[Bluez-devel] endianess question

[Bluez-devel] endianess question

[Martin Röhricht]

Hello together,

whenever I take data out of a socket buffer using the get_unaligned()
function like this:
	u16 control = get_unaligned((u16 *) skb->data);
do I have to worry about endianess correctness if I need to do some bit
fiddling stuff?
For example:
The specification states that all packet formats shall use little-endian
byte order, so an iframe is denoted like the following:
| 16 | 15 | 14 | 13 | 12 | 11 | 10 | 09 | 08 | 07 06 05 04 03 02 01
|---SAR---|------------ReqSeq-----------|-R--| ...

if I need to pull the ReqSeq value out of the 16 Bits, do I need to make
use of a function like __btoh(control):
	u16 reqseq = (__btoh16(control) & 0x3F00) >> 8;
or is it enough to say:
	u16 reqseq = (control & 0x3F00) >> 8;
when I took the get_unaligned() function?

Martin



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Re: [Bluez-devel] endianess question

[Marcel Holtmann]

Hi Martin,

> whenever I take data out of a socket buffer using the get_unaligned()
> function like this:
> 	u16 control = get_unaligned((u16 *) skb->data);
> do I have to worry about endianess correctness if I need to do some bit
> fiddling stuff?
> For example:
> The specification states that all packet formats shall use little-endian
> byte order, so an iframe is denoted like the following:
> | 16 | 15 | 14 | 13 | 12 | 11 | 10 | 09 | 08 | 07 06 05 04 03 02 01
> |---SAR---|------------ReqSeq-----------|-R--| ...
> 
> if I need to pull the ReqSeq value out of the 16 Bits, do I need to make
> use of a function like __btoh(control):
> 	u16 reqseq = (__btoh16(control) & 0x3F00) >> 8;
> or is it enough to say:
> 	u16 reqseq = (control & 0x3F00) >> 8;
> when I took the get_unaligned() function?

the get_unaligned() macro has nothing to do with the endian thing. So
you still need to convert from little endian into machine byte order.

Regards

Marcel




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