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linuxcan-bussocketcan

Send one-shot message using SocketCAN

Is it possible to send a "one-shot" CAN message using SocketCAN? This is basically a message that does not expect an acknowledgement back from the receiver. The protocol allows this ACK bit not to be set.

Solution

  • I'm not sure what you mean by "one-shot", but I send single CAN frames all the time using the cansend utility from can-utils, like this:

    $ cansend can0 135#02080426A10D112A

    I've also written C programs that do the same using an AF_CAN socket. (You can basically crib from cansend.c to figure out how.)

    Regarding retransmission of messages, this doesn't happen with cansend:

    $ timeout 20s tcpdump -i can0 -w out.pcap &                                                                                                                                               
[1] 16509
tcpdump: listening on can0, link-type CAN_SOCKETCAN (CAN-bus with SocketCAN headers), capture size 262144 bytes

$ cansend can0 135#02080426A10D112A

$ 1 packet captured
0 packets received by filter
0 packets dropped by kernel

[1]  + 16509 exit 124   timeout 20s tcpdump -i can0 -w out.pcap

    In the sequence above, there are no consumers online, so nobody ACKs the message, but I still only capture a single packet on can0. It seems safe to conclude that the SocketCAN stack isn't 'automagically' trying to re-send the message in the background after cansend exits.