# Various questions about libcanard

**URL:** <https://forum.opencyphal.org/t/various-questions-about-libcanard/928>\
**Category:** Libcanard\
**Created:** [August 25, 2020, 6:52am UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928 "2020-08-25T06:52:13Z")\
**Posts on this page:** 20\
**Page:** 1

<div class="post-metadata">

**Author:** ![newcomer](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/n/e5b9ba/32.png) [@newcomer](https://forum.opencyphal.org/u/newcomer)\
**Post date:** [August 25, 2020, 6:52am UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/1 "2020-08-25T06:52:13Z")

</div>

```auto
uint8_t mode = canardDSDLGetU8(heartbeat_transfer->payload, heartbeat_transfer->payload_size, 34, 3);
uint32_t uptime = canardDSDLGetU32(heartbeat_transfer->payload, heartbeat_transfer->payload_size, 0, 32);
uint32_t vssc = canardDSDLGetU32(heartbeat_transfer->payload, heartbeat_transfer->payload_size, 37, 19);
uint8_t health = canardDSDLGetU8(heartbeat_transfer->payload, heartbeat_transfer->payload_size, 32, 2);

```

I am confused about “heartbeat\_transfer-\>payload, heartbeat\_transfer-\>payload\_size”.  
How can i access the file “32085.Heartbeat.1.0.uavcan” to make it as a buffer(heartbeat\_transfer-\>payload)?

---

<div class="post-metadata">

**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:** [August 25, 2020, 6:58am UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/2 "2020-08-25T06:58:50Z")

</div>

Please read [the rest of the documentation](https://github.com/UAVCAN/libcanard):

```auto
CanardRxSubscription heartbeat_subscription;
(void) canardRxSubscribe(&ins, // Subscribe to messages uavcan.node.Heartbeat.
                         CanardTransferKindMessage,
                         32085, // The fixed Subject-ID of the Heartbeat message type (see DSDL definition).
                         7, // The maximum payload size (max DSDL object size) from the DSDL definition.
                         CANARD_DEFAULT_TRANSFER_ID_TIMEOUT_USEC,
                         &heartbeat_subscription);
CanardTransfer transfer;
const int8_t result = canardRxAccept(&ins,
                                     &received_frame, // The CAN frame received from the bus.
                                     redundant_interface_index, // If the transport is not redundant, use 0.
                                     &transfer);
if (result < 0)
{
    // An error has occurred: either an argument is invalid or we've ran out of memory.
    // It is possible to statically prove that an out-of-memory will never occur for a given application if
    // the heap is sized correctly; for background, refer to the Robson's Proof and the documentation for O1Heap.
    // Reception of an invalid frame is NOT an error.
    abort();
}
else if (result == 1)
{
    processReceivedTransfer(redundant_interface_index, &transfer); // A transfer has been received, process it.
    ins.memory_free(&ins, (void*)transfer.payload); // Deallocate the dynamic memory afterwards.
}
else
{
    // Nothing to do.
    // The received frame is either invalid or it's a non-last frame of a multi-frame transfer.
    // Reception of an invalid frame is NOT reported as an error because it is not an error.
}

```

---

<div class="post-metadata">

**Author:** ![newcomer](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/n/e5b9ba/32.png) [@newcomer](https://forum.opencyphal.org/u/newcomer)\
**Post date:** [August 26, 2020, 6:53am UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/3 "2020-08-26T06:53:20Z")

</div>

The contents of files suffixed with .uavcan are of serialization type?

---

<div class="post-metadata">

**Author:** ![newcomer](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/n/e5b9ba/32.png) [@newcomer](https://forum.opencyphal.org/u/newcomer)\
**Post date:** [August 26, 2020, 7:39am UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/4 "2020-08-26T07:39:47Z")

</div>

I still don’t know how to use “uint8\_t mode = canardDSDLGetU8(heartbeat\_transfer-\>payload, heartbeat\_transfer-\>payload\_size, 34, 3);” to get data from 32085.Heartbeat.1.0.uavcan. 😵

---

<div class="post-metadata">

**Author:** ![newcomer](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/n/e5b9ba/32.png) [@newcomer](https://forum.opencyphal.org/u/newcomer)\
**Post date:** [August 26, 2020, 7:47am UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/5 "2020-08-26T07:47:03Z")

</div>

```auto
CanardRxSubscription heartbeat_subscription;
(void) canardRxSubscribe(&ins, // Subscribe to messages uavcan.node.Heartbeat.
                         CanardTransferKindMessage,
                         32085, // The fixed Subject-ID of the Heartbeat message type (see DSDL definition).
                         7, // The maximum payload size (max DSDL object size) from the DSDL definition.
                         CANARD_DEFAULT_TRANSFER_ID_TIMEOUT_USEC,
                         &heartbeat_subscription)

```

Are these code is decode the data of 32085.Heartbeat.1.0.uavcan?

---

<div class="post-metadata">

**Author:** ![newcomer](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/n/e5b9ba/32.png) [@newcomer](https://forum.opencyphal.org/u/newcomer)\
**Post date:** [August 26, 2020, 7:50am UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/6 "2020-08-26T07:50:30Z")

</div>

```auto
uint8_t mode = canardDSDLGetU8(heartbeat_transfer->payload, heartbeat_transfer->payload_size, 34, 3);
uint32_t uptime = canardDSDLGetU32(heartbeat_transfer->payload, heartbeat_transfer->payload_size, 0, 32);
uint32_t vssc = canardDSDLGetU32(heartbeat_transfer->payload, heartbeat_transfer->payload_size, 37, 19);
uint8_t health = canardDSDLGetU8(heartbeat_transfer->payload, heartbeat_transfer->payload_size, 32, 2);

```

Does these code mean that i don’t need to use nunauvt to decode the 32085.Heartbeat.1.0.uavcan to a .h file?

---

<div class="post-metadata">

**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:** [August 26, 2020, 1:10pm UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/7 "2020-08-26T13:10:52Z")

</div>

> [@newcomer](#):
>
> get data from 32085.Heartbeat.1.0.uavcan

`*.uavcan` files do not contain any data. They merely specify what the data should look like. In order to publish a UAVCAN message you need to serialize it per the format defined in the respective `*.uavcan` file (vice versa when subscribing). You can do that manually with the help of the `canardDSDL...()` functions, or you could use Nunavut to generate the serialization/deserialization code automatically.

> [@newcomer](#):
>
> Are these code is decode the data of 32085.Heartbeat.1.0.uavcan?

No. The fragment you posted instructs the library to accept Heartbeat transfers from the network. It is up to you then to deserialize their payload using either of the methods I mentioned above.

---

<div class="post-metadata">

**Author:** ![newcomer](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/n/e5b9ba/32.png) [@newcomer](https://forum.opencyphal.org/u/newcomer)\
**Post date:** [August 27, 2020, 12:55am UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/8 "2020-08-27T00:55:17Z")

</div>

Thanks for your reply! I still have the following quesion:  
Quesion 1: Could you please tell me the common command of nnvg to serialize/deserialize?  
Quesion 2: What does serialization and deserialization mean? I think of serialization and deserialization as generating a file. Is that right?  
Quesion 3:Do i have to subscribe to messages uavcan.node.Heartbeat?  
I sincerely hope you can reply me. Thanks a lot !

---

<div class="post-metadata">

**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:** [August 27, 2020, 9:30pm UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/9 "2020-08-27T21:30:06Z")

</div>

> [@newcomer](#):
>
> Quesion 1: Could you please tell me the common command of nnvg to serialize/deserialize?

Nunavut cannot serialize or deserialize anything. It can only create code for you that performs serialization/deserialization of your data types. Please read the Nunavut documentation for further information; if you have any _specific_ questions about that, feel free to ask them here.

> [@newcomer](#):
>
> Quesion 2: What does serialization and deserialization mean? I think of serialization and deserialization as generating a file. Is that right?

No. Serialization means converting an application data object (or, more generally, any pertinent state of your application) into a well-defined representation. Deserialization is the inverse of serialization. Please read this:

> **[Serialization](https://en.wikipedia.org/wiki/Serialization)**
>
> In computing, serialization (or serialisation, also referred to as pickling in Python) is the process of translating a data structure or object state into a format that can be stored (e.g. files in secondary storage devices, data buffers in primary storage devices) or transmitted (e.g. data streams over computer networks) and reconstructed later (possibly in a different computer environment). When the resulting series of bits is reread according to the serialization format, it can be used to c Th...

> [@newcomer](#):
>
> Quesion 3:Do i have to subscribe to messages uavcan.node.Heartbeat?

No, it is entirely optional. You do have to publish them, though.

---

<div class="post-metadata">

**Author:** ![newcomer](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/n/e5b9ba/32.png) [@newcomer](https://forum.opencyphal.org/u/newcomer)\
**Post date:** [August 31, 2020, 8:41am UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/13 "2020-08-31T08:41:30Z")

</div>

- Do I need to serialize a string when I send it? For example “Hello UAVCAN”

```auto
const CanardTransfer transfer = {
			    .timestamp_usec = 0,      
			    .priority = CanardPriorityNominal,
			    .transfer_kind = CanardTransferKindMessage,
			    .port_id = 1234, // This is the subject-ID. 
			    .remote_node_id = CANARD_NODE_ID_UNSET, // Messages cannot be unicast, so use UNSET.
			    .transfer_id = my_message_transfer_id,
			    .payload_size = 13,
			    .payload = "Hello UAVCAN",
			};

```

---

<div class="post-metadata">

**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:** [August 31, 2020, 11:45am UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/14 "2020-08-31T11:45:12Z")

</div>

Yes, of course. Strings are like variable-length arrays of `uint8`: they are encoded in UTF-8 and prefixed with the length prefix (`uint8`/`uint16`/`uint32`). This is explained in the Specification.

---

<div class="post-metadata">

**Author:** ![newcomer](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/n/e5b9ba/32.png) [@newcomer](https://forum.opencyphal.org/u/newcomer)\
**Post date:** [September 2, 2020, 6:24am UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/16 "2020-09-02T06:24:08Z")

</div>

```auto
const CanardTransfer transfer = {
			    .timestamp_usec = 0,      
			    .priority = CanardPriorityNominal,
			    .transfer_kind = CanardTransferKindMessage,
			    .port_id = 1234, // This is the subject-ID. 
			    .remote_node_id = CANARD_NODE_ID_UNSET, // Messages cannot be unicast, so use UNSET.
			    .transfer_id = my_message_transfer_id,
			    .payload_size = 13,
			    .payload = "Hello UAVCAN",
			};

```

what is the difference between transfer\_id and port\_id ?

---

<div class="post-metadata">

**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:** [September 2, 2020, 2:42pm UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/17 "2020-09-02T14:42:06Z")

</div>

> [@newcomer](#):
>
> what is the difference between transfer\_id and port\_id ?

Please read chapter **2 “Basic concepts”** of the [UAVCAN Specification](https://uavcan.org/specification). Briefly:

- Port-ID (subject-ID, service-ID) specifies the purpose of the data. This is like the topic name in ROS or DDS or some MQs.

- Transfer-ID identifies transfers (that is, messages in case of pub/sub links) under the same port-ID. It allows you to distinguish multiple messages received from a given subject.

---

<div class="post-metadata">

**Author:** ![newcomer](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/n/e5b9ba/32.png) [@newcomer](https://forum.opencyphal.org/u/newcomer)\
**Post date:** [September 3, 2020, 7:47am UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/18 "2020-09-03T07:47:48Z")

</div>

![4ee763e4aedcc11e15307830b87e3b7](https://forum.opencyphal.org/uploads/default/original/1X/b2c409d88328b8477b7c283f6d1ce34d715dec2e.png)  
Does unique node-ID mean transfer-ID?

---

<div class="post-metadata">

**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:** [September 3, 2020, 4:10pm UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/19 "2020-09-03T16:10:52Z")

</div>

No. If they were the same thing, they would have been named identically. Please read the specification, every question you asked so far is answered there.

---

<div class="post-metadata">

**Author:** ![newcomer](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/n/e5b9ba/32.png) [@newcomer](https://forum.opencyphal.org/u/newcomer)\
**Post date:** [September 5, 2020, 5:49am UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/20 "2020-09-05T05:49:00Z")

</div>

```auto
use libcanard v1
platform: stm32h743 baremetal platforms
runs results: No matching subscription.

void UavcanRun(void)
{

	static uint8_t a = 0;
	CanardInstance ins = canardInit(&memAllocate, &memFree);
	ins.mtu_bytes = CANARD_MTU_CAN_FD; //Defaults to 64 (CAN FD); here we select Classic CAN.
	ins.node_id = 42;
    a++；
	if(a == 1) //subcribe once 
	{
	 a++;
	CanardRxSubscription heartbeat_subscription;
	(void) canardRxSubscribe(&ins, // Subscribe to messages uavcan.node.Heartbeat.
							 CanardTransferKindMessage,
							 32085, // The fixed Subject-ID of the Heartbeat message type (see DSDL definition).
							 7, // The maximum payload size (max DSDL object size) from the DSDL definition.
							 CANARD_DEFAULT_TRANSFER_ID_TIMEOUT_USEC,
							 &heartbeat_subscription);

	CanardRxSubscription mymsg_subscription;
	(void) canardRxSubscribe(&ins, // Subscribe to messages uavcan.node.Heartbeat.
							 CanardTransferKindMessage,
							 4416, // The fixed Subject-ID of the Heartbeat message type (see DSDL definition).
							 2, // The maximum payload size (max DSDL object size) from the DSDL definition.
							 CANARD_DEFAULT_TRANSFER_ID_TIMEOUT_USEC,
							 &mymsg_subscription);
	}

		if(cnt %1000 == 0){ //excute per 1000ms 
			uint8_t buffer[7] = {0};  
			canardDSDLSetUxx(&buffer[0], 34, 2, 3); // mode
			canardDSDLSetUxx(&buffer[0], 32, 2, 2); // health
			canardDSDLSetUxx(&buffer[0], 0, 0xDEADBEEF, 32); // uptime
			canardDSDLSetUxx(&buffer[0], 37, 0x7FFFF, 19); // vssc
			static uint8_t heartbeat_transfer_id;
			 
			const CanardTransfer transfer = {
				.timestamp_usec = HAL_GetTick(),      
				.priority = CanardPriorityNominal, 
				.transfer_kind = CanardTransferKindMessage,
				.port_id = 32085, // This is the subject-ID.
				.remote_node_id = CANARD_NODE_ID_UNSET, // Messages cannot be unicast, so use UNSET.
				.transfer_id = heartbeat_transfer_id,
				.payload_size = 7,   
				.payload = &buffer[0], 
			};
			++heartbeat_transfer_id; 
			int32_t result = canardTxPush(&ins, &transfer);
			printf("------------------send hearttransfer-------------------------\n");
			if (result < 0)
			{
				// An error has occurred: either an argument is invalid or we've ran out of memory.
				// It is possible to statically prove that an out-of-memory will never occur for a given application if the
				// heap is sized correctly; for background, refer to the Robson's Proof and the documentation for O1Heap.
		// abort();
				printf("Illegal parameter or Memory Exhausted \n");
	// return ;
			}
		}

		if(cnt %200 == 0){//excute per 200ms 
				uint8_t my_msg[2] = {0};
				canardDSDLSetUxx(&my_msg[0], 0, 120, 8);  
				canardDSDLSetUxx(&my_msg[0], 8, 110, 8);   
				static uint8_t mymsg_transfer_id;
				 
				const CanardTransfer mytransfer = {
					.timestamp_usec = HAL_GetTick(),      
					.priority = CanardPriorityNominal, 
					.transfer_kind = CanardTransferKindMessage,
					.port_id = 4416, // This is the subject-ID.
					.remote_node_id = CANARD_NODE_ID_UNSET, // Messages cannot be unicast, so use UNSET.
					.transfer_id = mymsg_transfer_id,
					.payload_size = 2,   
					.payload = &my_msg[0], 
				};
				++mymsg_transfer_id;  
				int32_t myresult = canardTxPush(&ins, &mytransfer); 
				printf("---------------------send mytransfer------------\n");
				if (myresult < 0)
				{
					// An error has occurred: either an argument is invalid or we've ran out of memory.
					// It is possible to statically prove that an out-of-memory will never occur for a given application if the
					// heap is sized correctly; for background, refer to the Robson's Proof and the documentation for O1Heap.
			// abort();
					printf("Illegal parameter or Memory Exhausted \n");
			// return ;
				}
		}
		    static uint8_t fail_count;
			FDCAN_TxHeaderTypeDef *txmsg = NULL;
			uint8_t sendbuf[7] = {0}; 

			for (const CanardFrame* txf = NULL; (txf = canardTxPeek(&ins)) != NULL;) // Look at the top of the TX queue.
			{
				printf("enter txmsg:\n");
				static uint8_t i = 0;
				txmsg->Identifier = txf->extended_can_id >> 8 & 0x00ffffU;  
				txmsg->DataLength = txf->payload_size << 16 & 0x000F0000U; 
				txmsg->IdType = FDCAN_EXTENDED_ID;
				txmsg->FDFormat = FDCAN_FD_CAN;
				txmsg->TxFrameType = FDCAN_DATA_FRAME;
				txmsg->BitRateSwitch = FDCAN_BRS_OFF;
				txmsg->TxEventFifoControl = FDCAN_STORE_TX_EVENTS;
				txmsg->ErrorStateIndicator = FDCAN_ESI_ACTIVE;
				txmsg->MessageMarker = 0x01;

				
				memcpy(sendbuf,txf->payload,txf->payload_size); 

			HAL_StatusTypeDef ret = HAL_FDCAN_AddMessageToTxFifoQ(&hfdcan1, txmsg, sendbuf); //stm32 hal library
			HAL_FDCAN_Start(&hfdcan1); 
			HAL_FDCAN_EnableTxBufferRequest(&hfdcan1, FDCAN_TX_BUFFER0);

			if(ret != HAL_OK)
			{

				// just exit and try again later. If we fail 8 times in a row
				// then start discarding to prevent the pool filling up
				if (fail_count < 8) {
					fail_count++;
					printf("send fail,fail_count:%d\n",fail_count);
				} else {
					canardTxPop(&ins);
					printf("fail_count 8 times, exit send\n");
					ins.memory_free(&ins, (CanardFrame*)txf); // Deallocate the dynamic memory afterwards.
				}
				return;
			}
			else{
				canardTxPop(&ins);
				fail_count = 0;
				ins.memory_free(&ins, (CanardFrame*)txf); // Deallocate the dynamic memory afterwards.
			}

		}

//////////////////////////	//receive serialized datat //////////////////////////

				FDCAN_RxHeaderTypeDef RxHeader={0};
				uint32_t RxLocation = 0;
				uint8_t recdata[8] = {0};  
			   	CanardFrame received_frame;
			   	received_frame.payload = &recdata;

				 if(HAL_FDCAN_GetRxMessage(&hfdcan1, FDCAN_RX_FIFO0, &RxHeader, recdata) == HAL_OK) 

				{
				printf("RxHeader.Identifier = %d\n",RxHeader.Identifier);

				CanardTransfer receive={0};

				if(RxHeader.Identifier == 32085)
					received_frame.payload_size = 7;
				else
					received_frame.payload_size = 2;

				received_frame.extended_can_id = RxHeader.Identifier;
				memcpy(received_frame.payload,recdata,received_frame.payload_size);

				uint8_t result = canardRxAccept(&ins,
										&received_frame, // The CAN frame received from the bus.
										0, // If the transport is not redundant, use 0.
										&receive);
				if (result < 0)
				{
					// An error has occurred: either an argument is invalid or we've ran out of memory.
					// It is possible to statically prove that an out-of-memory will never occur for a given application if
					// the heap is sized correctly; for background, refer to the Robson's Proof and the documentation for O1Heap.
					// Reception of an invalid frame is NOT an error.
				

					printf("RxAccept failure\n");

					printf("An error has occurred: either an argument is invalid or we've ran out of memory.\n");
					abort();
				}
				else if (result == 1) 
				{
					printf("Receive Port ID :%d\n",receive.port_id);
					//processReceivedTransfer(redundant_interface_index, &transfer); // A transfer has been received, process it.
					ins.memory_free(&ins, (void*)receive.payload); // Deallocate the dynamic memory afterwards.
				}
				else
				{
					// Nothing to do.
					// The received frame is either invalid or it's a non-last frame of a multi-frame transfer.
					// Reception of an invalid frame is NOT reported as an error because it is not an error.
					printf("nothing to do \n");

				}
}

```

Thanks for your reply.I run this code ,i found it return " No matching subscription. " at canardRxAccept.  
Is UAVCAN subscribed, sent and received in my code correct?  
–\>use libcanard v1 ------[https://github.com/UAVCAN/libcanard](https://github.com/UAVCAN/libcanard)  
–\>platform: stm32h743 baremetal platforms  
–\>runs results: No matching subscription.

---

<div class="post-metadata">

**Author:** ![newcomer](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/n/e5b9ba/32.png) [@newcomer](https://forum.opencyphal.org/u/newcomer)\
**Post date:** [September 5, 2020, 4:25pm UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/21 "2020-09-05T16:25:43Z")

</div>

And how to make rxAcceptFrame function return 1, so i can handle the received data.

 ![image](https://forum.opencyphal.org/uploads/default/original/1X/c9fb6f2eff4d8cfa4ea0df3448398c691c072974.png)

---

<div class="post-metadata">

**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:** [September 5, 2020, 9:31pm UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/22 "2020-09-05T21:31:05Z")

</div>

There are many logical errors in your code; I will mention only some of the most critical ones.

1. It would appear that the function `UavcanRun()` is to be invoked periodically while updating an external state. Yet the state is not retained between invocations – the library instance is reinitialized from scratch upon the entry.

2. Even if the above were to be done correctly, it still wouldn’t work because you would be destroying and re-initializing the subscription instances every 256th invocation due to the integer overflow.

3. This manipulation destroys the identifier: `txmsg->Identifier = txf->extended_can_id >> 8 & 0x00ffffU;`

4. This is incorrect because the original frame payload length information is overwritten by your hardcoded constants; you need to use the length information from the driver:

```auto
if (RxHeader.Identifier == 32085)
    received_frame.payload_size = 7;
else
    received_frame.payload_size = 2;

```

1. This memcpy copies the data back where it was – the source and the destination addresses are identical, which results in undefined behavior:

```auto
uint8_t recdata[8] = {0};
CanardFrame received_frame;
received_frame.payload = &recdata;
...
memcpy(received_frame.payload,recdata,received_frame.payload_size);

```

My ability to look through your code is limited because doing so is rather time-consuming. You seem to lack certain fundamental knowledge like understanding the concept of state, memory addressing, and memory management. This remark is not to pontificate but to help – consider practicing with simpler tasks first and build the complexity gradually. You may find help with that elsewhere.

---

<div class="post-metadata">

**Author:** ![newcomer](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/n/e5b9ba/32.png) [@newcomer](https://forum.opencyphal.org/u/newcomer)\
**Post date:** [September 6, 2020, 1:41pm UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/23 "2020-09-06T13:41:32Z")

</div>

Thank you very much! how about this? It that right to give the handled length?

```auto
txmsg->DataLength = txf->payload_size << 16 & 0x000F0000U; 

```

---

<div class="post-metadata">

**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:** [September 6, 2020, 3:44pm UTC](https://forum.opencyphal.org/t/various-questions-about-libcanard/928/24 "2020-09-06T15:44:01Z")

</div>

```auto
txmsg->DataLength = txf->payload_size;

```

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