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.