Sockets in Lisp

I'm trying to communicate from a Lisp script to another program by using TCP/IP sockets (with sbcl and the usocket library in a Linux system). Through some online sources I have managed to put together the following simple code:

(require 'asdf)
(require 'usocket)

(defun start-client (message)
"Connects to server."
(usocket:with-client-socket (socket stream "0.0.0.0" 30000)
(format stream message)
(force-output stream)))

(start-client "Hello!~%")


This code lets me send a message, (I have tested it and it works). My problem is that I need to split this code in two different functions, one for opening the socket connection and another to send different messages at different times. Also I need to add an additional function to receive messages from the other program. However, as I'm quite new with Lisp I have failed to do so.

1 Answer
1

The best way (I think) would be to have your entire script in the scope of with-client-socket. You might have something like a main function where this would fit. This avoids resource leaks. You might want to use a dynamic variable to avoid passing the socket stream manually through function arguments to wherever it is needed.

with-client-socket

main

Otherwise, you have to manage the closing of the socket yourself. Any call path that might lead to program termination needs to be protected by some unwind-protect that closes the socket using usocket:socket-close. For that, you open the socket using usocket:socket-connect with the same arguments as you used for usocket:with-client-socket. (You can take a look at the source for usocket:with-client-socket and usocket:with-connected-socket to see the interactions taking place.)

unwind-protect

usocket:socket-close

usocket:socket-connect

usocket:with-client-socket

usocket:with-client-socket

usocket:with-connected-socket

In order to be able to write to the socket stream (obtainable through (usocket:socket-stream socket)) and close the socket, you need to remember it somewhere, e. g. by binding a dynamic variable.

(usocket:socket-stream socket)

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