Auto storage class - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of auto variable inside a function

What is the output of this C program?

C

#include <stdio.h>

void func() {
    auto int x = 5;
    x++;
    printf("%d ", x);
}

int main() {
    func();
    func();
    return 0;
}

A 6 6

B 6 7

C 5 6

D 5 5

💡 Hint

Remember that 'auto' variables inside functions are reinitialized each call.

The variable 'x' is declared as auto inside func(), so it is created anew each time func() is called. It starts at 5, then increments to 6, and prints 6 each time.

❓ Predict Output

intermediate

Effect of auto keyword on global variables

What error or output does this C code produce?

C

auto int x = 10;

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

A 0

B 10

C Compilation error: 'auto' not allowed for global variables

D Undefined behavior

💡 Hint

Check if 'auto' can be used outside functions.

The 'auto' storage class is only valid for local variables inside functions. Using it for global variables causes a compilation error.

🔧 Debug

advanced

Identify the error with auto variable redeclaration

What error does this code produce?

C

int main() {
    auto int a = 5;
    auto int a = 10;
    printf("%d", a);
    return 0;
}

A Compilation error: redeclaration of 'a'

B 5

C 10

D Undefined behavior

💡 Hint

Can you declare the same variable twice in the same scope?

Declaring 'a' twice in the same scope causes a compilation error due to redeclaration.

🧠 Conceptual

advanced

Behavior of auto variables in loops

What is the output of this program?

C

#include <stdio.h>

int main() {
    for (int i = 0; i < 3; i++) {
        auto int x = 0;
        x += i;
        printf("%d ", x);
    }
    return 0;
}

A 0 1 3

B 0 1 2

C 0 0 0

D 1 2 3

💡 Hint

Each loop iteration creates a new 'x' starting at 0.

The variable 'x' is auto inside the loop, so it resets to 0 each iteration, then adds i, printing 0, 1, 2.

❓ Predict Output

expert

Auto variable and pointer behavior

What is the output of this C program?

C

#include <stdio.h>

int* func() {
    auto int x = 10;
    return &x;
}

int main() {
    int* p = func();
    printf("%d", *p);
    return 0;
}

A 10

B 0

C Compilation error

D Garbage value (undefined behavior)

💡 Hint

Think about the lifetime of 'x' after func() returns.

The function returns the address of an auto variable 'x' which is destroyed after the function ends. Accessing it causes undefined behavior and likely garbage output.