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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 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 getprotobyname.

The function returns a packed string representing the requested socket option, or undef on error, with the reason for the error placed in $!. 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 unpack with the i (or 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";