Calculate checksum for an xbee frame on iOS

xbee.c

void checksum(uint8_t *buf, int length, uint8_t *dstbuf, int *retlength) {
  uint16_t sum = 0;
  // then calculate checksum
  for (int i=3;i<length-1;i++) {
    sum+=  buf[i];
  }
  
  sum = CFSwapInt16HostToBig(sum);
  uint8_t right = (sum >> 8) & 0xff;
  uint8_t checksum = 0xFF - right;
  buf[length -1] = checksum;
  
  // We escape everything but the 1st 0x7D
  // START_BYTE (0x7e), ESCAPE (0x7d), XON (0x11) and XOFF (0x13)
  NSUInteger pos = 1;
  uint8_t *ptrb = buf;
  // skip 1st char
  ptrb++;
  dstbuf[0] = XBEE_START_MARK;
  for (int i=1;i<length;i++) {
    // escape if needed (api mode 2)
    // ESCAPE (0x7d)
    if (*ptrb == XBEE_START_MARK || *ptrb == 0x7d|| *ptrb == 0x11 || *ptrb == 0x13) {
      dstbuf[pos] = 0x7d;
      pos++;
      dstbuf[pos] = *ptrb ^ 0x20;
    } else {
      dstbuf[pos] = *ptrb;
    }
    pos++;
    ptrb++;
  }
  *retlength = pos;
}