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Error binding socket
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Date: 2005-11-09 (12:33)
From: Maurizio Colucci <maurizio.colucci@g...>
Subject: Error binding socket
Hi there. For my free tennis game (http://freetennis.sf.net), I get an
error binding the socket to a port. (Unix_error 50, "bind"). The error
description is "address already in use".

This error only happens the *second* time I start the program. It is
as if the port had not been freed and were still in use. The system is
GNU/Linux Ubuntu Breezy.

However, if I wait about 1minute, or I start the program specifying a
different port, the problem does not happen. It is as if Linux were
freeing the port with a delay.

At first glance, I thought I was not using Unix.close or Unix.socket
correctly, but this does not seem to be the case.

The code is freely available, however here is the relevant part of the code:

(* Here is the socket initialization phase, for both client and server: *)

	      let serverData =
		  let rec tryToConnectNTimes n ~soc ~inet_a ~port=
			  Unix.connect  soc (Unix.ADDR_INET  (inet_a, port) )
		      with Unix.Unix_error _ ->
			  if n = 0 then
			      raise CouldNotConnectToServer
			      ( print_endline ( "The server is down. Retrying " ^
string_of_int (n-1) ^ " times.");
				Unix.sleep 1;
				tryToConnectNTimes (n-1) ~soc ~inet_a ~port )
		  if !server  then
		      let soc = Unix.socket Unix.PF_INET Unix.SOCK_STREAM 0 in
			      Unix.bind soc (Unix.ADDR_INET (Unix.inet_addr_any, !port))
			  with Unix.Unix_error (err, _, _) ->
			      (print_endline ("Error: " ^(Unix.error_message err) ^ ". This
is a known bug. Please wait a few seconds or simply change the port
number with the -port option");
			       exit 0 ));
			  Unix.listen soc 5;
			  print_endline "waiting for client to connect...";
			  let clientSocket, _ = Unix.accept soc in
			  Server ( (soc, clientSocket), Unix.in_channel_of_descr
clientSocket, Unix.out_channel_of_descr clientSocket)

		  else  if 0 != compare !client  ""  then
		      let soc = Unix.socket Unix.PF_INET Unix.SOCK_STREAM 0 in
		      let inet_a = Unix.inet_addr_of_string !client in
		      print_endline "Connecting to server...";
		      ( tryToConnectNTimes 60 ~soc ~inet_a ~port:!port;
			print_endline "Connected to server";
			Client (soc, Unix.in_channel_of_descr  soc, Unix.out_channel_of_descr soc) )

(*... and here is the socket cleanup code *)

	      (match serverData with
		       (* the rule is to shutdown before you close,
			  but this is often automatic. see sockets

		   | Server( (sock, clientSocket), inc, outc) ->
			 print_endline "Shutting down socket";
  			 Unix.shutdown clientSocket Unix.SHUTDOWN_ALL ;
  			 Unix.shutdown sock Unix.SHUTDOWN_ALL ;
  			 Unix.close clientSocket;
  			 Unix.close sock;
		   | Client ( sock, inc, outc) ->
			 print_endline "Shutting down socket";
  			 Unix.shutdown sock Unix.SHUTDOWN_ALL  ;
 			 Unix.close sock;

		   | NeitherServerNorClient -> ());

One doubt I have is the following: in the server code, must I call
close on sock or in clientSocket?

If I am doing something wrong, could you please tell me. :-) Thank you