On 14/03/14 21:29, Phoebe Buckheister wrote:
> On Fri, March 14, 2014 10:07 pm, Martin Townsend wrote:
>> See below
> […]
>>> +struct ieee802154_hdr_fc {
>>> +#if defined(__LITTLE_ENDIAN_BITFIELD)
>>> + u16 type:3,
>>> + security_enabled:1,
>>> + frame_pending:1,
>>> + ack_request:1,
>>> + intra_pan:1,
>>> + reserved:3,
>>> + dest_addr_mode:2,
>>> + version:2,
>>> + source_addr_mode:2;
>>> +#elif defined(__BIG_ENDIAN_BITFIELD)
>>> + u16 reserved:1,
>>> + intra_pan:1,
>>> + ack_request:1,
>>> + frame_pending:1,
>>> + security_enabled:1,
>>> + type:3,
>>> + source_addr_mode:2,
>>> + version:2,
>>> + dest_addr_mode:2,
>>> + reserved2:2;
>>> +#else
>>> +#error "Please fix <asm/byteorder.h>"
>>> +#endif
>> I'm wondering if using 2 u8's might read better so at least the reserved
>> fields match?
>> like in the following:
>> http://lxr.free-electrons.com/source/drivers/net/ethernet/sun/niu.h#L2706
> They would look contiguous, but they won't be contiguous in memory.
> Declarig the reserved field as u8:3 will cause the allocator to move that
> three-bit field to the next byte in memory, so that's certainly unwanted.
I don't follow, I was thinking that there would be reserved and
reserved2 in both versions
> Using two u8s would also betray that this field is treated as a 16 bit
> entity by the standard and the protocol, and we couldn't cast the header
> pointer to u16* to get the raw value of the fc field, because u16 forces
> the compiler to allocate the struct as though the bitfield sequence was
> one u16.
>
union? maybe I need to do some reading on bitfields :)
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