Perl in a Nutshell, 2nd Edition

Initializing a Socket

Both client and server use the socket call to create a socket and associate it with a filehandle. The socket function takes several arguments: the name of the filehandle, the network domain, an indication of whether the socket is stream-oriented or record-oriented, and the network protocol to be used. For example, HTTP (web) transactions require stream-oriented connections running TCP. The following lines create a socket for this case and associates it with the filehandle SH:

use Socket;
socket(SH, PF_INET, SOCK_STREAM, getprotobyname('tcp')) || die $!;

The PF_INET argument indicates that the socket will connect to addresses in the Internet domain (i.e., IP addresses). Sockets with a Unix domain address use PF_UNIX.

Because this is a streaming connection using TCP, we specify SOCK_STREAM for the second argument. The alternative would be to specify SOCK_DGRAM for a packet-based UDP connection.

The third argument indicates the protocol used for the connection. Each protocol has a number assigned to it by the system; that number is passed to socket as the third argument. In the scalar context, getprotobyname returns the protocol number.

Finally, if the socket call fails, the program will die, printing the error message found in $!.