Hello Patrick,
On 04.02.21 18:47, Patrick Menschel wrote:
Am 04.02.21 um 16:38 schrieb Patrick Menschel:
Hi,
is anyone writing to BCMSocket on the Raspberry Pi from Python3 ?
I'm digging through an endianess / alignment issue on armhf platform.
My testcode [1] that I wrote years ago on works on X86_64 platform but
fails on armhf platform with OSERROR 22 "invalid argument".
Some more details.
The length on X86_64 results in 72 bytes which are consumed by bcm.
On armhf it results in 52 bytes which cause OSError: [Errno 22] Invalid
argument.
Then I started concatenating bytes by hand instead of using ctypes.
What I came around is that frames[0] is somehow expected to be 8 bytes
length although it should be 16 bytes.
struct can_frames[0] is used in this struct definition to point out,
that this struct is followed by a number (0..257) struct can_frame's.
This creates a padding at the end of struct bcm_head, so that the struct
can_frame (which is always 64 bit aligned) can be directly concatenated.
Regards,
Oliver
struct bcm_msg_head {
...
struct can_frame frames[0];
};
I ended up inserting padding 8 bytes instead of frames[0] value and that
actually works.
I have to amend that, it's 4 padding bytes.
struct.pack("=IIIllllII4xIB3x8s",opcode,flags,count,ival1_sec,ival1_usec,ival2_sec,ival2_usec,can_id,nframes,can_id,can_dlc,data)
^^ padding bytes
Sending the resulting 56bytes to bcmsocket gets me a cyclic message on
armhf.
That makes my overall BCMHead 40 bytes and the complete bcm message
including the can frame 56bytes.
[1] https://github.com/menschel/pysocketcan
Thanks,
Patrick
