'\"
'\" Copyright (c) 1996 Sun Microsystems, Inc.
'\" Copyright (c) 1998-1999 by Scriptics Corporation.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\"
'\" RCS: @(#) $Id: socket.n,v 1.7.2.3 2004/10/27 14:23:58 dkf Exp $
.so man.macros
.TH socket n 8.0 Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
socket \- Open a TCP network connection
.SH SYNOPSIS
.sp
\fBsocket \fR?\fIoptions\fR? \fIhost port\fR
.sp
\fBsocket\fR \fB\-server \fIcommand\fR ?\fIoptions\fR? \fIport\fR
.BE

.SH DESCRIPTION
.PP
This command opens a network socket and returns a channel
identifier that may be used in future invocations of commands like
\fBread\fR, \fBputs\fR and \fBflush\fR.
At present only the TCP network protocol is supported;  future
releases may include support for additional protocols.
The \fBsocket\fR command may be used to open either the client or
server side of a connection, depending on whether the \fB\-server\fR
switch is specified.
.PP
Note that the default encoding for \fIall\fR sockets is the system
encoding, as returned by \fBencoding system\fR.  Most of the time, you
will need to use \fBfconfigure\fR to alter this to something else,
such as \fIutf\-8\fR (ideal for communicating with other Tcl
processes) or \fIiso8859\-1\fR (useful for many network protocols,
especially the older ones).
.SH "CLIENT SOCKETS"
.PP
If the \fB\-server\fR 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.
\fIPort\fR and \fIhost\fR specify a port
to connect to;  there must be a server accepting connections on
this port.  \fIPort\fR is an integer port number
(or service name, where supported and understood by the host operating
system) and \fIhost\fR
is either a domain-style name such as \fBwww.tcl.tk\fR or
a numerical IP address such as \fB127.0.0.1\fR.
Use \fIlocalhost\fR to refer to the host on which the command is invoked.
.PP
The following options may also be present before \fIhost\fR
to specify additional information about the connection:
.TP
\fB\-myaddr\fI addr\fR
\fIAddr\fR 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.
.TP
\fB\-myport\fI port\fR
\fIPort\fR 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 software.
.TP
\fB\-async\fR
The \fB\-async\fR option will cause the client socket to be connected
asynchronously. This means that the socket will be created immediately but
may not yet be connected to the server, when the call to \fBsocket\fR
returns. When a \fBgets\fR or \fBflush\fR 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 completed or fails. If the
socket is in nonblocking mode and a \fBgets\fR or \fBflush\fR is done on
the socket before the connection attempt succeeds or fails, the operation
returns immediately and \fBfblocked\fR on the socket returns 1.
.SH "SERVER SOCKETS"
.PP
If the \fB\-server\fR option is specified then the new socket
will be a server for the port given by \fIport\fR (either an integer
or a service name, where supported and understood by the host
operating system; if \fIport\fR is zero, the operating system will
allocate a free port to the server socket which may be discovered by
using \fBfconfigure\fR to read the \fB\-sockname\fR 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 \fIcommand\fR
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.
.PP
The following additional option may also be specified before \fIhost\fR:
.TP
\fB\-myaddr\fI addr\fR
\fIAddr\fR 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.
.PP
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.
.PP
Server sockets depend on the Tcl event mechanism to find out when
new connections are opened.  If the application doesn't enter the
event loop, for example by invoking the \fBvwait\fR command or
calling the C procedure \fBTcl_DoOneEvent\fR, then no connections
will be accepted.
.PP
If \fIport\fR is specified as zero, the operating system will allocate
an unused port for use as a server socket.  The port number actually
allocated may be retrieved from the created server socket using the
\fBfconfigure\fR command to retrieve the \fB\-sockname\fR option as
described below.
.SH "CONFIGURATION OPTIONS"
The \fBfconfigure\fR command can be used to query several readonly
configuration options for socket channels:
.TP
\fB\-error\fR
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.
.TP
\fB\-sockname\fR
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.
.TP
\fB\-peername\fR
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.
.PP
.SH "EXAMPLES"
Here is a very simple time server:
.CS
proc Server {channel clientaddr clientport} {
   puts "Connection from $clientaddr registered"
   puts $channel [clock format [clock seconds]]
   close $channel
}

\fBsocket\fR -server Server 9900
vwait forever
.CE
.PP
And here is the corresponding client to talk to the server:
.CS
set server localhost
set sockChan [\fBsocket\fR $server 9900]
gets $sockChan line
close $sockChan
puts "The time on $server is $line"
.CE

.SH "SEE ALSO"
fconfigure(n), flush(n), open(n), read(n)

.SH KEYWORDS
bind, channel, connection, domain name, host, network address, socket, tcp