pvPortMalloc and vPortFree in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this FreeRTOS memory allocation code?

Consider this FreeRTOS snippet using pvPortMalloc and vPortFree. What will be printed?

FreeRTOS

void *ptr = pvPortMalloc(100);
if(ptr != NULL) {
    printf("Allocated 100 bytes\n");
    vPortFree(ptr);
    printf("Memory freed\n");
} else {
    printf("Allocation failed\n");
}

A

Allocated 100 bytes
Memory freed

B Allocation failed

C Allocated 100 bytes

D Memory freed

💡 Hint

Think about what happens if pvPortMalloc returns a valid pointer.

If pvPortMalloc succeeds, it returns a non-NULL pointer, so both messages print. Otherwise, only the failure message prints.

🧠 Conceptual

intermediate

What happens if you call vPortFree with a NULL pointer?

In FreeRTOS, what is the expected behavior when vPortFree(NULL) is called?

A It frees the entire heap memory

B It causes a runtime error and crashes the system

C It safely does nothing and returns immediately

D It allocates new memory instead

💡 Hint

Think about how standard C free(NULL) behaves.

Calling vPortFree(NULL) is safe and does nothing, similar to standard C free(NULL).

🔧 Debug

advanced

Why does this FreeRTOS code cause a heap corruption error?

Analyze the code below. Why might it cause heap corruption?

FreeRTOS

void *ptr = pvPortMalloc(50);
// ... some code ...
vPortFree(ptr);
vPortFree(ptr);

A pvPortMalloc does not allocate memory properly

B Double free of the same pointer causes heap corruption

C vPortFree requires size parameter which is missing

D Freeing memory twice is allowed and safe

💡 Hint

What happens if you free the same memory twice?

Calling vPortFree twice on the same pointer causes heap corruption because the memory is freed twice.

📝 Syntax

advanced

Which option correctly allocates and frees memory in FreeRTOS?

Choose the code snippet that correctly uses pvPortMalloc and vPortFree without syntax errors.

A

char *buf = pvPortMalloc(128);
if(buf) {
  vPortFree(buf);
}

B

char *buf = pvPortMalloc(128)
if(buf) {
  vPortFree(buf)
}

C

char *buf = pvPortMalloc(128);
if(buf) {
  vPortFree();
}

D

char *buf = pvPortMalloc(128);
if(buf) {
  // use buf
  vPortFree(buf);
}

💡 Hint

Check for missing semicolons and correct function calls.

Option D has correct syntax with semicolons and proper vPortFree call. Option D misses semicolons, A is syntactically correct, D calls vPortFree without argument.

🚀 Application

expert

How many items are in the heap after this sequence of allocations and frees?

Given the following FreeRTOS code, how many allocated blocks remain in the heap?

FreeRTOS

void *p1 = pvPortMalloc(20);
void *p2 = pvPortMalloc(30);
vPortFree(p1);
void *p3 = pvPortMalloc(40);
vPortFree(p2);

A 1

B 2

C 3

D 0

💡 Hint

Count how many pointers are still allocated and not freed.

p1 and p2 are freed, p3 is allocated and not freed, so only one block remains allocated.