Programming Perl, 4th Edition

getsockopt

getsockopt SOCKET, LEVEL, OPTNAME

This function queries the option named OPTNAME associated with SOCKET at a given LEVEL. Options may exist at multiple protocol levels depending on 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 for TCP, which you can get using getprotobyname.

The function returns a packed string representing the requested socket option, or undef if there is an error with the reason for that error in $!. Just what is in the packed string depends on the LEVEL and OPTNAME; see getsockopt(2) for details. But often the option is an integer, in which case the result is a packed integer, which you can unpack with the “i” (or I) format.

For example, to test whether Nagle’s algorithm is enabled on a socket:

use Socket qw(:all); # assume that $socket hold the handle of a connected socket $tcp = IPPROTO_TCP; $packed = getsockopt($socket, $tcp, TCP_NODELAY) || die ...