[Qemu-devel] Re: [PATCH 1/2] Add TAILQ_FOREACH_SAFE

[Jan Kiszka <jan.kiszka@siemens.com>]

Anthony Liguori wrote:
> Jan Kiszka wrote:
>> Add TAILQ iterator that allows to safely remove elements while walking
>> the list.
>>
>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>> ---
>>
>>  sys-queue.h |    6 ++++++
>>  1 files changed, 6 insertions(+), 0 deletions(-)
>>
>> diff --git a/sys-queue.h b/sys-queue.h
>> index 3d0773e..b92a77f 100644
>> --- a/sys-queue.h
>> +++ b/sys-queue.h
>> @@ -210,6 +210,12 @@ struct
>> {                                                                \
>>                 
>> (var);                                                  \
>>                  (var) = ((var)->field.tqe_next))
>>  
>> +#define TAILQ_FOREACH_SAFE(var, head, field,
>> next_var)                  \
>> +        for ((var) =
>> ((head)->tqh_first);                               \
>> +                (var) ? ({ (next_var) = ((var)->field.tqe_next); 1;
>> })  \
>> +                      :
>> 0;                                              \
>> +                (var) = (next_var))
>>   
> 
> So the Linux implementation of this is:
> 
> #define list_for_each_safe(pos, n, head) \
>    for (pos = (head)->next, n = pos->next; pos != (head); \
>        pos = n, n = pos->next)

This would relate to CIRCLEQ, not TAILQ.

> 
> I'd prefer something similar:
> 
> #define TAILQ_FOREACH_SAFE(var, head, field, next_var)
>             for (var = (head)->tqh_first, next_var = var->field.tqe_next;
>                   var;
>                   var = next_var, next_var = var->field.tqe_next)
> 
> As it's functionally equivalent and avoids using a GCC-ism ({}).

Won't fly, next_var can become NULL and would be dereferenced without a
prior check. Unless I'm totally blind now, there is no
TAILQ_FOREACH_SAFE without "GCC-ism". But I think I could switch to
CIRCLEQ and add CIRCLEQ_FOREACH_SAFE instead.

Jan

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