2024-07-24-struct-group

__struct_group

浏览IPv6代码时候，看到这样一个新玩法，
__struct_group

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118 struct ipv6hdr {
...
132         __u8                    hop_limit;
133
134         __struct_group(/* no tag */, addrs, /* no attrs */,
135                 struct  in6_addr        saddr;
136                 struct  in6_addr        daddr;
137         );
138 };

###用法
再看一下用法：

 922 /* copy IPv6 saddr & daddr to flow_keys, possibly using 64bit load/store
 923  * Equivalent to :      flow->v6addrs.src = iph->saddr;
 924  *                      flow->v6addrs.dst = iph->daddr;
 925  */
 926 static inline void iph_to_flow_copy_v6addrs(struct flow_keys *flow,
 927                                             const struct ipv6hdr *iph)
 928 {
 ...
 932         memcpy(&flow->addrs.v6addrs, &iph->addrs, sizeof(flow->addrs.v6addrs));
 ...
 
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###


 11 /**
 12  * __struct_group() - Create a mirrored named and anonyomous struct
 13  *
 14  * @TAG: The tag name for the named sub-struct (usually empty)
 15  * @NAME: The identifier name of the mirrored sub-struct
 16  * @ATTRS: Any struct attributes (usually empty)
 17  * @MEMBERS: The member declarations for the mirrored structs
 18  *
 19  * Used to create an anonymous union of two structs with identical layout
 20  * and size: one anonymous and one named. The former's members can be used
 21  * normally without sub-struct naming, and the latter can be used to
 22  * reason about the start, end, and size of the group of struct members.
 23  * The named struct can also be explicitly tagged for layer reuse, as well
 24  * as both having struct attributes appended.
 25  */
 26 #define __struct_group(TAG, NAME, ATTRS, MEMBERS...) \
 27         union { \
 28                 struct { MEMBERS } ATTRS; \
 29                 struct TAG { MEMBERS } ATTRS NAME; \
 30         } ATTRS
```