The names of the union fields in Linux's ifaddrs are named incorrectly.

[dlangBugzillaToGithub]

Jonathan M Davis (@jmdavis) reported this on 2024-02-23T07:01:27Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=24404

Description

core.sys.linux.ifaddrs's ifaddrs struct incorrectly names the ifu_dstaddr field if_dstaddr.

https://www.man7.org/linux/man-pages/man3/getifaddrs.3.html

[dlangBugzillaToGithub]

issues.dlang (@jmdavis) commented on 2024-02-23T07:39:37Z

Actually, the situation is a bit more nuanced, though druntime still has the name wrong.

```
struct ifaddrs {
               struct ifaddrs  *ifa_next;    /* Next item in list */
               char            *ifa_name;    /* Name of interface */
               unsigned int     ifa_flags;   /* Flags from SIOCGIFFLAGS */
               struct sockaddr *ifa_addr;    /* Address of interface */
               struct sockaddr *ifa_netmask; /* Netmask of interface */
               union {
                   struct sockaddr *ifu_broadaddr;
                                    /* Broadcast address of interface */
                   struct sockaddr *ifu_dstaddr;
                                    /* Point-to-point destination address */
               } ifa_ifu;
           #define              ifa_broadaddr ifa_ifu.ifu_broadaddr
           #define              ifa_dstaddr   ifa_ifu.ifu_dstaddr
               void            *ifa_data;    /* Address-specific data */
           };
```

The union's field is suppposed to be ifu_dstaddr rather than if_dstaddr, though you would have to access it through the union to use that name. The #defined names - ifa_broadaddr and ifa_dstaddr - match what other POSIX platforms use, and I'm not sure that it's actually intended that anyone would use the ifu_* names via the union.

So, the correct fix here would either be to make it so that druntime names the union and provides function wrappers (since we can't use aliases to access the ifa_ifu's members) so that it's both possible to access the ifu_* names via the union name and possible to use the ifa_* names - OR we should just use the ifa_* names. I'm inclined to go with the second, since it doesn't require helper functions and matches what other platforms have.

[dlangBugzillaToGithub]

dlang-bot commented on 2024-02-23T07:53:35Z

@jmdavis created dlang/dmd pull request #16233 "Fix Bugzilla issue 24404 - ifaddrs union fields incorrectly name on Linux." fixing this issue:

- Fix Bugzilla issue 24404 - ifaddrs union fields incorrectly name on Linux.
  
  You can see the correct declaration for ifaddrs on Linux here:
  
  https://www.man7.org/linux/man-pages/man3/getifaddrs.3.html
  
  What the fields are supposed to be named is ifa_broadaddr and
  ifa_dstaddr. However, because Linux defines them using a union, it gives
  them different names within the union - ifu_broadaddr and ifu_dstaddr -
  and then #defines the proper names to access the union names.
  
  What druntime did was use the union names - and then incorrectly name
  ifu_dtsaddr as if_dstaddr. So, it was doubly wrong for that field.
  
  The two approaches that we could take here would be to either
  
  1. Turn the union into a type so that we could have an ifa_ifu field to
     allow accessing the ifu_* names that way - as is technically possible
     in C - and then add wrapper functions with the ifa_* names (since we
     couldn't use an alias to access members of the union member
     variable).
  
  2. Just rename the fields to ifa_* and ignore the fact that you can
     technically access the ifu_* fields via the union name in C.
  
  The simpler approach is #2, so that's what this commit does. I'm pretty
  sure that the ability to access the ifu_* fields via the union name is
  just an implementation detail - particularly since other platforms just
  declare the ifa_* names without using a union at all.
  
  Either way, deprecated aliases are provided so that existing code
  doesn't break.

https://github.com/dlang/dmd/pull/16233