free function - Mini Project: Build & Apply

Using the free Function in C

📖 Scenario: Imagine you are writing a small program in C that creates a dynamic array to store some numbers. After using the array, you need to clean up the memory to keep your program healthy and avoid memory leaks.

🎯 Goal: You will create a dynamic array using malloc, then use the free function to release the memory when you are done.

📋 What You'll Learn

Create a pointer variable to hold dynamically allocated memory for an array of integers

Allocate memory for 5 integers using malloc

Use the free function to release the allocated memory

Print a message confirming the memory has been freed

💡 Why This Matters

🌍 Real World

Dynamic memory management is important in programs that need flexible storage, like games, databases, or any software handling variable data sizes.

💼 Career

Understanding how to allocate and free memory safely is a key skill for C programmers working in systems programming, embedded systems, or performance-critical applications.

1

Create a pointer for dynamic memory

Declare a pointer variable called numbers of type int * and set it to NULL.

C

# Declare the pointer variable 'numbers' and set it to NULL
# Your code here

Hint

Use int *numbers = NULL; to declare a pointer that points to nothing yet.

2

Allocate memory for 5 integers

Use malloc to allocate memory for 5 integers and assign it to the pointer numbers.

C

#include 
#include 

int *numbers = NULL;

// Allocate memory for 5 integers
# Your code here

Hint

Use malloc(5 * sizeof(int)) to allocate space for 5 integers.

3

Free the allocated memory

Use the free function to release the memory pointed to by numbers.

C

#include 
#include 

int *numbers = NULL;

numbers = malloc(5 * sizeof(int));

// Free the allocated memory
# Your code here

Hint

Call free(numbers); to release the memory.

4

Print confirmation message

Write a printf statement to display the message "Memory has been freed.".

C

#include 
#include 

int *numbers = NULL;

numbers = malloc(5 * sizeof(int));

free(numbers);

// Print confirmation message
# Your code here

Hint

Use printf("Memory has been freed.\n"); to show the message.

Practice

1. What is the main purpose of the free function in C?

easy

A. To initialize a pointer to NULL

B. To allocate new memory dynamically

C. To copy data from one memory location to another

D. To release memory previously allocated by malloc or similar functions

Solution

1. Step 1: Understand what free does

   The free function releases memory that was allocated dynamically, returning it to the system.

2. Step 2: Differentiate from other memory functions

   malloc allocates memory, free releases it. Copying or initializing pointers is unrelated.

3. Final Answer:

   To release memory previously allocated by malloc or similar functions -> Option D

4. Quick Check:

   free releases memory = A [OK]

Hint: Remember: free returns memory, malloc gets memory [OK]

Common Mistakes:

• Confusing free with malloc or calloc
• Thinking free allocates memory
• Using free to copy data

2. Which of the following is the correct way to free memory allocated by malloc?

easy

A. free(pointer); pointer = NULL;

B. free(pointer); free(pointer);

C. free(pointer); pointer++;

D. free(pointer); pointer = malloc(10);

Solution

1. Step 1: Free the allocated memory

   Calling free(pointer); releases the memory correctly.

2. Step 2: Avoid dangling pointer

   Setting pointer = NULL; after freeing prevents accidental use of freed memory.

3. Final Answer:

   free(pointer); pointer = NULL; -> Option A

4. Quick Check:

   Free then NULL pointer = D [OK]

Hint: Always set pointer to NULL after free to avoid bugs [OK]

Common Mistakes:

• Calling free twice on the same pointer
• Incrementing pointer after free
• Reassigning pointer without freeing first

3. What will be the output of this C code?

char *ptr = malloc(5);
strcpy(ptr, "test");
free(ptr);
printf("%s", ptr);

medium

A. test

B. Undefined behavior (may print garbage or crash)

C. Compilation error

D. Empty string

Solution

1. Step 1: Analyze memory allocation and free

   Memory is allocated and string "test" copied. Then memory is freed.

2. Step 2: Using pointer after free

   Printing ptr after free causes undefined behavior; it may print garbage or crash.

3. Final Answer:

   Undefined behavior (may print garbage or crash) -> Option B

4. Quick Check:

   Using freed pointer = undefined behavior [OK]

Hint: Never use pointer after free without reassigning [OK]

Common Mistakes:

• Assuming freed pointer still holds valid data
• Expecting compilation error
• Thinking it prints empty string

4. Identify the error in this code snippet:

int *arr = malloc(3 * sizeof(int));
free(arr + 1);

medium

A. free called on pointer not returned by malloc

B. Missing free call

C. Incorrect size in malloc

D. No error, code is correct

Solution

1. Step 1: Understand pointer passed to free

   Memory was allocated at arr, but free is called on arr + 1, which is not the original pointer.

2. Step 2: Free must be called on original pointer

   Only the pointer returned by malloc (or NULL) can be passed to free safely.

3. Final Answer:

   free called on pointer not returned by malloc -> Option A

4. Quick Check:

   Free must use original pointer = A [OK]

Hint: Free only original malloc pointer, not offset pointers [OK]

Common Mistakes:

• Freeing pointer offset from malloc result
• Not freeing allocated memory
• Assuming free can take any pointer inside block

5. You have a function that reallocates memory for an integer array:

int *resize(int *arr, int new_size) {
    int *temp = realloc(arr, new_size * sizeof(int));
    if (temp == NULL) {
        free(arr);
        return NULL;
    }
    return temp;
}

What is the best practice to avoid memory leaks when using this function?

hard

A. Always free the original pointer before calling resize

B. Call free on the returned pointer immediately after resize

C. Assign the return value to a temporary pointer before updating original pointer

D. Do not check if realloc returns NULL

Solution

1. Step 1: Understand realloc behavior

   If realloc fails, it returns NULL and original memory remains allocated.

2. Step 2: Use temporary pointer to avoid losing original memory

   Assign realloc result to a temp pointer first; if NULL, free original to avoid leaks.

3. Final Answer:

   Assign the return value to a temporary pointer before updating original pointer -> Option C

4. Quick Check:

   Use temp pointer for realloc to prevent leaks = B [OK]

Hint: Use temp pointer for realloc result before replacing original [OK]

Common Mistakes:

• Overwriting original pointer without checking realloc result
• Freeing original pointer before realloc
• Ignoring NULL return from realloc