[sheepdog] [PATCH 2/2] fix array index error in function add_new_participant

[Ruoyu]

On 2014年08月12日 13:28, Hitoshi Mitake wrote:
> At Tue, 12 Aug 2014 09:25:49 +0800,
> Ruoyu wrote:
>>
>> On 2014年08月11日 21:45, Hitoshi Mitake wrote:
>>> At Mon, 11 Aug 2014 18:11:44 +0800,
>>> Ruoyu wrote:
>>>> Nothing wrong with fixing it, alhough I don't know exactly
>>>> what will be happened if not patching it.
>>>>
>>>> Signed-off-by: Ruoyu <liangry at ucweb.com>
>>>> ---
>>>>   sheep/vdi.c | 3 ++-
>>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/sheep/vdi.c b/sheep/vdi.c
>>>> index 05cae7b..77dc253 100644
>>>> --- a/sheep/vdi.c
>>>> +++ b/sheep/vdi.c
>>>> @@ -372,11 +372,12 @@ static bool add_new_participant(struct vdi_state_entry *entry,
>>>>   		return true;
>>>>   	}
>>>>   
>>>> -	idx = entry->nr_participants++;
>>>> +	idx = entry->nr_participants;
>>>>   	memcpy(&entry->participants[idx], owner, sizeof(*owner));
>>>>   	entry->participants_state[idx] =
>>>>   		is_modified(entry) ?
>>>>   		SHARED_LOCK_STATE_INVALIDATED : SHARED_LOCK_STATE_SHARED;
>>>> +	entry->nr_participants++;
>>> I think this patch doesn't change the function. Does this fixes a bug?
>> It fixes a logical error. Otherwise, there should be a hole in the array 
>> if new participant is added.
>> For example,
>>
>> 	// supposed entry->nr_participants is 2;
>> 	idx = entry->nr_participants++;
>> 	// idx is 3 now;
> No, idx is 2 in this case. Specification of C guarantees it. e.g.
>
> #include <stdio.h>
>
> int main(void)
> {
> 	int n = 2, m;
>
> 	m = n++;
> 	printf("n: %d, m: %d\n", n, m);
>
> 	return 0;
> }
>
> if you execute the above program, you can see an output:
> n: 3, m: 2
>
> If the expression of increment is put before the incremented variable,
> the assigned variable will be equal to the incremented result.
OK, you are right. The incremented variable confused me.
>
> Thanks,
> Hitoshi