free function - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code using free()?

Consider the following C code snippet. What will be printed when it runs?

C

#include <stdio.h>
#include <stdlib.h>

int main() {
    int *ptr = malloc(sizeof(int));
    *ptr = 42;
    free(ptr);
    printf("%d\n", *ptr);
    return 0;
}

A 0

B Compilation error

C 42

D Undefined behavior (likely garbage or crash)

💡 Hint

Think about what happens to memory after free() is called.

After calling free(ptr), the memory pointed to by ptr is released back to the system. Accessing *ptr after free() causes undefined behavior, which may print garbage, cause a crash, or anything unpredictable.

🧠 Conceptual

intermediate

What does free() do in C?

Which of the following best describes what the free() function does?

A Deallocates previously allocated memory

B Copies memory from one location to another

C Allocates memory on the heap

D Initializes memory to zero

💡 Hint

Think about how you return memory to the system after using malloc.

The free() function releases memory that was previously allocated by malloc(), calloc(), or realloc(). It does not allocate or initialize memory.

🔧 Debug

advanced

Identify the error related to free() in this code

What error will this code cause when run?

C

#include <stdlib.h>

int main() {
    int x = 10;
    int *ptr = &x;
    free(ptr);
    return 0;
}

A Segmentation fault at malloc

B Freeing memory not allocated by malloc causes undefined behavior

C No error, runs fine

D Double free error

💡 Hint

Consider what kind of pointers can be passed to free().

free() must only be called on pointers returned by malloc, calloc, or realloc. Here, ptr points to a local variable on the stack, so calling free(ptr) causes undefined behavior.

📝 Syntax

advanced

Which option correctly frees a dynamically allocated array?

Given int *arr = malloc(5 * sizeof(int));, which option correctly frees the memory?

A free(arr[0]);

B free(&arr);

C free(arr); arr = NULL;

D free(*arr);

💡 Hint

Remember what free() expects as its argument.

free() expects a pointer to the start of the allocated memory block. Option C correctly frees arr and then sets it to NULL to avoid dangling pointer. Options A, B, and C pass invalid pointers causing errors or undefined behavior.

🚀 Application

expert

How many times is free() called and what is the output?

Analyze the code below. How many times is free() called and what is printed?

C

#include <stdio.h>
#include <stdlib.h>

int main() {
    int *a = malloc(sizeof(int));
    int *b = a;
    *a = 100;
    free(a);
    free(b);
    printf("%d\n", *b);
    return 0;
}

A free() called twice; causes undefined behavior (likely crash)

B free() called twice; prints 100

C free() called once; prints 100

D free() called once; causes segmentation fault

💡 Hint

Consider what happens when you free the same memory twice.

free() is called twice on the same pointer (a and b point to the same memory). Double free causes undefined behavior, often a crash. Accessing *b after free is also invalid.