Memory leak concepts - Mini Project: Build & Apply

Memory Leak Concepts

📖 Scenario: You are writing a simple C program that uses dynamic memory allocation to store numbers. You want to understand how forgetting to free memory causes memory leaks.

🎯 Goal: Build a small C program that allocates memory for an integer array, uses it, and then properly frees the memory to avoid memory leaks.

📋 What You'll Learn

Create a pointer variable to hold dynamically allocated memory

Allocate memory for 5 integers using malloc

Assign values to the allocated memory

Free the allocated memory using free

Print the values stored in the allocated memory

💡 Why This Matters

🌍 Real World

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

💼 Career

Understanding memory management and avoiding leaks is crucial for software developers working in C or systems programming to write efficient and stable applications.

1

Create a pointer and allocate memory

Create a pointer variable called numbers of type int *. Use malloc to allocate memory for 5 integers and assign it to numbers.

C

#include 
#include 

int main() {

    // Create pointer and allocate memory for 5 integers
    
    // Your code here

    return 0;
}

Hint

Use int *numbers = malloc(5 * sizeof(int)); to allocate memory for 5 integers.

2

Assign values to allocated memory

Use a for loop with variable i from 0 to 4 to assign the value i * 10 to numbers[i].

C

#include 
#include 

int main() {

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

    // Assign values to numbers array
    
    // Your code here

    return 0;
}

Hint

Use a for loop to assign values like numbers[i] = i * 10;.

3

Print the values stored in the allocated memory

Use a for loop with variable i from 0 to 4 to print each value in numbers[i] using printf. Print each number on its own line.

C

#include 
#include 

int main() {

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

    for (int i = 0; i < 5; i++) {
        numbers[i] = i * 10;
    }

    // Print values stored in numbers
    
    // Your code here

    return 0;
}

Hint

Use a for loop and printf("%d\n", numbers[i]); to print each number.

4

Free the allocated memory

Use free(numbers); to release the memory allocated by malloc before the program ends.

C

#include 
#include 

int main() {

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

    for (int i = 0; i < 5; i++) {
        numbers[i] = i * 10;
    }

    for (int i = 0; i < 5; i++) {
        printf("%d\n", numbers[i]);
    }

    // Free the allocated memory
    
    // Your code here

    return 0;
}

Hint

Use free(numbers); to release the memory.

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