Programming Perl, 4th Edition

shutdown

shutdown SOCKET, HOW

This function shuts down a socket connection in the manner indicated by HOW. If HOW is 0, further receives are disallowed. If HOW is 1, further sends are disallowed. If HOW is 2, everything is disallowed.

shutdown(SOCK, 0);    # no more reading
shutdown(SOCK, 1);    # no more writing
shutdown(SOCK, 2);    # no more I/O at all

This is useful with sockets when you want to tell the other side you’re done writing but not done reading, or vice versa. It’s also a more insistent form of close because it disables any copies of those file descriptors held in forked processes.

Imagine a server that wants to read its client’s request until end-of-file, then send an answer. If the client calls close, that socket is now invalid for I/O, so no answer would ever come back. Instead, the client should use shutdown to half-close the connection:

say SERVER "my request";            # send some data
shutdown(SERVER, 1);                # send eof; no more writing
$answer = <SERVER>;                 # but you can still read

(If you came here trying to figure out how to shut down your system, you’ll have to execute an external program to do that. See system.)