Feature Request: Provide if_nametoindex in <net/if.h>

[edubart]

Feature Description

Provide if_nametoindex and if_indextoname functions in <net/if.h>, see https://man7.org/linux/man-pages/man3/if_nametoindex.3.html

Motivation

I was trying to compile libslirp but I got the error:

slirp.c: In function 'get_dns_addr_resolv_conf':
slirp.c:455:28: error: implicit declaration of function 'if_nametoindex' [-Wimplicit-function-declaration]
  455 |                 if_index = if_nametoindex(c + 1);

Possible Implementation

If not easy to implement, at worst case it could fail and set an error.

[jart]

This one is a little tricky, SIOCGIFINDEX is only defined on Linux, FreeBSD, and NetBSD. The indices returned by it are inconsistent from the ordering returned by SIOCGIFCONF. I can't find a definition for these functions in Apple's XNU or Libc codebases. I'm also not entirely sure what this function even does or why it's useful. I would need help understanding it better in order to support it.

[jart]

Here's a definition from Musl you can use that will work on Linux.

unsigned if_nametoindex(const char *name) {
  int fd, rc;
  struct ifreq ifr;
  if ((fd = socket(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC, 0)) < 0)
    return 0;
  strncpy(ifr.ifr_name, name, sizeof ifr.ifr_name);
  rc = ioctl(fd, SIOCGIFINDEX, &ifr);
  close(fd);
  return rc < 0 ? 0 : ifr.ifr_ifindex;
}

char *if_indextoname(unsigned index, char *name) {
  int fd, rc;
  struct ifreq ifr;
  if ((fd = socket(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC, 0)) < 0)
    return 0;
  ifr.ifr_ifindex = index;
  rc = ioctl(fd, SIOCGIFNAME, &ifr);
  close(fd);
  if (rc < 0) {
    if (errno == ENODEV)
      errno = ENXIO;
    return 0;
  }
  return strncpy(name, ifr.ifr_name, IF_NAMESIZE);
}