UNIX for Advanced & Expert Users

/** Reverses bit order. MSB -> LSB and LSB -> MSB. */
unsigned int reverse(unsigned int x) {
    unsigned int ret = 0;
    for (int i=0; i<32; ++i) {
        if (x&0x1 != 0) {
            ret |= (0x80000000 >> i);
        }
        else {}
        x = (x >> 1);
    }
    return ret;
}

unsigned int crc32(unsigned char* message, unsigned int msgsize, unsigned int crc) {
    unsigned int i, j; // byte counter, bit counter
    unsigned int byte;
    unsigned int poly = 0x04C11DB7;
    i = 0;
    for (i=0; i<msgsize; ++i) {
        byte = message[i];       // Get next byte.
        byte = reverse(byte);    // 32-bit reversal.
        for (j=0; j<= 7; ++j) {  // Do eight times.
            if ((int)(crc ^ byte) < 0)
                crc = (crc << 1) ^ poly;
            else crc = crc << 1;
            byte = byte << 1;    // Ready next msg bit.
        }
    }
    return reverse(~crc);
}