goes_in_main.c

// The actual ISR, modify this to your needs
// Run HAL_CAN_ActivateNotification(&hcan1, CAN_IT_RX_FIFO0_MSG_PENDING) once to set up the ISR 
void HAL_CAN_RxFifo0MsgPendingCallback(CAN_HandleTypeDef *hcan) {
	// Receiving
	CanardCANFrame rx_frame;

	const uint64_t timestamp = HAL_GetTick() * 1000ULL;
	const int16_t rx_res = canardSTM32Recieve(hcan, CAN_RX_FIFO0, &rx_frame);

	if (rx_res < 0) {
		printf("Receive error %d\n", rx_res);
	}
	else if (rx_res > 0)        // Success - process the frame
	{
		canardHandleRxFrame(&canard, &rx_frame, timestamp);
	}
}

// CAN Filter setup, modify this to your needs
// Run this once before calling HAL_CAN_Start()
void setupCANFilter(CAN_HandleTypeDef *hcan) {
    CAN_FilterTypeDef filter;
    filter.FilterBank = 0;
	filter.FilterMode = CAN_FILTERMODE_IDMASK;
	filter.FilterFIFOAssignment = CAN_RX_FIFO0;
	filter.FilterIdHigh = 0;
	filter.FilterIdLow = 0;
	filter.FilterMaskIdHigh = 0;
	filter.FilterMaskIdLow = 0;
	filter.FilterScale = CAN_FILTERSCALE_32BIT;
	filter.FilterActivation = ENABLE;
	filter.SlaveStartFilterBank = 14;

	HAL_CAN_ConfigFilter(hcan, &filter);
}

// Processes the canard Tx queue and attempts to transmit the messages
// Call this function very often to check if there are any Tx to process
// Calling it once every cycle of the while(1) loop is not a bad idea
void processCanardTxQueue(CAN_HandleTypeDef *hcan) {
	// Transmitting

	for (const CanardCANFrame *tx_frame ; (tx_frame = canardPeekTxQueue(&canard)) != NULL;) {
		const int16_t tx_res = canardSTM32Transmit(hcan, tx_frame);

		if (tx_res <= 0) {
			printf("Transmit error %d\n", tx_res);
		} else if (tx_res > 0) {
			printf("Successfully transmitted message\n");
		}

		// Pop canardTxQueue either way
		canardPopTxQueue(&canard);
	}
}