Stack vs heap in embedded context - Hands-On Comparison

Understanding Stack vs Heap in Embedded C

📖 Scenario: In embedded systems, memory is limited and managing it well is very important. Two main areas where memory is used are the stack and the heap. The stack is used for temporary data like function variables, and the heap is used for dynamic memory allocation.Imagine you are programming a small embedded device that needs to store some data temporarily and also allocate memory dynamically for a buffer.

🎯 Goal: You will write a simple embedded C program that shows how to use the stack and the heap. You will create a local variable on the stack and allocate memory on the heap using malloc. Then you will print their addresses to see the difference.

📋 What You'll Learn

Create a local variable on the stack

Allocate memory dynamically on the heap using malloc

Print the addresses of the stack variable and the heap memory

Understand the difference in memory locations

💡 Why This Matters

🌍 Real World

Embedded devices like sensors and controllers have limited memory. Understanding stack and heap helps write efficient and safe programs.

💼 Career

Embedded software engineers must manage memory carefully to avoid crashes and bugs in devices like medical equipment, automotive systems, and IoT gadgets.

1

Create a local variable on the stack

Write a line of code to declare an integer variable called stack_var and set it to 10 inside the main function.

Embedded C

#include 
#include 

int main() {
    // Declare an integer variable called stack_var and set it to 10
    // Your code here
    return 0;
}

Hint

Use int stack_var = 10; inside main.

2

Allocate memory dynamically on the heap

Add a line of code to allocate memory for an integer pointer called heap_var using malloc. Allocate space for one integer.

Embedded C

#include 
#include 

int main() {
    int stack_var = 10;
    // Allocate memory for an integer pointer called heap_var using malloc
    // Your code here
    return 0;
}

Hint

Use int *heap_var = (int *)malloc(sizeof(int)); to allocate memory on the heap.

3

Store a value in the heap memory

Add a line of code to store the value 20 in the memory pointed to by heap_var.

Embedded C

#include 
#include 

int main() {
    int stack_var = 10;
    int *heap_var = (int *)malloc(sizeof(int));
    // Store 20 in the memory pointed to by heap_var
    // Your code here
    return 0;
}

Hint

Use *heap_var = 20; to store the value in heap memory.

4

Print the addresses of stack and heap variables

Add two printf lines to print the addresses of stack_var and heap_var using %p. Then free the allocated heap memory.

Embedded C

#include 
#include 

int main() {
    int stack_var = 10;
    int *heap_var = (int *)malloc(sizeof(int));
    *heap_var = 20;
    // Print the address of stack_var
    // Print the address stored in heap_var
    // Free the allocated heap memory
    // Your code here
    return 0;
}

Hint

Use printf("Address of stack_var: %p\n", (void *)&stack_var); and printf("Address of heap_var: %p\n", (void *)heap_var);. Don't forget to free(heap_var);.

Practice

1. In embedded C, where are local variables inside functions typically stored?

easy

A. On the stack

B. On the heap

C. In flash memory

D. In CPU registers only

Solution

1. Step 1: Understand local variable storage

   Local variables inside functions are temporary and exist only during function execution.

2. Step 2: Identify stack usage in embedded C

   The stack is used to store temporary data like local variables and return addresses.

3. Final Answer:

   On the stack -> Option A

4. Quick Check:

   Local variables = stack [OK]

Hint: Local variables go on stack, dynamic memory on heap [OK]

Common Mistakes:

• Confusing heap with stack for local variables
• Thinking local variables are stored in flash
• Assuming all variables are in CPU registers

2. Which of the following is the correct way to allocate memory dynamically in embedded C?

easy

A. int p = malloc(sizeof(int));

B. int *p = sizeof(int);

C. int *p = malloc(sizeof(int));

D. malloc(int p);

Solution

1. Step 1: Recall malloc syntax

   malloc returns a pointer to allocated memory, so pointer variable is needed.

2. Step 2: Check each option

   int *p = malloc(sizeof(int)); correctly assigns malloc's return to an int pointer. Others misuse types or syntax.

3. Final Answer:

   int *p = malloc(sizeof(int)); -> Option C

4. Quick Check:

   malloc returns pointer, assign to pointer [OK]

Hint: malloc returns pointer, assign to pointer variable [OK]

Common Mistakes:

• Assigning malloc result to non-pointer variable
• Using sizeof without malloc
• Incorrect function call syntax

3. What will happen if you declare a large array inside a function in embedded C on a system with limited stack size?

medium

A. Stack overflow may occur causing a crash

B. Heap memory will be used automatically

C. The array will be stored in flash memory

D. The compiler will convert it to a global variable

Solution

1. Step 1: Understand stack size limits

   Stack size is limited in embedded systems; large local arrays consume stack space.

2. Step 2: Consequence of large stack allocation

   Allocating large arrays locally can exceed stack size, causing overflow and crash.

3. Final Answer:

   Stack overflow may occur causing a crash -> Option A

4. Quick Check:

   Large local arrays risk stack overflow [OK]

Hint: Large local arrays risk stack overflow in embedded [OK]

Common Mistakes:

• Assuming heap is used automatically
• Thinking large arrays go to flash
• Believing compiler auto-converts locals to globals

4. Consider this code snippet in embedded C:

void func() {
  int *p = malloc(10 * sizeof(int));
  // ... use p ...
  // missing free(p);
}

What is the main problem here?

medium

A. Pointer p is not initialized

B. Stack overflow due to large local array

C. Syntax error in malloc usage

D. Memory leak because allocated heap memory is not freed

Solution

1. Step 1: Analyze malloc usage

   Memory is allocated on the heap with malloc but never freed.

2. Step 2: Identify consequences of missing free

   Not freeing heap memory causes memory leak, wasting limited embedded memory.

3. Final Answer:

   Memory leak because allocated heap memory is not freed -> Option D

4. Quick Check:

   Missing free causes memory leak [OK]

Hint: Always free malloc memory to avoid leaks [OK]

Common Mistakes:

• Confusing stack overflow with memory leak
• Thinking malloc syntax is wrong
• Assuming pointer is uninitialized

5. In an embedded system with 2KB stack and 4KB heap, which approach is best to safely allocate a 1KB buffer for temporary data?

hard

A. Declare a local array of 1KB inside a function

B. Use malloc to allocate 1KB on the heap and free after use

C. Declare a global 1KB array to avoid stack and heap

D. Use a static local array inside the function

Solution

1. Step 1: Evaluate stack usage for large buffer

   1KB local array uses half the stack, risking overflow in embedded systems.

2. Step 2: Consider heap allocation safety

   Allocating 1KB on heap with malloc is safer; heap is larger and managed manually.

3. Step 3: Compare global and static arrays

   Global/static arrays use fixed memory but reduce flexibility and increase RAM usage always.

4. Final Answer:

   Use malloc to allocate 1KB on the heap and free after use -> Option B

5. Quick Check:

   Heap allocation safer for large temporary buffers [OK]

Hint: Use heap malloc for large temporary buffers in embedded [OK]

Common Mistakes:

• Using large local arrays risking stack overflow
• Ignoring freeing heap memory
• Assuming global arrays are always best