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Questions about the transfer-ID

I think that I don’t exactly understand how I should work with transfer-ID when using Libcanard. I have read official repository, specification v1.0 and several open source libraries and repositories (like 107-Arduino-UAVCAN). But there is still one little question: should I create unique transfer-ID for each subject? In the official example of Libcanard one can find this comment:

++my_message_transfer_id;  // The transfer-ID shall be incremented after every transmission on this subject.

But in this example only one message is transferring. So suppose that if I have 3 publishing functions:

void publishHeartbeat(CanardInstance* const canard, const uint32_t uptime);
void publishTemperature(CanardInstance* const canard, float temperature);
void publishSpeed(CanardInstance* const canard, float speed);

Should I create static variable for storing transfer-ID counter in each of this functions ?

Also is it correct that Libcanard takes care of the transfer-ID overflow (modulo 32)? If yes then is it necessary for user to do something like this (ArduinoUAVCAN.cpp):

CanardTransferID ArduinoUAVCAN::getNextTransferId(CanardPortID const port_id)
  CanardTransferID const next_transfer_id = (_tx_transfer_map.count(port_id) > 0) ? ((_tx_transfer_map[port_id] + 1) % CANARD_TRANSFER_ID_MAX) : 0;
  _tx_transfer_map[port_id] = next_transfer_id;
  return next_transfer_id;

Thanks in advance.

Yes. Please find the relevant statement highlighted:


Not necessary, but it’s not an error either.

1 Like

Thank you very much, Pavel!