However, while the ip6_hdrctl union is 64 bits,
it's largest member is only 32 bits.
So it is forced to align to 32 bits, not 64.
On 64-bit platforms which care about 64-bit alignment
this may need to be enforced via a 64-bit dummy
union member or pragma.
Rgds, Stuart.
Stuart Remphrey
RMIT ITS Infrastructure Services - Unix Systems
Phone (03) 992 55 070 (or extension 55070)
>>> Hans Werner Strube <strube@physik3.gwdg.de> 06/06/07 5:40 PM >>>
Geoff Adams wrote:
> struct ip6_hdr {
> union {
> struct ip6_hdrctl {
> u_int32_t ip6_un1_flow; /* 20 bits of flow-
> ID */
> u_int16_t ip6_un1_plen; /* payload length */
> u_int8_t ip6_un1_nxt; /* next header */
> u_int8_t ip6_un1_hlim; /* hop limit */
> } ip6_un1;
> u_int8_t ip6_un2_vfc; /* 4 bits version, top 4
> bits class */
> } ip6_ctlun;
> struct in6_addr ip6_src; /* source address */
> struct in6_addr ip6_dst; /* destination address */
> } __attribute__((__packed__));
>
> Sure enough, that 8-bit ip6_un2_vfc un-aligns ip6_src.
Hardly. ip6_ctlun is a union, not a struct. ip6_un2_vfc is just made
equivalent to the first 8 bits of ip6_un1, which has size 64 bits, which
is
thus also the size of union ip6_ctlun. This kind of union appears to
depend
on the byte order, but neither ip6_un1_flow nor ip6_un2_vfc seem to be
used
anywhere in the ipfilter sources.
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