free function - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to free the memory allocated to pointer ptr.

C

free([1]);

Drag options to blanks, or click blank then click option'

A*ptr

B&ptr

Cptr

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the address of the pointer instead of the pointer itself.

Passing a dereferenced pointer instead of the pointer.

The free function releases the memory pointed to by ptr. You should pass the pointer itself, not its address or dereferenced value.

2fill in blank

medium

Complete the code to allocate and then free an integer pointer.

C

int *p = malloc(sizeof(int));
if (p != NULL) {
    *p = 10;
    [1](p);
}

Drag options to blanks, or click blank then click option'

Adelete

Bfree

Crelease

Ddispose

Attempts:

3 left

💡 Hint

Common Mistakes

Using C++ keyword delete in C code.

Using non-existent functions like release or dispose.

In C, free is used to release memory allocated by malloc. Other options are not valid C functions.

3fill in blank

hard

Fix the error in the code to properly free the allocated memory.

C

char *buffer = malloc(100);
if (buffer != NULL) {
    // use buffer
    [1](buffer);
}

Drag options to blanks, or click blank then click option'

Afree

Bfree_ptr

Cfree(buffer)

Dfree(buffer);

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the address of the pointer instead of the pointer.

Including parentheses or semicolon inside the blank.

The free function should be called with the pointer itself, not its address. So use free(buffer); but since the blank replaces the function call, just free is correct to fill in.

4fill in blank

hard

Fill both blanks to correctly allocate and free memory for an integer array.

C

int *arr = [1](5 * sizeof(int));
if (arr != NULL) {
    // use arr
    [2](arr);
}

Drag options to blanks, or click blank then click option'

Amalloc

Bfree

Ccalloc

Drealloc

Attempts:

3 left

💡 Hint

Common Mistakes

Using calloc instead of malloc.

Using realloc without an existing allocation.

Forgetting to free the allocated memory.

malloc allocates memory, and free releases it. calloc also allocates but initializes memory to zero, realloc resizes allocated memory.

5fill in blank

hard

Fill all three blanks to allocate memory for a struct, assign a value, and then free the memory.

C

typedef struct {
    int id;
} Item;

Item *item = [1](sizeof(Item));
if (item != NULL) {
    item->id = [2];
    [3](item);
}

Drag options to blanks, or click blank then click option'

Amalloc

B42

Cfree

Dcalloc

Attempts:

3 left

💡 Hint

Common Mistakes

Using calloc instead of malloc.

Assigning a pointer instead of an integer to id.

Not freeing the allocated memory.

malloc allocates memory, 42 is assigned to the id field, and free releases the memory.

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