What does it mean the following C statement? ((int*) & var)

Question

I have the following code in C to calculate a CRC16-USB for some data flows:

uint16_t DRV_CANFDSPI_CalculateCRC16(uint8_t* data, uint16_t size)
{
    uint16_t init = CRCBASE;
    uint8_t index;

    while (size-- != 0) {
        index = ((uint8_t*) & init)[1] ^ *data++;
        init = (init << 8) ^ crc16_table[index];
    }

    return init;
}

Where crc16_table is an array of some hex values of 2 bytes (like 0xAFF3) , and data is an array of hex values of 1 byte (like 0xA3) representing the data stream (aqcuired by other means). Size is the length of the data array.

I want to reproduce this piece of code in Python, but I don't know that this statement means:

index = ((uint8_t*) & init)[1] ^ *data++;

I would like to understand that does this statament means and does, so I can reproduce it in Python. I am not an expert in C but have some knowledge, and I mostly undestand the rest of the code, but this line is giving me a headache.

Thanks and have a nice day!

Answer 1

init has type uint16_t, so the expression &init has type "pointer to uint16_t", or uint16_t *. The expression (uint8_t *) &init means "get the address of init, but treat that as the address of a uint8_t object, rather than a uint16_t object".

That address is then used with a subscript operator - ((uint8_t *) &init)[1], which is basically equivalent to "treat init as an array of two uint8_t objects, and give me the value of the second element in that array".

Graphically:

      +---+
init: |   | <-- ((uint8_t *) &init)[0]
      +---+
      |   | <-- ((uint8_t *) &init)[1]
      +---+

So, basically, you're grabbing the lower 8 bits of init, bitwise XORing that with the value of the current byte of the input message, and then advancing data to point to the next byte of the input message.