: may give garbage values or cause a crash.

c Run in compiler
#include <stdio.h>
int main() {
    int a = 0;
    if (a)
        printf("True");
    else
        printf("False");
    return 0;
}

text
False

• In C, 0 is equivalent to false.

• A pointer that points to nothing (address 0).

• malloc(): Allocates memory.
• free(): Releases allocated memory.

c Run in compiler
#include <stdio.h>
int main() {
    printf("%d", sizeof('A'));
    return 0;
}

text
4

• In C, character literals are treated as int.

c Run in compiler
#include <stdio.h>
// Call by Value
void swapValue(int a, int b) {
    int temp = a;
    a = b;
    b = temp;
}
// Call by Reference
void swapRef(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}
int main() {
    int x = 5, y = 10;
    swapValue(x, y);
    printf("After Call by Value: x=%d y=%d\n", x, y); // x=5 y=10
    swapRef(&x, &y);
    printf("After Call by Reference: x=%d y=%d", x, y); // x=10 y=5
    return 0;
}

Memory trick: Value - copy (no change). Reference - address (change happens).

• Stack: Automatic memory for local variables.
• Heap: Dynamic memory allocated using malloc() and free().

• Accessing invalid memory causes a program crash (segmentation fault).

c Run in compiler
#include <stdio.h>
int main() {
    int *ptr = NULL;
    printf("%d", *ptr); // Invalid memory access
    return 0;
}

c Run in compiler
#include <stdio.h>
int main() {
    int a = 5;
    printf("%d", a+++a);
    return 0;
}

text
11

• Interpreted as: (a++) + a → 5 + 6 = 11

• No; for execution, main() is required.

• Informs compiler not to optimize variable as its value may change unexpectedly.

c Run in compiler
#include <stdio.h>
int main() {
    int a = 3;
    printf("%d", a << 1);
    return 0;
}

text
6

• Left shift operator multiplies the value by 2.

1. Use int main() for standard C programs.
2. Be careful with assignments and comparisons inside conditions.
3. Always release dynamically allocated memory with free().
4. Misusing array indices or uninitialized pointers may lead to crashes.
5. Understanding call by value vs. reference is vital for debugging code effects.

1. Write a C function to show the difference between call by value and call by reference.
2. Predict the output: int x = 0; if (x) printf("One"); else printf("Zero");
3. Explain what happens when you use break and continue in a loop.
4. What is the output of printf("%d", sizeof('B'))?
5. Show a C code example leading to a segmentation fault.

1. Always use int main() and include a return statement.
2. Use == for comparisons and = for assignments.
3. Avoid modifying variables more than once in a single expression.
4. Never dereference NULL or uninitialized pointers.
5. Release all dynamically allocated memory using free().
6. Understand how pre- and post-increment operators work.