Searching for value in pointer: Reading 4 bytes given uint8_t pointer memory start location and size

Question

I'm trying to search through a chunk of memory for a 4 byte hex value (0xAABBAABB) and then copy out the 4 bytes before that into a separate variable.

0xAABBAABB is the message terminator and I'm after the 4 bytes that precede this.

I'm given the start of the location of the data in memory from a *uint8_t and a message size. So the *uint8 holds the first 2 bytes of the message.

Any Help would be appreciated.

Thanks

Answer 1

Try something like this:

uint8_t *msg = ...;
int msgsize = ...;

...

uint8_t bytes[4];
bool found = false;

msg += 4;
msgsize -= 4;
while (msgsize >= 4)
{
    if (*(uint32_t*)msg == 0xAABBAABB)
    {
        memcpy(bytes, msg-4,  4);
        found = true;
        break;
    }

    ++msg;
    --msgsize;
}

Or this:

uint8_t *msg = ...;
int msgsize = ...;

...

const uint8_t term[4] = {0xAA, 0xBB, 0xAA, 0xBB};
uint8_t bytes[4];
bool found = false;

msg += 4;
msgsize -= 4;
while (msgsize >= 4)
{
    if (memcmp(msg, term, 4) == 0)
    {
        memcpy(bytes, msg-4,  4);
        found = true;
        break;
    }

    ++msg;
    --msgsize;
}