I am using the USB CDC driver code provided by ST in stm32cubeide, and am trying to figure out a proper minimum heap size. I have found various slightly conflicting answers, but the consensus seems to be that 512 bytes is not enough.
In the same application, I also use FATFS, but I'm not 100% sure if that is supposed to use the heap.
Now I wanted to test just how much of the heap is in fact overwritten. To do this, I brutally stored the byte sequence AA BB CC DD over and over in the first 1 KB of the heap. I did that with this piece of code inserted in main.c, immediately after the entry point:
uint32_t *heapPtr;
uint32_t *heapOrigin;
(...)
/**
* @brief The application entry point.
* @retval int
*/
int main(void)
{
/* USER CODE BEGIN 1 */
uint16_t i;
heapOrigin = malloc(4);
free(heapOrigin);
heapPtr = heapOrigin;
slDebug6 = (uint32_t)heapOrigin;
for (i = 0; i < (1024 / sizeof(*heapOrigin)); i++)
{
*heapPtr = 0xDDCCBBAA;
heapPtr++;
}
/* USER CODE END 1 */
The malloc(4) function call in line 16 is there just to give me an address to the heap, as I know of no other way to do this. The for loop fills 1 kB with the byte sequence, from the beginning of the heap.
In the application, I can request a readout of the heap memory contents. Imagine my surprise when I discovered that, even though I'm communicating via the USB port, not a single byte in the heap is overwritten. My byte sequence is intact from the first byte (at 0x20006D80), through the following 1 KB.
Readout of SRAM from address 0x20006D80:
AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD
AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD
AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD
AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD
AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD
AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD
AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD
AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD
AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD AA BB CC DD
AA BB CC DD...
And so it continues.
Can someone explain what is going on here?
Have I completely misunderstood the heap? Or does the USB device driver actually not use the heap after all?
