socket - Open a TCP network connection 

 socket ?options? host port
 socket -server command ?options? port  

This command opens a network socket and returns a channel identifier that may be used in future invocations of commands like read, puts and flush. At present only the [TCP] network protocol is supported;  future releases may include support for additional protocols like [UDP] and [IPX]. The socket command may be used to open either the client or server side of a connection, depending on whether the -server switch is specified.  See also the official documentation at http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/socket.htm

Server sockets can only accept incoming connections when Tcl has an event loop running.  All sockets use the system encoding by default; this is probably not what you want, but it is up to the protocol to define or negotiate something more suitable.
----
How to use the -async option? Pasted from the Tcl'ers chat

 dgp	proc verify s {
	    set msg [fconfigure $s -error]
	    if {$msg ne ""} {
	        # an error connecting
	    }
	   # set up to use $s
	}
        set s [socket -async $host $port]
	fileevent $s writable [list verify $s]
 dgp	over the Tcl versions and platforms, I've found it helpful to test
 dgp	[puts -nonewline $s ""] and [fconfigure $s -peername] to be *really* 
        sure you have a successful socket connection.
----
[Cameron Laird] wrote in [the comp.lang.tcl newsgroup]:

The most widely-used solution ''(for ensuring only a single instance of an app is running on one machine)'', I think, is to set up a do-nothing networking service.  Have your application
  socket -server ... 5678
when it first starts.  If it's unable to do so because another
service is already on 5678, take that as a previous instance
of the application.  Otherwise, continue. For an implementation, see [How do I manage lock files in a cross platform manner in Tcl]

Notice this approach is reasonably portable.

CL himself notes, somewhat later, that this is a bit "tricky", and not on the main stream of socket use; it's really socket's contribution to the theme of [singleton application]s.

----

Sockets are also one way on [How can Tcl programs on two different machines communicate].  [JH] writes on sockets in http://aspn.activestate.com/ASPN/Cookbook/Tcl/Recipe/65436 .

More possibilities are listed in the [Inventory of IPC methods].  Yet
another example of socket service appears in
http://moogy.unstable.org:8080/simple.sockets.tar.gz .


----

One little-known corner of TCP/IP is "[port 0 networking]".


----
Several challenges in effective network programming have to do with
[DNS] pragmatics.  [DKF] wrote,
"numeric addresses are never resolved on socket creation
on Unix, but they *are* on Windows (but I do not know if it is *really* doing
it, as that depends on the Winsock implementation)."






----
One question that seems to arise often is about
[how to send a file through a socket connection].



----
As of 8.5, there's apparently no clean socket [introspection].
Socket handles '''do''' show up in "[file] channels sock*", though,
so, as long as one is willing to rely on this slightly dirty match, the answer
is computable.

[DGP] See Tcl Feature Request 455867
[http://sourceforge.net/tracker/index.php?func=detail&aid=455867&group_id=10894&atid=360894].

----

[GPS]: from the [Tcl Chatroom]:
* xmav000	i'm trying to implement a game client/server architecture. can anyone give me some hints which projects i could look at to get some more ideas. there probably are some chat client/server (which will have to be a part of course)

* GPS	xmav: I have some code for you.	 Server tclsh:
	% proc accept {s addr port} {puts $s "Hi and bye!"; close $s}
	% socket -server accept 8888
Client tclsh:
	% set s [socket localhost 8888]
	sock3
	% gets $s
Now the client will block, because the server isn't running its event loop.  So go to the server tclsh and type: update and you will see in the client: Hi and bye! (Wikified soon)

* GPS	I'm not sure if that's enough of what you need.  Everything for a game server should be built upon this foundation though.

* GPS	fileevent is the command you will use to process sockets when they are readable or writable.

* suchenwi	GPS: hey, that looks cool! PLEASE wikify it (plus discussion)!

[GPS]: I would like to clarify a bit.  [fileevent] interacts with the event loop allowing us to do other things (rather than just blocking) and have an event callback invoked when data is ready to be send/received on a [socket] channel.
----
[Tcl syntax help] - [Arts and crafts of Tcl-Tk programming] - [Category Command]