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'\" RCS: @(#) $Id: socket.n,v 1.1.1.1 2002-01-16 10:25:25 markom Exp $

.so man.macros

.TH socket n 7.5 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.

.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 and \fIhost\fR

is either a domain-style name such as \fBwww.sunlabs.com\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 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.

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.

.SH CONFIGURATION OPTIONS

The \fBfconfigure\fR command can be used to query several readonly

configuration options for socket channels:

.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 "SEE ALSO"

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

.SH KEYWORDS

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