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      unpack "x2 n x4 a16", $_[0]
   } elsif ($af == AF_UNIX) {
      ((Socket::unpack_sockaddr_un $_[0] ^ $sa_un_zero), pack "S", AF_UNIX)
   } else {
      Carp::croak "unpack_sockaddr: unsupported protocol family $af";
   }
}

=item AnyEvent::Socket::resolve_sockaddr $node, $service, $proto, $family, $type, $cb->([$family, $type, $proto, $sockaddr], ...)

Tries to resolve the given nodename and service name into protocol families
and sockaddr structures usable to connect to this node and service in a
protocol-independent way. It works remotely similar to the getaddrinfo
posix function.

For internet addresses, C<$node> is either an IPv4 or IPv6 address, an
internet hostname (DNS domain name or IDN), and C<$service> is either
a service name (port name from F</etc/services>) or a numerical port
number. If both C<$node> and C<$service> are names, then SRV records
will be consulted to find the real service, otherwise they will be
used as-is. If you know that the service name is not in your services
database, then you can specify the service in the format C<name=port>
(e.g. C<http=80>).

If a host cannot be found via DNS, then it will be looked up in
F</etc/hosts> (or the file specified via C<< $ENV{PERL_ANYEVENT_HOSTS}
>>). If they are found, the addresses there will be used. The effect is as
if entries from F</etc/hosts> would yield C<A> and C<AAAA> records for the
host name unless DNS already had records for them.

For UNIX domain sockets, C<$node> must be the string C<unix/> and
C<$service> must be the absolute pathname of the socket. In this case,
C<$proto> will be ignored.

C<$proto> must be a protocol name, currently C<tcp>, C<udp> or
C<sctp>. The default is currently C<tcp>, but in the future, this function
might try to use other protocols such as C<sctp>, depending on the socket
type and any SRV records it might find.

C<$family> must be either C<0> (meaning any protocol is OK), C<4> (use
only IPv4) or C<6> (use only IPv6). The default is influenced by
C<$ENV{PERL_ANYEVENT_PROTOCOLS}>.

C<$type> must be C<SOCK_STREAM>, C<SOCK_DGRAM> or C<SOCK_SEQPACKET> (or
C<undef> in which case it gets automatically chosen to be C<SOCK_STREAM>
unless C<$proto> is C<udp>).

The callback will receive zero or more array references that contain
C<$family, $type, $proto> for use in C<socket> and a binary
C<$sockaddr> for use in C<connect> (or C<bind>).

The application should try these in the order given.

Example:

   resolve_sockaddr "google.com", "http", 0, undef, undef, sub { ... };

=cut

our %HOSTS;          # $HOSTS{$nodename}[$ipv6] = [@aliases...]
our @HOSTS_CHECKING; # callbacks to call when hosts have been loaded
our $HOSTS_MTIME;

sub _parse_hosts($) {
   %HOSTS = ();

   for (split /\n/, $_[0]) {
      s/#.*$//;
      s/^[ \t]+//;
      y/A-Z/a-z/;

      my ($addr, @aliases) = split /[ \t]+/;
      next unless @aliases;

      if (my $ip = parse_ipv4 $addr) {
         ($ip) = $ip =~ /^(.*)$/s if AnyEvent::TAINT;
         push @{ $HOSTS{$_}[0] }, $ip
            for @aliases;
      } elsif (my $ip = parse_ipv6 $addr) {
         ($ip) = $ip =~ /^(.*)$/s if AnyEvent::TAINT;
         push @{ $HOSTS{$_}[1] }, $ip
            for @aliases;
      }
   }
}

# helper function - unless dns delivered results, check and parse hosts, then call continuation code
sub _load_hosts_unless(&$@) {
   my ($cont, $cv, @dns) = @_;

   if (@dns) {
      $cv->end;
   } else {
      my $etc_hosts = length $ENV{PERL_ANYEVENT_HOSTS} ? $ENV{PERL_ANYEVENT_HOSTS}
                      : AnyEvent::WIN32                ? "$ENV{SystemRoot}/system32/drivers/etc/hosts"
                      :                                  "/etc/hosts";

      push @HOSTS_CHECKING, sub {
         $cont->();
         $cv->end;
      };

      unless ($#HOSTS_CHECKING) {
         # we are not the first, so we actually have to do the work
         require AnyEvent::IO;

         AnyEvent::IO::aio_stat ($etc_hosts, sub {
            if ((stat _)[9] ne $HOSTS_MTIME) {
               AE::log 8 => "(re)loading $etc_hosts.";
               $HOSTS_MTIME = (stat _)[9];
               # we might load a newer version of hosts,but that's a harmless race,
               # as the next call will just load it again.
               AnyEvent::IO::aio_load ($etc_hosts, sub {
                  _parse_hosts $_[0];
                  (shift @HOSTS_CHECKING)->() while @HOSTS_CHECKING;
               });
            } else {
               (shift @HOSTS_CHECKING)->() while @HOSTS_CHECKING;
            }
         });
      }