socket(n)						       Tcl Built-In Commands							 socket(n)

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NAME

       socket - Open a TCP network connection

SYNOPSIS

       socket ?options? host port

       socket -server command ?options? port
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DESCRIPTION

       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.   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.

       Note  that the default encoding for all sockets is the system encoding, as returned by encoding system.	Most of the time, you will need to
       use fconfigure to alter this to something else, such as utf-8 (ideal for communicating with other Tcl processes) or iso8859-1  (useful  for
       many network protocols, especially the older ones).

CLIENT SOCKETS

       If  the	-server  option is not specified, then the client side of a connection is opened and the command returns a channel identifier that
       can be used for both reading and writing.  Port and host specify a port to connect to;  there must be a	server	accepting  connections	on
       this  port.   Port  is  an  integer  port number (or service name, where supported and understood by the host operating system) and host is
       either a domain-style name such as www.tcl.tk or a numerical IP address such as 127.0.0.1.  Use localhost to refer to the host on which the
       command is invoked.

       The following options may also be present before host to specify additional information about the connection:

       -myaddr addr
	      Addr  gives  the	domain-style  name  or	numerical IP address of the client-side network interface to use for the connection.  This
	      option may be useful if the client machine has multiple network interfaces.  If the option is omitted then the client-side interface
	      will be chosen by the system software.

       -myport port
	      Port  specifies an integer port number (or service name, where supported and understood by the host operating system) to use for the
	      client's side of the connection.	If this option is omitted, the client's port number will be chosen at random by the  system  soft-
	      ware.

       -async The  -async  option  will cause the client socket to be connected asynchronously. This means that the socket will be created immedi-
	      ately but may not yet be connected to the server, when the call to socket returns. When a gets or flush is done on the socket before
	      the  connection  attempt	succeeds or fails, if the socket is in blocking mode, the operation will wait until the connection is com-
	      pleted or fails. If the socket is in nonblocking mode and a gets or flush is done on the socket before the connection  attempt  suc-
	      ceeds  or  fails, the operation returns immediately and fblocked on the socket returns 1. Synchronous client sockets may be switched
	      (after they have connected) to operating in asynchronous mode using:
		     fconfigure chan -blocking 0

	      See the fconfigure command for more details.

SERVER SOCKETS

       If the -server option is specified then the new socket will be a server for the port given by port (either an integer or  a  service  name,
       where  supported and understood by the host operating system; if port is zero, the operating system will allocate a free port to the server
       socket which may be discovered by using fconfigure to read the -sockname option).  Tcl will automatically accept connections to	the  given
       port.   For  each  connection Tcl will create a new channel that may be used to communicate with the client.  Tcl then invokes command with
       three additional arguments: the name of the new channel, the address, in network address notation, of the client's host, and  the  client's
       port number.

       The following additional option may also be specified before port:

       -myaddr addr
	      Addr  gives  the	domain-style  name  or	numerical IP address of the server-side network interface to use for the connection.  This
	      option may be useful if the server machine has multiple network interfaces.  If the option is omitted  then  the	server	socket	is
	      bound to the special address INADDR_ANY so that it can accept connections from any interface.

       Server  channels  cannot  be  used  for	input or output; their sole use is to accept new client connections. The channels created for each
       incoming client connection are opened for input and output. Closing the server channel shuts down the server so	that  no  new  connections
       will be accepted;  however, existing connections will be unaffected.

       Server  sockets depend on the Tcl event mechanism to find out when new connections are opened.  If the application does not enter the event
       loop, for example by invoking the vwait command or calling the C procedure Tcl_DoOneEvent, then no connections will be accepted.

       If port is specified as zero, the operating system will allocate an unused port for use as a server socket.  The port number actually allo-
       cated may be retrieved from the created server socket using the fconfigure command to retrieve the -sockname option as described below.

CONFIGURATION OPTIONS

       The fconfigure command can be used to query several readonly configuration options for socket channels:

       -error This option gets the current error status of the given socket.  This is useful when you need to determine if an asynchronous connect
	      operation succeeded.  If there was an error, the error message is returned.  If there was no error, an empty string is returned.

       -sockname
	      This option returns a list of three elements, the address, the host name and the port number for the socket. If the host name cannot
	      be computed, the second element is identical to the address, the first element of the list.

       -peername
	      This option is not supported by server sockets. For client and accepted sockets, this option returns a list of three elements; these
	      are the address, the host name and the port to which the peer socket is connected or bound. If the host name cannot be computed, the
	      second element of the list is identical to the address, its first element.

EXAMPLES

       Here is a very simple time server:
	      proc Server {channel clientaddr clientport} {
		 puts "Connection from $clientaddr registered"
		 puts $channel [clock format [clock seconds]]
		 close $channel
	      }

	      socket -server Server 9900
	      vwait forever

       And here is the corresponding client to talk to the server:
	      set server localhost
	      set sockChan [socket $server 9900]
	      gets $sockChan line
	      close $sockChan
	      puts "The time on $server is $line"

SEE ALSO

       fconfigure(n), flush(n), open(n), read(n)

KEYWORDS

       bind, channel, connection, domain name, host, network address, socket, tcp

Tcl									8.0								 socket(n)