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.
Service details
Port numbers are integers but depending upon your system configuration you may be able to use symbolic names for some services. The calls the getservbyname sockets API function which uses a services database file to convert between port numbers and service names. This function is quite limited on certain platforms (eg:Windows). slebetman: this is partially untrue (see comment below).
Some lists of protocols and their associated port numbers are:
On unix-type systems you very likely have an /etc/services file which is used by this function. It typically has the same syntax as the official IANA list referenced above slebetman: this is also not true, similar syntax but not the same. The typical /etc/services file has the syntax:
# comment
service_name port_number/protocol aliaswhere comments can appear at the end of line and aliases can be used instead of the service_name when requesting for the port number (for example both "smtp" and "mail" refer to port 25). The IANA list on the other hand has the syntax:
service_name port_number/protocol description/comment
# person who requested the port for the service <his@email> OR relevant documentationIn addition, for the IANA list, all text above the lines:
Port Assignments:
Keyword Decimal Description References
------- ------- ----------- ----------are to be considered as comments.
slebetman:On Windows, starting from Windows 2000 you have a C:\WINNT\SYSTEM32\DRIVERS\ETC\SERVICES or C:\WINDOWS\SYSTEM32\DRIVERS\ETC\SERVICES file serving the exact same function as the Unix counterpart and has exactly the same syntax. The currently shipping services file for Windows XP is dated 1999 so it doesn't have newer registered ports such as tcpmux and rje. Nonetheless this file is editable and can be easily updated by downloading the current 2006 IANA definitions from the net.
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 applications.
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 .
http://www.tcl.tk/scripting/netserver.html is an excellent tutorial showing how to write both server and client sides of an echo service.
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 [L3 ].
GPS: from the Tcl Chatroom:
% proc accept {s addr port} {puts $s "Hi and bye!"; close $s}
% socket -server accept 8888Client tclsh:
% set s [socket localhost 8888]
sock3
% gets $sNow 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 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.
[ ... ceptcl ...]
kostix Issues with unexpected remote disconnects and buffered blocking sockets
I've faced a problem while working with blocking sockets on the client side: in my case the remote server could intentionally interrupt the connection. It was a simple command/response prototol, so it was native for a client to use puts and gets on blocking socket to sent/receive ASCII stanzas.
The problem looked like this: when we send a command to the server using puts, it works OK even if the connection was reset by the server. The same thing applies to flush: it completes successfully even if the output buffer has data but there's no TCP connection.
That's all due to the buffering that is on on sockets by default.
There are these ways to know whether the connection was reset by peer:
calls to eof will return true. This is bad in that respect that if we send commands to a server and then reads responses, such "preventive" gets will block forever.
In line buffering mode it will buffer one line. Unfortunately, in either case the first puts will not fail. In the line buffering mode the second call to puts will fail, in the full buffering mode you should call puts untill the buffer is full. This solution is bad when your protocol is line-oriented since you must output data by lines and wait for ACK for that lines.
So you only have to wrap the call to puts in the catch.
Some people on Tcl Chatroom advised to switch to async (non-blocking ) sockets or use expect but I think such simple line-based ASCII protocols can be implemented in pure Tcl and w/o complexities of non-blocking sockets. Just take this buffering issue in mind.
Tcl syntax help - Arts and crafts of Tcl-Tk programming - Category Command - Category Networking