Re: CAN libpcap capture endianess

[Marc Kleine-Budde <mkl@pengutronix.de>]

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On 09/12/2013 10:47 AM, Ico Doornekamp wrote:
> * On 2013-09-12 10:00:10 +0200, Marc Kleine-Budde wrote:
>  
>> ...but your CAN frames don't look correct in wireshark. I suggest you
>> bring up the command line tools and use candump to display the raw CAN
>> frames. Raw CAN frames, as they enter the application, are in host
>> order. To be precise the can_id is in host order, as all other members
>> are u8. "data" is an array of 8 x u8, which is in fact big endian
>> ordered, if you want to access it with 2x32 or 64 bit.
>>
>>> struct can_frame {
>>>         canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
>>>         __u8    can_dlc; /* frame payload length in byte (0 .. CAN_MAX_DLEN) */
>>>         __u8    data[CAN_MAX_DLEN] __attribute__((aligned(8)));
>>> };
> 
> I just did this. candump shows this:
> 
>   slcan0  58A   [8]  4B 12 20 04 D6 00 00 00
> 
> The raw data is shown in wireshark as
> 
>   8a 05 00 00 08 00 00 00  4b 12 20 04 d6 00 00 00

"8a 05 00 00" in little endian is 0x0000058a, "08 00 00 00" is the dlc
of 8. Looks correct.

> The CAN dissector parses this as identifier 0x0a050000 with the
> 'extended' flag set , which seems to have the wrong endianess.
> 
> The CANopen dissector also interprets the identifier with wrong endian
> order, and is thus not able to decode the rest of the packet.
> 
> If I'm not mistaking, both the CAN and CANopen dissector in wireshark
> are thus assuming wrong endianess. I'll check the source and see if I
> can come up with a proper patch for the wireshark team.

They are assuming the wrong endianes for your capture file. Yegor's
capture file, which comes in another format, is interpreted correctly.

Marc
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