pvPortMalloc and vPortFree in FreeRTOS - Step-by-Step Execution

Concept Flow - pvPortMalloc and vPortFree

Call pvPortMalloc(size)

↓

Check if enough heap memory

Yes / No↓

Allocate memory

↓

Return pointer to allocated memory

↓

Use memory

↓

Call vPortFree(pointer)

↓

Return memory to heap

↓

Memory available for future allocations

pvPortMalloc tries to allocate memory from the FreeRTOS heap and returns a pointer or NULL if not enough memory. vPortFree releases the memory back to the heap for reuse.

Execution Sample

FreeRTOS

void *ptr = pvPortMalloc(100);
if(ptr != NULL) {
  // use memory
  vPortFree(ptr);
}

This code allocates 100 bytes from the FreeRTOS heap, uses it if successful, then frees it.

Execution Table

Step	Action	Heap State Before	Result	Heap State After
1	Call pvPortMalloc(100)	Heap free: 1024 bytes	Allocates 100 bytes	Heap free: 924 bytes
2	Check if pointer is NULL	Pointer: valid	Pointer not NULL, proceed	Heap free: 924 bytes
3	Use allocated memory	Heap free: 924 bytes	Memory used	Heap free: 924 bytes
4	Call vPortFree(ptr)	Heap free: 924 bytes	Memory freed	Heap free: 1024 bytes
5	End	Heap free: 1024 bytes	All memory returned	Heap free: 1024 bytes

💡 Memory freed and heap restored to original free size

Variable Tracker

Variable	Start	After pvPortMalloc	After vPortFree	Final
ptr	NULL	Valid pointer (address)	NULL or invalid (freed)	NULL or invalid (freed)
Heap free	1024 bytes	924 bytes	1024 bytes	1024 bytes

Key Moments - 3 Insights

Why does pvPortMalloc sometimes return NULL?

pvPortMalloc returns NULL if there is not enough free heap memory to allocate the requested size, as shown in step 1 of the execution_table.

What happens if we forget to call vPortFree?

If vPortFree is not called, the allocated memory stays reserved and the heap free size does not increase, causing memory leaks and less available memory for future allocations.

Can we use the pointer after calling vPortFree?

No, after calling vPortFree, the pointer points to freed memory and using it can cause errors. The pointer should be considered invalid as shown after step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the heap free size after pvPortMalloc(100) is called?

A 1024 bytes

B 100 bytes

C 924 bytes

D 0 bytes

💡 Hint

Check the 'Heap State After' column in step 1 of the execution_table.

At which step does the memory get returned to the heap?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look for the action 'Call vPortFree(ptr)' in the execution_table.

If pvPortMalloc is called with a size larger than available heap, what will be the result?

A Returns NULL

B Frees memory automatically

C Returns a valid pointer

D Crashes the system

💡 Hint

Refer to key_moments about pvPortMalloc returning NULL when memory is insufficient.

Concept Snapshot

pvPortMalloc(size): Allocates memory from FreeRTOS heap.
Returns pointer or NULL if no memory.
vPortFree(ptr): Frees allocated memory back to heap.
Always free memory to avoid leaks.
Check pointer != NULL before use.

Full Transcript

This visual execution trace shows how pvPortMalloc and vPortFree work in FreeRTOS. First, pvPortMalloc is called with a size to allocate memory from the heap. If enough memory is available, it returns a pointer and reduces the heap free size. The program uses the memory, then calls vPortFree to release it back to the heap, restoring the free size. If memory is insufficient, pvPortMalloc returns NULL. Forgetting to free memory causes leaks. Using a pointer after freeing is invalid. The trace helps beginners see each step and variable change clearly.

Practice

1. What is the primary purpose of pvPortMalloc in FreeRTOS?

easy

A. To suspend a running task temporarily

B. To free allocated memory back to the system

C. To create a new task in FreeRTOS

D. To allocate memory safely for tasks and resources

Solution

1. Step 1: Understand the role of pvPortMalloc

   pvPortMalloc is designed to allocate memory safely in FreeRTOS, especially in multitasking environments.

2. Step 2: Differentiate from other functions

   Functions like vPortFree free memory, and task creation or suspension are unrelated to memory allocation.

3. Final Answer:

   To allocate memory safely for tasks and resources -> Option D

4. Quick Check:

   pvPortMalloc = allocate memory [OK]

Hint: pvPortMalloc always allocates memory safely [OK]

Common Mistakes:

• Confusing pvPortMalloc with vPortFree
• Thinking pvPortMalloc creates tasks
• Assuming it suspends tasks

2. Which of the following is the correct syntax to free memory allocated by pvPortMalloc?

easy

A. vPortFree(pointer);

B. pvPortMalloc(pointer);

C. free(pointer);

D. vPortMalloc(pointer);

Solution

1. Step 1: Identify the function to free memory

   Memory allocated by pvPortMalloc must be freed using vPortFree to avoid memory leaks.

2. Step 2: Check syntax correctness

   The correct syntax is vPortFree(pointer);. Other options either allocate memory or use standard C free, which is not recommended in FreeRTOS.

3. Final Answer:

   vPortFree(pointer); -> Option A

4. Quick Check:

   Free memory with vPortFree [OK]

Hint: Use vPortFree to release memory allocated by pvPortMalloc [OK]

Common Mistakes:

• Using free() instead of vPortFree()
• Calling pvPortMalloc to free memory
• Incorrect function names like vPortMalloc

3. Consider the following FreeRTOS code snippet:

void *ptr = pvPortMalloc(100);
if(ptr != NULL) {
    vPortFree(ptr);
    ptr = NULL;
}
printf("%p", ptr);

What will be the output of the printf statement?

medium

A. A memory address (non-NULL pointer)

B. Compilation error

C. NULL (0x0 or equivalent)

D. Random garbage value

Solution

1. Step 1: Analyze memory allocation and freeing

   pvPortMalloc(100) allocates memory and returns a pointer. Then vPortFree(ptr) frees it, and ptr is set to NULL.

2. Step 2: Understand the printf output

   Since ptr is explicitly set to NULL, printf will print 0x0 or equivalent NULL pointer representation.

3. Final Answer:

   NULL (0x0 or equivalent) -> Option C

4. Quick Check:

   Pointer set to NULL after free = NULL output [OK]

Hint: Pointer set to NULL after free prints NULL [OK]

Common Mistakes:

• Assuming pointer still holds address after free
• Expecting compilation error due to free
• Thinking printf prints garbage

4. Identify the error in the following FreeRTOS code snippet:

void *ptr = pvPortMalloc(50);
// Some code using ptr
vPortFree(ptr);
vPortFree(ptr);

medium

A. Double free of the same pointer

B. Missing check if pvPortMalloc returned NULL

C. Incorrect function name vPortFree

D. Memory leak due to not freeing ptr

Solution

1. Step 1: Review memory allocation and freeing

   pvPortMalloc(50) allocates memory and returns a pointer stored in ptr. The first vPortFree(ptr) frees this memory.

2. Step 2: Identify the problem with second free

   The second vPortFree(ptr) attempts to free the same memory again, causing a double free error which can crash or corrupt memory.

3. Final Answer:

   Double free of the same pointer -> Option A

4. Quick Check:

   Freeing pointer twice causes error [OK]

Hint: Never free the same pointer twice without resetting it [OK]

Common Mistakes:

• Ignoring double free risks
• Not checking for NULL after malloc
• Confusing function names

5. You want to allocate memory for an array of 10 integers using FreeRTOS memory management. Which code snippet correctly allocates and frees this memory?

hard

A. int *arr = (int *)pvPortMalloc(sizeof(int));
if(arr != NULL) {
// use arr
vPortFree(arr);
}

B. int *arr = (int *)pvPortMalloc(10 * sizeof(int));
if(arr != NULL) {
// use arr
vPortFree(arr);
}

C. int *arr = (int *)malloc(10 * sizeof(int));
if(arr != NULL) {
// use arr
free(arr);
}

D. int *arr = (int *)vPortFree(10 * sizeof(int));
if(arr != NULL) {
// use arr
pvPortMalloc(arr);
}

Solution

1. Step 1: Allocate correct memory size with pvPortMalloc

   To store 10 integers, allocate 10 times the size of int using pvPortMalloc(10 * sizeof(int)). This ensures enough space.

2. Step 2: Free memory properly with vPortFree

   After using the array, free the memory with vPortFree(arr). Also, check if allocation succeeded by verifying arr != NULL.

3. Final Answer:

   Correct allocation and freeing using pvPortMalloc and vPortFree -> Option B

4. Quick Check:

   Allocate 10 ints with pvPortMalloc, free with vPortFree [OK]

Hint: Multiply size by count in pvPortMalloc, free with vPortFree [OK]

Common Mistakes:

• Allocating only size of one int instead of 10
• Using standard malloc/free instead of pvPortMalloc/vPortFree
• Confusing allocation and free function calls