Re: [Qemu-devel] [PATCHv7 4/9] slirp: Factorizing tcpiphdr structure with an union

[Thomas Huth]

On 10.02.2016 10:28, Samuel Thibault wrote:
> That one is tricky, yes :)
> 
> Thomas Huth, on Wed 10 Feb 2016 09:05:32 +0100, wrote:
>>> -#define SLIRP_MSIZE (IF_MTU + IF_MAXLINKHDR + offsetof(struct mbuf, m_dat) 
>>> + 6)
>>> +#define SLIRP_MSIZE\
>>> +    (offsetof(struct mbuf, m_dat) + IF_MAXLINKHDR + TCPIPHDR_DELTA + 
>>> IF_MTU)
>>
>> I'm somehow having a hard time to understand why TCPIPHDR_DELTA is used
>> here. As far as I understand, TCPIPHDR_DELTA is the difference between
>> the size of struct tcpiphdr and the size of the IPv4 + TCP header. But
>> if it's just the difference, where does the base size of the headers
>> come from in this define, since the headers are stored in the mbuf, too,
>> aren't they? ... I've got the feeling that I miss something here, could
>> you enlighten me?
> 
> TCP/IP headers are within IF_MTU.

Ah, of course, that makes sense, thanks! ... so in the old definition of
SLIRP_MSIZE, the TCP/IP headers were counted twice, I guess? Anyway, the
definition looks fine to me now.

 Thomas