--- /dev/null
+'\"
+'\" Copyright (c) 1996 Sun Microsystems, Inc.
+'\" Copyright (c) 1998-1999 Scriptics Corporation.
+'\"
+'\" See the file "license.terms" for information on usage and redistribution
+'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
+'\"
+.TH socket n 8.6 Tcl "Tcl Built-In Commands"
+.so man.macros
+.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 over IPv4 and IPv6; 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 \fBchan configure\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 IPv4 or IPv6 address such as \fB127.0.0.1\fR or \fB2001:DB8::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
+.
+This 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.
+.RS
+.PP
+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. Synchronous client sockets may
+be switched (after they have connected) to operating in asynchronous
+mode using:
+.PP
+.CS
+\fBchan configure \fIchan \fB\-blocking 0\fR
+.CE
+.PP
+See the \fBchan configure\fR command for more details.
+.PP
+The Tcl event loop should be running while an asynchronous connection
+is in progress, because it may have to do several connection attempts
+in the background. Running the event loop also allows you to set up a
+writable channel event on the socket to get notified when the
+asynchronous connection has succeeded or failed. See the \fBvwait\fR
+and the \fBchan\fR commands for more details on the event loop and
+channel events.
+.PP
+The \fBchan configure\fR option \fB-connecting\fR may be used to check if the connect is still running. To verify a successful connect, the option \fB-error\fR may be checked when \fB-connecting\fR returned 0.
+.PP
+Operation without the event queue requires at the moment calls to \fBchan configure\fR to advance the internal state machine.
+.RE
+.SH "SERVER SOCKETS"
+.PP
+If the \fB\-server\fR option is specified then the new socket will be
+a server that listens on the given \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
+\fBchan configure\fR to read the \fB\-sockname\fR option). If the host
+supports both, IPv4 and IPv6, the socket will listen on both address
+families. 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 (properly
+a command prefix list, see the \fBEXAMPLES\fR below) 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 \fIport\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
+wildcard address so that it can accept connections from any
+interface. If \fIaddr\fR is a domain name that resolves to multiple IP
+addresses that are available on the local machine, the socket will
+listen on all of them.
+.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 does not 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
+\fBchan configure\fR command to retrieve the \fB\-sockname\fR option as
+described below.
+.SH "CONFIGURATION OPTIONS"
+.PP
+The \fBchan configure\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.
+.RS
+.PP
+Note that the error status is reset by the read operation; this mimics
+the underlying getsockopt(SO_ERROR) call.
+.RE
+.TP
+\fB\-sockname\fR
+.
+For client sockets (including the channels that get created when a
+client connects to a server socket) 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.
+.RS
+.PP
+For server sockets this option returns a list of a multiple of three
+elements each group of which have the same meaning as described
+above. The list contains more than one group when the server socket
+was created without \fB\-myaddr\fR or with the argument to
+\fB\-myaddr\fR being a domain name that resolves multiple IP addresses
+that are local to the invoking host.
+.RE
+.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.
+.TP
+\fB\-connecting\fR
+.
+This option is not supported by server sockets. For client sockets, this option returns 1 if an asyncroneous connect is still in progress, 0 otherwise.
+.PP
+.SH "EXAMPLES"
+.PP
+Here is a very simple time server:
+.PP
+.CS
+proc Server {startTime channel clientaddr clientport} {
+ puts "Connection from $clientaddr registered"
+ set now [clock seconds]
+ puts $channel [clock format $now]
+ puts $channel "[expr {$now - $startTime}] since start"
+ close $channel
+}
+
+\fBsocket -server\fR [list Server [clock seconds]] 9900
+vwait forever
+.CE
+.PP
+And here is the corresponding client to talk to the server and extract
+some information:
+.PP
+.CS
+set server localhost
+set sockChan [\fBsocket\fR $server 9900]
+gets $sockChan line1
+gets $sockChan line2
+close $sockChan
+puts "The time on $server is $line1"
+puts "That is [lindex $line2 0]s since the server started"
+.CE
+.SH "HISTORY"
+Support for IPv6 was added in Tcl 8.6.
+.SH "SEE ALSO"
+chan(n), flush(n), open(n), read(n)
+.SH KEYWORDS
+asynchronous I/O, bind, channel, connection, domain name, host, network address, socket, tcp
+'\" Local Variables:
+'\" mode: nroff
+'\" End:
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