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As we all know, in the CAN bus communication protocol, sender know whether the data was successfully sent. I send socketcan data as follows.

ret = write (socket, frame, sizeof (struct can_frame));

However, even if the CAN communication cable is disconnected, the return value of ret is still 16(=sizeof (struct can_frame)).I queried the information and found that the problem was due to the tx_queue of the network stack used by socketcan. When write is called multiple times, the buffer is full and the return value of ret is -1. But this is not the behavior I expect, I hope that every frame of data sent will immediately get the status of success or failure. By

echo 0> / sys / class / net / can0 / tx_queue_len

I want to cancel the tx_queue, but it does not work. What I want to ask is, is there a way to cancel the tx_queue of socketcan, or to get the status of the each sending frame about controller through the API (such as libsocketcan). Thanks.

wyyghst
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  • What do you mean "cable is disconnected"? Is there any receiving node on the bus or not? If there is no receiving node, you cannot have CAN communication. The bus relies on at least one other node acknowledging each sent frame. – Lundin Mar 18 '20 at 11:41
  • @Lundin "cable is disconnected" means that there are no other nodes on the bus. Of course, socketcan will fail to send frame. The behavior I expect is to get the status of the data frame sending failure, but I don't know how to get the status. – wyyghst Mar 19 '20 at 02:33

1 Answers1

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You cannot use write() itself to discover whether a CAN frame was successfully put on the bus, because all it does is write the frame to the in-kernel socket buffer. The kernel then moves the frame to the transmit queue of the SocketCAN network interface, followed by the driver moving it to the transmit buffer of the CAN controller, which finally puts the frame on the bus. What you want is a direct write which bypasses all those buffers, but that's not possible with SocketCAN, even if you set the transmit queue length to 0.

However, there is another way to get confirmation. If you enable the CAN_RAW_RECV_OWN_MSGS socket option (see section 4.1.4 and 4.1.7 in the SocketCAN documentation), you will receive frames that were successfully sent. You'll need to use recvmsg() so you get the message flags. msg_flags will have the MSG_CONFIRM bit set for a frames that was successfully sent by the same socket on which it is received. You won't be informed of failures, but you can detect them by using a timeout for the confirmation.

It's not an ideal solution because it mixes the read and write logic in your application. One way to avoid this would be to use two sockets. One for writing and reading MSG_CONFIRM frames, the other for reading all other frames. You could then create a (blocking) write function that does a write() followed by multiple calls to recvmsg() with an appropriate timeout.

Finally, it is useful to enable error frames (through the CAN_RAW_ERR_FILTER socket option). If you send a frame on a socket with a disconnected cable, this will typically result in a bus off state, which will be reported in an error frame.

Jos Seldenthuis
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