Memory leak concepts - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to allocate memory for an integer pointer.

C

int *ptr = (int *)[1](sizeof(int));

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

Amalloc

Bfree

Cprintf

Dsizeof

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'free' instead of 'malloc' to allocate memory.

Using 'sizeof' as a function to allocate memory.

The malloc function allocates memory dynamically. Here, it allocates enough space for one integer.

2fill in blank

medium

Complete the code to release the allocated memory.

C

free([1]);

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

Amalloc

Bptr

Csizeof

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Passing 'malloc' or 'sizeof' instead of the pointer.

Not freeing allocated memory causing leaks.

The free function releases the memory previously allocated by malloc. You must pass the pointer to the allocated memory.

3fill in blank

hard

Fix the error in the code to avoid memory leak by freeing the allocated memory.

C

int *arr = (int *)malloc(5 * sizeof(int));
// Missing [1] here
return 0;

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

Afree(arr);

Bmalloc(arr);

Csizeof(arr);

Darr = NULL;

Attempts:

3 left

💡 Hint

Common Mistakes

Not calling free causing memory leaks.

Calling malloc instead of free to release memory.

To avoid memory leaks, you must call free on the allocated pointer before the program ends or when the memory is no longer needed.

4fill in blank

hard

Fill both blanks to correctly allocate and free memory for a character array.

C

char *buffer = (char *)[1](100 * sizeof(char));
// Use the buffer
[2](buffer);

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

Amalloc

Bfree

Csizeof

Dprintf

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'sizeof' or 'printf' instead of 'malloc' or 'free'.

Forgetting to free allocated memory.

First, allocate memory with malloc. After using the buffer, release the memory with free to avoid leaks.

5fill in blank

hard

Fill all three blanks to create a memory leak by allocating memory without freeing it.

C

int *data = (int *)[1](10 * [2](int));
// Forgot to [3] data, causing a memory leak.

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

Amalloc

Bsizeof

Cfree

Dprintf

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to free allocated memory.

Using wrong functions for allocation or size.

The code allocates memory with malloc and uses sizeof to get the size of int. The comment correctly identifies that it forgot to call free(data), causing a memory leak.

Practice

1. What is a memory leak in C programming?

easy

A. When allocated memory is not freed after use

B. When a program runs out of variables

C. When a program crashes due to syntax errors

D. When memory is allocated too quickly

Solution

1. Step 1: Understand memory allocation

   Memory is allocated dynamically using functions like malloc().

2. Step 2: Identify what causes leaks

   If allocated memory is not freed using free(), it causes a memory leak.

3. Final Answer:

   When allocated memory is not freed after use -> Option A

4. Quick Check:

   Memory leak = not freeing allocated memory [OK]

Hint: Memory leak means forgetting to free allocated memory [OK]

Common Mistakes:

• Confusing memory leak with syntax errors
• Thinking memory leaks happen only on crashes
• Believing memory leaks are about variable count

2. Which of the following is the correct way to free dynamically allocated memory in C?

easy

A. free(ptr);

B. release(ptr);

C. delete(ptr);

D. dispose(ptr);

Solution

1. Step 1: Recall C memory management functions

   C uses malloc() to allocate and free() to release memory.

2. Step 2: Identify correct syntax

   free(ptr); is the standard function to free allocated memory.

3. Final Answer:

   free(ptr); -> Option A

4. Quick Check:

   Use free() to release memory [OK]

Hint: Use free() to release memory allocated by malloc() [OK]

Common Mistakes:

• Using delete which is C++ syntax
• Using non-existent functions like release or dispose
• Forgetting to pass the pointer to free()

3. What will be the output of the following C code snippet?

int *ptr = malloc(sizeof(int));
*ptr = 10;
printf("%d\n", *ptr);
// Missing free(ptr);

medium

A. Runtime crash

B. 10

C. Compilation error

D. 0

Solution

1. Step 1: Analyze memory allocation and assignment

   Memory is allocated and 10 is stored at that location.

2. Step 2: Check output and memory freeing

   printf prints 10 correctly; missing free() causes leak but no output error.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   Output prints stored value despite missing free() [OK]

Hint: Missing free() leaks memory but doesn't affect immediate output [OK]

Common Mistakes:

• Assuming missing free causes crash or wrong output
• Confusing memory leak with runtime errors
• Expecting zero or garbage output

4. Identify the problem in this C code snippet:

char *str = malloc(20);
strcpy(str, "Hello World");
// No free(str) called
str = malloc(30);
strcpy(str, "New String");
free(str);

medium

A. No problem, code is correct

B. Syntax error in strcpy usage

C. Double free error

D. Memory leak due to first malloc not freed

Solution

1. Step 1: Trace memory allocations

   First malloc(20) is assigned to str, then overwritten by second malloc(30).

2. Step 2: Check freeing of memory

   First allocated memory is lost without free, causing memory leak.

3. Final Answer:

   Memory leak due to first malloc not freed -> Option D

4. Quick Check:

   Overwriting pointer without free causes leak [OK]

Hint: Always free old memory before reassigning pointer [OK]

Common Mistakes:

• Thinking strcpy causes syntax error
• Assuming double free without repeated free calls
• Believing code is leak-free because free is called once

5. You have a function that allocates memory for an integer array and returns it. Which practice helps prevent memory leaks when using this function multiple times?

hard

A. Use global variables to store allocated memory

B. Never call free() because the function handles it internally

C. Always call free() on the returned pointer before reassigning it

D. Allocate memory only once and never reuse pointers

Solution

1. Step 1: Understand ownership of allocated memory

   The caller owns the memory returned and must free it to avoid leaks.

2. Step 2: Prevent leaks by freeing before reassignment

   Calling free() before reassigning pointer prevents losing track of allocated memory.

3. Final Answer:

   Always call free() on the returned pointer before reassigning it -> Option C

4. Quick Check:

   Free before reuse prevents leaks [OK]

Hint: Free returned memory before pointer reuse [OK]

Common Mistakes:

• Assuming function frees memory automatically
• Using global variables without freeing
• Avoiding reuse but causing inefficient memory use