Re: [Qemu-devel] [PULL 00/13] vhost, virtio, pci, pxe

[Samuel Thibault <samuel.thibault@gnu.org>]

Hello,

Peter Maydell, on Fri 19 Feb 2016 12:09:17 +0000, wrote:
> We can do that, I think, by updating slirp to use C99 flexible
> arrays:

Indeed.

> diff --git a/slirp/mbuf.h b/slirp/mbuf.h
> index 38fedf4..ef5a4f7 100644
> --- a/slirp/mbuf.h
> +++ b/slirp/mbuf.h
> @@ -81,11 +81,9 @@ struct mbuf {
>         Slirp *slirp;
>         bool    resolution_requested;
>         uint64_t expiration_date;
> +       char    *m_ext;
>         /* start of dynamic buffer area, must be last element */
> -       union {
> -               char    m_dat[1]; /* ANSI don't like 0 sized arrays */
> -               char    *m_ext;
> -       };
> +       char    m_dat[];
>  };
> 
>  #define ifq_prev m_prev
> 
> [compile-tested, but need to audit the slirp code to check that
> it doesn't care that m_dat and m_ext no longer have the same
> offset within the struct...

I believe that's true. It needs to be cross-checked though.

> I'm assuming we don't care about
> the fact we've used an extra 4 bytes of struct.]

That's probably fine, yes.

> However that then needs us to add a new disable-warning flag
> -Wno-gnu-variable-sized-type-not-at-end, because slirp.h uses
> these 'struct mbuf's embedded inside another structure,

Uh. It does so for not so useful reasons, it actually really needs a list head
(i.e. a struct quehead), that could probably be reworked.

Samuel