=over

=item getsockopt SOCKET,LEVEL,OPTNAME
X<getsockopt>

Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
Options may exist at multiple protocol levels depending on the socket
type, but at least the uppermost socket level SOL_SOCKET (defined in the
C<Socket> module) will exist.  To query options at another level the
protocol number of the appropriate protocol controlling the option
should be supplied.  For example, to indicate that an option is to be
interpreted by the TCP protocol, LEVEL should be set to the protocol
number of TCP, which you can get using C<getprotobyname>.

The function returns a packed string representing the requested socket
option, or C<undef> on error, with the reason for the error placed in
C<$!>.  Just what is in the packed string depends on LEVEL and OPTNAME;
consult getsockopt(2) for details.  A common case is that the option is an
integer, in which case the result is a packed integer, which you can decode
using C<unpack> with the C<i> (or C<I>) format.

Here's an example to test whether Nagle's algorithm is enabled on a socket:

    use Socket qw(:all);

    defined(my $tcp = getprotobyname("tcp"))
        or die "Could not determine the protocol number for tcp";
    # my $tcp = IPPROTO_TCP; # Alternative
    my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
        or die "getsockopt TCP_NODELAY: $!";
    my $nodelay = unpack("I", $packed);
    print "Nagle's algorithm is turned ",
           $nodelay ? "off\n" : "on\n";

Portability issues: L<perlport/getsockopt>.

=back