Re: [RFC v2 1/7] hashtable: introduce a small and naive hashtable

Sasha Levin
2012-08-03T22:20:02Z

On 08/03/2012 11:48 PM, Tejun Heo wrote:
> Hello,
>
> On Fri, Aug 03, 2012 at 11:41:34PM +0200, Sasha Levin wrote:
>> I forgot to comment on that one, sorry.
>>
>> If we put hash entries after struct hash_table we don't take the
>> bits field size into account, or did I miss something?
>
> So, if you do the following,
>
> struct {
> struct {
> int i;
> long ar[];
> } B;
> long __ar_storage[32];
> } A;

struct A should have been an union, right?

> It should always be safe to dereference A.B.ar[31]. I'm not sure
> whether this is something guaranteed by C tho. Maybe compilers are
> allowed to put members in reverse order but I think we already depend
> on the above.

why is accessing A.B.ar[31] safe?

__ar_storage is only 32*sizeof(long) bytes long, while struct B would need to be 32*sizeof(long) + sizeof(int) bytes long so that A.B.ar[31] access would be safe.

> Thanks.