在 2013-1-11，0:42，"Yann Droneaud" <ydroneaud@xxxxxxxxxx> 写道：
>> Hello kernel hackers,
>>
>>
>> I noticed that in [linux]/include/uapi/linux/nbd.h the structure
>>
>> struct nbd_request {
>> __be32 magic;
>> __be32 type; /* == READ || == WRITE */
>> char handle[8];
>> __be64 from;
>> __be32 len;
>> } __attribute__((packed));
>>
>> is packed
>
> Given a target where ABI requires types to be aligned on their size (1 =>
> 1, 2 => 2, 4 => 4, 8 => 8, etc.)
>
>
> 4 + 4 + 8 + 8 + 4 = 28 bytes
>
> To satisfy __be64 alignement (the strictest), it must be aligned on a 8
> bytes boundary, but 28 is not multiple of 8. 32 is. Padding is required.
>
> [The padding is required so that in a array of struct nbd_request[],
> all members of all structures are aligned to their requirement alignement]
>
>
>> but its reply counter part
>>
>> struct nbd_reply {
>> __be32 magic;
>> __be32 error; /* 0 = ok, else error */
>> char handle[8]; /* handle you got from request */
>> };
>
> 4 + 4 + 8 = 16 bytes.
> To satisfy __be32 alignement (the strictest), it must be aligned on a 4
> bytes boundary and 16 is a multiple of 4: no need for padding.
>
>> is not. Since Linux seems to read sizeof(nbd_reply) bytes from the
>> network (see [1]), the number of bytes read varies with the number of
>> bytes of the structure.
>
>
> It won't.
>
>>
>> So my understanding is that if the size of struct nbd_reply varies from
>> platform/compiler to another that means trouble. I wonder:
>>
>> - Is there anything about struct nbd_reply that would keep its size
>> equal everywhere or a reason why variance in size is no problem here?
>>
>> - Any ideas for a platform where you would expect struct nbd_reply to
>> be other than 4 + 4 + 8 = 16 bytes in size?
>
> Look for an ABI that require 'char' with either property:
> - size > 1 byte
> - alignment > 8 (16 bytes !)
>
>
Do you mean this structure size will vary on different platform? If yes, why Linux does not packed here? Thanks
> Regards.
>
> --
> Yann Droneaud
> OPTEYA
>
>
>
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