Register storage class - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of code using register variable

What is the output of this C code snippet that uses a register variable inside a loop?

C

#include <stdio.h>

int main() {
    register int i;
    int sum = 0;
    for(i = 1; i <= 5; i++) {
        sum += i;
    }
    printf("%d", sum);
    return 0;
}

A Undefined behavior

B 10

C 15

D Compilation error due to register variable

💡 Hint

Remember that register variables are stored in CPU registers for faster access but behave like normal variables in this context.

The register keyword suggests storing the variable in a CPU register for speed, but it does not change the logic. The loop sums numbers 1 to 5, so output is 15.

❓ Predict Output

intermediate

Effect of register keyword on address operator

What happens when you try to print the address of a register variable in C?

C

#include <stdio.h>

int main() {
    register int x = 10;
    printf("%p", (void*)&x);
    return 0;
}

A Compilation error: cannot take address of register variable

B Prints the memory address of x

C Runtime error

D Prints 0

💡 Hint

The register keyword restricts taking the address of the variable.

Register variables are stored in CPU registers and their address cannot be taken, so the compiler throws an error when & operator is used.

🔧 Debug

advanced

Identify the error with register variable usage

What error will this code produce and why?

C

#include <stdio.h>

int main() {
    register int count = 5;
    int *ptr = &count;
    printf("%d", *ptr);
    return 0;
}

A Prints 5

B Compilation error: cannot take address of register variable

C Runtime segmentation fault

D Undefined behavior but compiles

💡 Hint

Check if the address of a register variable can be taken.

Taking the address of a register variable is not allowed, so the compiler will produce an error at the line int *ptr = &count;

🧠 Conceptual

advanced

Behavior of register variables in recursive functions

Consider a recursive function using a register variable as a counter. What is the behavior of the register variable across recursive calls?

C

int factorial(int n) {
    register int count = 0;
    count++;
    if (n <= 1) return 1;
    return n * factorial(n - 1);
}

A The count variable is shared globally among all calls

B The register variable count keeps incrementing across all recursive calls

C Compilation error due to register variable in recursion

D Each recursive call has its own separate register variable count initialized to 0

💡 Hint

Think about variable scope and lifetime in recursive calls.

Each recursive call creates a new instance of the register variable count initialized to 0, so count does not accumulate across calls.

❓ Predict Output

expert

Number of items in a register variable array

What is the output of this code that declares a register array and prints its size?

C

#include <stdio.h>

int main() {
    register int arr[4] = {1, 2, 3, 4};
    printf("%lu", sizeof(arr)/sizeof(arr[0]));
    return 0;
}

A Compilation error: register variable cannot be an array

B 4

C 1

D Undefined behavior

💡 Hint

Check if arrays can be declared with register storage class.

In C, register storage class cannot be applied to arrays, so this code causes a compilation error.