Perl Cd Bookshelf [Electronic resources] نسخه متنی

17.9.1. Problem

When a process forks, the child has copies of the parent's open
filehandles, including sockets. When you close a
file or socket, you close only the current process's copy. If another
process (parent or child) still has the socket open, the operating
system doesn't consider their file or socket closed.

Take the case of a socket that data is being sent to. If two
processes have this socket open, one can close it but, because the
other still has it open, the socket isn't considered closed by the
operating system. Until the other process closes
the socket, the process reading from the socket won't get an
end-of-file. This can lead to confusion and deadlock.

To avoid this, either close unused filehandles
after a fork or use shutdown.
The shutdown function is a more insistent form of
close—it tells the operating system that
even though other processes have copies of this filehandle, it should
be marked as closed, and the other end should get an end-of-file if
the processes read from it or a SIGPIPE if they write to it.

The numeric argument to shutdown lets you specify
which sides of the connection are closed. An argument of
0 says that we're done reading data, so the other
end of the socket will get a SIGPIPE if they try writing.
1 says that we're done writing data, so the other
end of the socket will get an end-of-file if they try reading.
2 says we're done reading and writing.

Imagine a server that wants to read its client's request until
end-of-file, and 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.

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

The close and shutdown
functions in Chapter 29 of Programming Perl
and in perlfunc(1); your system's
shutdown(2) manpage (if you have it)

17.8. Finding Your Own Name and Address		17.10. Writing Bidirectional Clients