CRC16-TVB checksum custom dissector CRC

Question

Hi Iam developing custom dissector using wieshark 1.113 , iam trying for CRC tree uusing crc16_ccitt_tvb() (my polynominal is (x16 + x12 + x5 + 1))and i can able to see results correctly.

But its showing reverse (eg. my 2 byte CRC is 52 AC my wireshark shows as CRC 0X52AC [CORRECT] but actually i suppose to get CRC 0XAC52 [CORRECT] here below my code.

checksum_offset = reported_length - 2;
checksum = tvb_get_ntohs(tvb, checksum_offset);
checksum_calculated = crc16_ccitt_tvb(tvb, checksum_offset);
checksum_calculated = g_htons(checksum_calculated);
  /* Note: g_htons() macro may eval arg multiple times */
if (checksum == checksum_calculated) {
checksum_ti = proto_tree_add_uint_format_value(PARENT_tree, hf_PARENT_CRC, tvb, checksum_offset, 2, 0, "0x%04x [correct]", checksum, ENC_LITTLE_ENDIAN);
checksum_tree = proto_item_add_subtree(checksum_ti, ett_PARENT_CRC);
proto_tree_add_boolean(checksum_tree, hf_PARENT_cksum_gd, tvb,checksum_offset, 2, TRUE);
proto_tree_add_boolean(checksum_tree, hf_PARENT_cksum_bd, tvb, checksum_offset, 2, FALSE);
} else {
checksum_ti = proto_tree_add_uint_format_value(PARENT_tree, hf_PARENT_CRC, tvb, checksum_offset, 2, 0, "0x%04x [incorrect, should be 0x%04x]", checksum, checksum_calculated, ENC_LITTLE_ENDIAN);
checksum_tree = proto_item_add_subtree(checksum_ti, ett_PARENT_CRC);
proto_tree_add_boolean(checksum_tree, hf_PARENT_cksum_gd, tvb, checksum_offset, 2, FALSE);
proto_tree_add_boolean(checksum_tree, hf_PARENT_cksum_bd, tvb, checksum_offset, 2, TRUE);
}

Answer 1

Just a wild guess: Please try ENC_BIG_ENDIAN instead of ENC_LITTLE_ENDIAN.

Regards
Kurt

Answer 2

proto_tree_add_uint_format_value() takes, as the value argument and as arguments to the format string, values in the host byte order, so don't do

checksum_calculated = g_htons(checksum_calculated);

Also, all arguments to proto_tree_add_uint_format_value() after the format string are arguments to the format string, and proto_tree_add_uint_format_value() doesn't fetch any values, so leave the ENC_LITTLE_ENDIAN argument out.