On Wed, Nov 21, 2007 at 02:23:52PM +0000, Daniel Drake wrote: > network = ieee80211softmac_get_network_by_bssid_locked(mac, > resp->header.addr3); > > addr3 is offset 20 bytes in the struct and is 6 bytes long. Because 20 > is not evenly divisible by 6 does that make it an unaligned access? > Sort of. An "unaligned" access is one where the referenced address is not aligned to the size of the data type being accessed. For example, a 4-byte load must be accessed on 4-byte aligned boundaries: 0xffff0000 0xffff0004 0xffff0008, and so forth. Obviously a single byte load is always aligned. Unaligned accesses are universally bad. On some architectures (x86, ppc) they will be emulated in hardware at a performance cost. On others (sparc, parisc, ia64) they will trap, giving you the option of software emulation (very slow) or sending a SIGBUS. On still others (arm, I think[1]) they'll "align" in hardware, resulting in loads or stores to the wrong address. The Linux networking code is a pretty good example of when to use the support macros (get|put)_unaligned to do the right thing. For something like a packed structure accessed in a data stream, you're invariably going to have to use these. > Is there any documentation I can read on this topic? In my current > uneducated state I'm likely to write further code with these problems... > cheers, Kyle 1. Just a guess based on <asm-arm/unaligned.h> providing macros that actually do something, and the lack of an obvious unaligned trap handler. - To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
