Re: rfc: "canonical" radiotap parser

On Sat, Dec 08, 2007 at 11:39:57AM +0100, Johannes Berg wrote:
> Hi Dave,
> 
> I just took a quick look at the parser, it looks pretty good. Andy has
> done something similar and was willing to relicense it under BSD last I
> asked him, his code is part of the Linux kernel right now:
> 
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=net/wireless/radiotap.c;hb=HEAD

I will take a look at that.

> Small question on the code: why did you declare that radiotap_field is
> aligned? It can potentially come from anywhere unaligned, no?

I was trying to give the compiler a hint to help it generate better
code on architectures, such as ARM, where unaligned loads may take
many instructions.  Making the whole structure 64-bit aligned doesn't
sit very well with me.  I believe this is better,

union radiotap_field { 
        int8_t          f_i8[8];
        uint8_t         f_u8[8];
        uint16_t        f_u16[4];
        uint32_t        f_u32[2];
        uint64_t        f_u64;
        struct radiotap_xchan {
                uint32_t        xc_flags;
                uint16_t        xc_mhz;
                uint8_t         xc_chan;
                int8_t          xc_dbm;
        } f_xchan __attribute__((__packed__, __aligned__(4)));
        struct radiotap_chan {
                uint16_t        c_mhz; 
                uint16_t        c_flags;
        } f_chan __attribute__((__packed__, __aligned__(2)));
};
 
Dave

-- 
David Young             OJC Technologies
[email protected]      Urbana, IL * (217) 278-3933 ext 24