Checking volatile value of address in C++

Question

I'm trying to implement a mailbox write for the Raspberry Pi. According to the info I found, I can write to address 0x2000B8A0 when mailbox is empty, meaning 0x2000B898 has not the last bit set. I wrote it like this:

uint32_t *mailbox = reinterpret_cast<uint32_t*>(0x2000B880);

while((mailbox[6] & 0x80000000) != 0);

mailbox[8] = value + channel;

But the disassembly shows that the value at mailbox[6] is only loaded once, before the loop, then it just repeats the check with that one value.

I could not find a solution because I don't even know the proper words for this problem. I'm sure it's simple but googling brought nothing for this special case.

Answer 1

Answer lies in title of your question. You should use the following:

volatile uint32_t *mailbox = const_cast<volatile uint32_t *>(reinterpret_cast<uint32_t*>(0x2000B880));

This will make sure the value is loaded each time in your loop. If you see any application not responding, consider adding some sleep or delay or yield in while.