# Custom message signature fail **URL:** \ **Category:** Libcanard\ **Created:** [February 25, 2019, 3:50pm UTC](https://forum.opencyphal.org/t/custom-message-signature-fail/372 "2019-02-25T15:50:25Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Sergey\_Frolov](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/s/b19c9b/32.png) [@Sergey\_Frolov](https://forum.opencyphal.org/u/Sergey_Frolov)\ **Post date:** [February 25, 2019, 3:50pm UTC](https://forum.opencyphal.org/t/custom-message-signature-fail/372/1 "2019-02-25T15:50:25Z")
Hello. I use Libcanard with STM32F4. And I compiled my own custom message. GUI looks it fine. I have no troubles when I send messages less than 7 bytes, but when I try to send more bytes per 1 message, bus monitor print this: ![crcfail](https://forum.opencyphal.org/uploads/default/original/1X/c660630a658d24d51e8128ab18718f68ac68fd92.png) “CRC mismatch: expected 697d, got 57b5…” In canard core I see that crc calculated using ‘signature’-field (if size \> 7 bytes). It seems, that calculated signature (through dsdl) is wrong. How can I calculate it correct (manually)?? So GUI tool still send errors. Thanks ---
**Author:** ![pavel.kirienko](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/pavel.kirienko/32/9_2.png) [@pavel.kirienko](https://forum.opencyphal.org/u/pavel.kirienko)\ **Post date:** [February 25, 2019, 4:05pm UTC](https://forum.opencyphal.org/t/custom-message-signature-fail/372/2 "2019-02-25T16:05:07Z")
The most likely reason is that the message definition used by the GUI tool differs from the one used by your libcanard-based system. Please ensure that the definitions are exactly identical. Also, you can’t define vendor-specific messages inside the UAVCAN namespace. You should use a separate root namespace for your custom data types (although this is not directly related to your question). ---
**Author:** ![Sergey\_Frolov](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/s/b19c9b/32.png) [@Sergey\_Frolov](https://forum.opencyphal.org/u/Sergey_Frolov)\ **Post date:** [February 25, 2019, 5:14pm UTC](https://forum.opencyphal.org/t/custom-message-signature-fail/372/3 "2019-02-25T17:14:52Z")
Thanks. But this works fine in another board with Nuttx and libuavcan. That says that signature-field is correct. And we able to include own message to uavcan-namespace… Hm… As for libcanard, troubles are still not solved. ---
**Author:** ![pavel.kirienko](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/pavel.kirienko/32/9_2.png) [@pavel.kirienko](https://forum.opencyphal.org/u/pavel.kirienko)\ **Post date:** [February 25, 2019, 7:27pm UTC](https://forum.opencyphal.org/t/custom-message-signature-fail/372/4 "2019-02-25T19:27:51Z")
> [@Sergey\_Frolov](#): > > As for libcanard, troubles are still not solved. Let’s have a look at your code. ---
**Author:** ![Sergey\_Frolov](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/s/b19c9b/32.png) [@Sergey\_Frolov](https://forum.opencyphal.org/u/Sergey_Frolov)\ **Post date:** [February 26, 2019, 7:00am UTC](https://forum.opencyphal.org/t/custom-message-signature-fail/372/5 "2019-02-26T07:00:37Z")
Ok. I hava a message template, described as: ```auto uint8 id void5 UvrValue value ``` where UveValue is: ```auto @union # Tag is 3 bit long, so outer structure has 5-bit prefix to ensure proper alignment uint8 bool_value int8 byte_value # 8-bit value is used for alignment reasons int16 short_value int32 integer_value int64 long_value ``` And function, where I try to send int64 value (tag=4 from union above) ```auto uint8_t n = 0; uint8_t tag=4; int offset = 0; uint8_t buffer[32]; static uint8_t transferId = 0; uint8_t id=1; int64_t abs= 0x0000FF55; memset(buffer,0x00,32); canardEncodeScalar(buffer, offset, 8, &id); //id offset += 8; canardEncodeScalar(buffer, offset, 5, &n); //void5 offset offset += 5; canardEncodeScalar(buffer, offset, 3, &tag); //tag to union member offset += 3; canardEncodeScalar(buffer, offset, 64, &abs); //int64 value canardBroadcast(&g_canard, UVR_ARRAY_SIGNATURE, UVR_ARRAY_ID, &transferId, CANARD_TRANSFER_PRIORITY_HIGH, buffer, 10); ``` I have doubts for last argument of canardBroadcast (length of packet), but I see - 8+5(void)+3+64=80 (10 bytes). But CRC is calculated with fail. If i try send for example int32 value (tag=3), length=6 (\<7) than the transmission is OK ---
**Author:** ![Sergey\_Frolov](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/s/b19c9b/32.png) [@Sergey\_Frolov](https://forum.opencyphal.org/u/Sergey_Frolov)\ **Post date:** [February 26, 2019, 7:45am UTC](https://forum.opencyphal.org/t/custom-message-signature-fail/372/6 "2019-02-26T07:45:34Z")
So, it seems that signature is wrong. In libuavcan i cannot find using of signature-field in ‘add\_crc’-function (in contrast of libcanard). Libuavcan uses separate CRC-table (not Signature-field) That’s why libuavcan works fine… I’m clearly sure, that signature is wrong. And CRC-value also returns with fail ```auto if (payload_len > 7) { crc = crcAddSignature(crc, data_type_signature); crc = crcAdd(crc, payload, payload_len); } ``` ---
**Author:** ![pavel.kirienko](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/pavel.kirienko/32/9_2.png) [@pavel.kirienko](https://forum.opencyphal.org/u/pavel.kirienko)\ **Post date:** [February 26, 2019, 9:17am UTC](https://forum.opencyphal.org/t/custom-message-signature-fail/372/7 "2019-02-26T09:17:59Z")
Yes, indeed, the serialization code seems correct; you need to check the value of `UVR_ARRAY_SIGNATURE`. You can use this script to compute and display the signature: [https://github.com/UAVCAN/libcanard/blob/master/show\_data\_type\_info.py](https://github.com/UAVCAN/libcanard/blob/master/show_data_type_info.py) ---
**Author:** ![Sergey\_Frolov](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/s/b19c9b/32.png) [@Sergey\_Frolov](https://forum.opencyphal.org/u/Sergey_Frolov)\ **Post date:** [February 26, 2019, 10:54am UTC](https://forum.opencyphal.org/t/custom-message-signature-fail/372/8 "2019-02-26T10:54:27Z")
Good day!! O, great thanks.! I launched script and got a valid ‘signature’-field for my message. And now it works clearly fine. I want specify this moment (from replies below): > “Also, you can’t define vendor-specific messages inside the UAVCAN namespace. You should use a separate root namespace for your custom data types.” I little confused …how can I define my own namespace? I see extension of file like \*.uavcan. Is it means that i need use another extension (as own namespace) for custom messages? ---
**Author:** ![pavel.kirienko](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/pavel.kirienko/32/9_2.png) [@pavel.kirienko](https://forum.opencyphal.org/u/pavel.kirienko)\ **Post date:** [February 26, 2019, 11:27am UTC](https://forum.opencyphal.org/t/custom-message-signature-fail/372/9 "2019-02-26T11:27:12Z")
> [@Sergey\_Frolov](#): > > I see extension of file like \*.uavcan. Is it means that i need use another extension (as own namespace) for custom messages? Nope, you still use the same `*.uavcan` extension but you put your files into a separate root namespace. This tutorial here should clarify things (ignore the fact that it’s written for libuavcan; same principles apply for any implementation): [Open technology for real-time intravehicular distributed computing and communication based on modern networking standards](https://uavcan.org/Implementations/Libuavcan/Tutorials/8._Custom_data_types/)