FreeRTOS heap implementations (heap_1 to heap_5) - Step-by-Step Execution

Concept Flow - FreeRTOS heap implementations (heap_1 to heap_5)

Start: Choose heap implementation

↓

heap_1: Simple malloc/free

↓

heap_2: malloc/free with coalescing

↓

heap_3: Uses standard malloc/free

↓

heap_4: malloc/free with best fit + coalescing

↓

heap_5: Multiple memory regions

↓

End

The flow shows selecting one of the five FreeRTOS heap implementations, each with different memory management features.

Execution Sample

FreeRTOS

/* Example: Using heap_4.c */
void *ptr = pvPortMalloc(100);  // Allocate 100 bytes
vPortFree(ptr);                 // Free allocated memory

This code allocates 100 bytes using heap_4 and then frees it, showing basic allocation and deallocation.

Execution Table

Step	Action	Heap Implementation	Memory State	Notes
1	Call pvPortMalloc(100)	heap_4	Block of 100 bytes allocated, free blocks adjusted	Best fit block chosen, free blocks coalesced if needed
2	Call vPortFree(ptr)	heap_4	Block of 100 bytes freed, adjacent free blocks merged	Memory coalescing reduces fragmentation
3	Call pvPortMalloc(50)	heap_1	Block of 50 bytes allocated, no coalescing	Simple allocation, no merging of free blocks
4	Call vPortFree(ptr)	heap_1	Block of 50 bytes freed, no coalescing	Memory fragmentation possible
5	Call pvPortMalloc(200)	heap_3	Standard malloc allocates 200 bytes	Uses system malloc/free
6	Call vPortFree(ptr)	heap_3	Standard free called	Relies on system heap
7	Call pvPortMalloc(150)	heap_5	Allocates 150 bytes from multiple regions	Supports multiple memory regions
8	Call vPortFree(ptr)	heap_5	Block freed in correct region	Memory managed per region
9	Exit	-	-	All allocations and frees completed

💡 All memory allocations and frees completed for different heap implementations

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	Final
ptr	NULL	Pointer to 100-byte block (heap_4)	NULL (freed)	Pointer to 50-byte block (heap_1)	NULL (freed)	Pointer to 200-byte block (heap_3)	NULL (freed)	Pointer to 150-byte block (heap_5)	NULL (freed)	NULL

Key Moments - 3 Insights

Why does heap_1 not merge free blocks after freeing memory?

heap_1 only frees memory but does not coalesce adjacent free blocks, so fragmentation can occur as shown in steps 3 and 4 of the execution_table.

How does heap_4 reduce fragmentation compared to heap_1?

heap_4 uses best fit allocation and merges adjacent free blocks when freeing memory, reducing fragmentation as seen in steps 1 and 2.

What is special about heap_5 compared to other heap implementations?

heap_5 manages multiple separate memory regions, allocating and freeing memory in the correct region, demonstrated in steps 7 and 8.

Visual Quiz - 3 Questions

Look at the execution_table, what happens to the pointer 'ptr' after step 2?

A It points to the freed 100-byte block

B It points to a new allocated block

C It becomes NULL after freeing

D It remains unchanged

💡 Hint

Check variable_tracker column 'After Step 2' for 'ptr' value

At which step does the heap implementation use system malloc/free?

A Step 1

B Step 5

C Step 3

D Step 7

💡 Hint

Look at execution_table 'Heap Implementation' column for 'heap_3'

If heap_1 was changed to coalesce free blocks, which step's memory state would change?

A Step 4

B Step 2

C Step 6

D Step 8

💡 Hint

Refer to key_moments about heap_1 behavior and execution_table step 4

Concept Snapshot

FreeRTOS offers 5 heap implementations:
heap_1: Simple malloc/free, no coalescing
heap_2: malloc/free with coalescing
heap_3: Uses system malloc/free
heap_4: Best fit malloc/free with coalescing
heap_5: Supports multiple memory regions
Choose based on fragmentation and memory needs.

Full Transcript

This visual execution shows how FreeRTOS heap implementations manage memory differently. Starting from choosing a heap type, heap_1 simply allocates and frees memory without merging free blocks, which can cause fragmentation. heap_2 improves by coalescing adjacent free blocks. heap_3 relies on the system's malloc and free functions. heap_4 uses a best fit strategy and merges free blocks to reduce fragmentation. heap_5 supports multiple memory regions, allocating and freeing memory in the correct region. The execution table traces allocation and freeing steps for each heap type, showing pointer changes and memory state. Variable tracking follows the pointer 'ptr' through allocations and frees. Key moments clarify why coalescing matters and how heap_5 differs. The quiz tests understanding of pointer states, heap types, and effects of coalescing. The snapshot summarizes the five heap implementations and their key features.

Practice

1. Which FreeRTOS heap implementation does NOT support freeing allocated memory?

easy

A. heap_1

B. heap_2

C. heap_3

D. heap_4

Solution

1. Step 1: Understand heap_1 behavior

   heap_1 only allows allocation but does not provide a function to free memory once allocated.

2. Step 2: Compare with other heaps

   heap_2, heap_3, and heap_4 all support freeing memory, with heap_3 using standard malloc/free.

3. Final Answer:

   heap_1 -> Option A

4. Quick Check:

   heap_1 no free support = heap_1 [OK]

Hint: heap_1 never frees memory, others do [OK]

Common Mistakes:

• Confusing heap_1 with heap_2 which supports free
• Assuming all heaps support freeing memory
• Mixing heap_3's malloc/free with heap_1's behavior

2. Which of the following is the correct way to include the heap_3 implementation in your FreeRTOS project?

easy

A. #include "heap3.c"

B. #include "heap_3.c"

C. #include "FreeRTOSHeap3.c"

D. #include

Solution

1. Step 1: Check FreeRTOS heap file naming

   Heap implementations are included by adding the corresponding .c file, e.g., "heap_3.c".

2. Step 2: Verify correct syntax

   Using #include "heap_3.c" is the standard way to include heap_3 in the build.

3. Final Answer:

   #include "heap_3.c" -> Option B

4. Quick Check:

   Include heap_3.c with quotes = D [OK]

Hint: Include heap source file with quotes, not angle brackets [OK]

Common Mistakes:

• Using angle brackets instead of quotes
• Incorrect file name spelling or casing
• Trying to include a header instead of the .c file

3. Given the following code snippet using heap_4, what will be the output if the allocation fails?

void *ptr = pvPortMalloc(1000);
if(ptr == NULL) {
    printf("Allocation failed\n");
} else {
    printf("Allocation succeeded\n");
}

medium

A. Compilation error

B. Allocation succeeded

C. Allocation failed

D. No output

Solution

1. Step 1: Understand heap_4 allocation behavior

   heap_4 supports allocation and freeing with coalescing. If memory is insufficient, pvPortMalloc returns NULL.

2. Step 2: Analyze the if condition

   If ptr is NULL, the code prints "Allocation failed"; otherwise, it prints "Allocation succeeded".

3. Final Answer:

   Allocation failed -> Option C

4. Quick Check:

   pvPortMalloc NULL means failure = A [OK]

Hint: pvPortMalloc returns NULL on failure [OK]

Common Mistakes:

• Assuming allocation always succeeds
• Confusing NULL with zero pointer
• Ignoring the if condition logic

4. You are using heap_2 in your FreeRTOS project but notice memory is not being freed properly. Which of the following is the most likely cause?

medium

A. heap_2 does not support freeing memory

B. heap_2 uses standard malloc/free which is not thread-safe

C. heap_2 requires manual coalescing of free blocks

D. You forgot to call vPortFree() after allocation

Solution

1. Step 1: Recall heap_2 features

   heap_2 supports freeing memory via vPortFree(), but does not coalesce free blocks.

2. Step 2: Identify common mistake

   If memory is not freed, the most common cause is forgetting to call vPortFree() after allocation.

3. Final Answer:

   You forgot to call vPortFree() after allocation -> Option D

4. Quick Check:

   Missing vPortFree causes leaks = B [OK]

Hint: Always call vPortFree() to release heap_2 memory [OK]

Common Mistakes:

• Assuming heap_2 lacks free support
• Confusing heap_2 with heap_1 or heap_3
• Thinking heap_2 uses malloc/free internally

5. You want to use a FreeRTOS heap implementation that supports both allocation and freeing, minimizes fragmentation by coalescing adjacent free blocks, and is suitable for real-time systems. Which heap should you choose?

hard

A. heap_4

B. heap_2

C. heap_1

D. heap_5

Solution

1. Step 1: Review heap features

   heap_1 does not support freeing; heap_2 supports free but no coalescing; heap_4 supports free and coalesces adjacent free blocks; heap_5 extends heap_4 with multiple memory regions.

2. Step 2: Match requirements

   For minimizing fragmentation and real-time suitability, heap_4 is ideal as it coalesces free blocks and is deterministic.

3. Final Answer:

   heap_4 -> Option A

4. Quick Check:

   heap_4 coalesces free blocks = A [OK]

Hint: heap_4 coalesces free blocks, best for fragmentation [OK]

Common Mistakes:

• Choosing heap_5 without needing multiple regions
• Assuming heap_2 coalesces free blocks
• Picking heap_1 which lacks free support