How to use TCP in Racket?

Question

I was trying to send a message from a client to a server, and print the message on the server.

server.rkt:

#lang racket
(define the-listener (tcp-listen 9876))
(define-values (in out) (tcp-accept the-listener))
(displayln (read in))
(tcp-close the-listener)

client.rkt:

#lang racket
(define-values (in out) (tcp-connect "localhost" 9876))
(write "Hello" out)

I ran server.rkt and then client.rkt in the terminal. But the server only prints #<eof> instead of the Hello message.

Why is that? And how to do it correctly?

Answer 1

You need to flush the output on the client side with flush-output after sending the message. Don't forget to cleanly close the ports with close-input-port and close-output-port after usage too, on both the client and the server.

Edit: To answer the first part of your question, you get #<eof> because your client finishes before its output port has been flushed, which closes this output port, and therefore the server receives this end-of-file message (but means "end-of-stream" rather, here), which you can test with eof-object?.