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

[Samuel Thibault]

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.

Samuel