From mboxrd@z Thu Jan  1 00:00:00 1970
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Date: Sun, 18 Feb 2007 19:24:02 +0100
From: Jan Kiszka <jan.kiszka@domain.hid>
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Subject: [Xenomai-core] Re: Magics of [CAN] message filtering
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To: Wolfgang Grandegger <wg@domain.hid>
Cc: socketcan-core@domain.hid, Oliver Hartkopp <socketcan@domain.hid>, xenomai-core <xenomai@xenomai.org>

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Jan Kiszka wrote:
> Wolfgang Grandegger wrote:
>> Jan Kiszka wrote:
>>> Oliver Hartkopp wrote:
>>>> When you're touching anything inside your API, have you ever thought=
 to add
>>>>
>>>> __attribute__ ((aligned(8)))
>>>>
>>>> to the data[8] element of the struct can_frame?
>>>>
>>>> This would enable you to make 64 bit compares directly in the data
>>>> section of the can_frame ...
>>>>
>>>> typedef __u32 canid_t;
>>>>
>>>> struct can_frame {
>>>>     canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
>>>>     __u8    can_dlc; /* data length code: 0 .. 8 */
>>>>     __u8    data[8] __attribute__ ((aligned(8)));
>>>> };
>>> [Swallowing down my well-known opinion on typeof(can_dlc) :)]
>>>
>>> Yes, this should be done, already for the more urging sake of
>>> unambiguous layout of the structure across the kernel/user space bord=
er.
>> Is this not already the case? At least the size of struct can_frame is=
=20
>> 16 bytes.
>=20
> On all target archs? With all supported compilers? Better make it expli=
cit.

gcc -Os is such an example, toasting the assumed can_frame layout by
moving that 3-byte hole *after* data.

So this was an urging ABI bug, and I applied the alignment to both
stable and trunk Xenomai SVNs. Thanks to Oliver for insisting on this!

Jan


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